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lamune

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PostSubject: chugga chugga   Mon Aug 03, 2015 2:36 am

Here's another "my wagon runs weird" thread.

Any speculation as to why mine "pulsates" at low RPM under load. Example: crawling into the garage riding the brakes at idle. The car chugs along jerkily, like a cylinder may be weaker or significantly stronger than the rest.

The car always did this, but today it was really bad. Well for a while. The last thing I did to the motor before I took the car off the road for rear-end work was change the fuel injectors. And when I drove it today for the first time in a few weeks, whatever is wrong with it was really terribly noticable. I got the car to a store, went shopping for 20 or 30 minutes, came out and the car ran "normally" after that. Weird.

So off the top of my head, I have some ideas. First, a cylinder may not be firing properly. Possible, but I have replaced every ignition system component- some more than once- so perhaps not terribly likely.

Changing the fuel injectors probably threw the fuel trims off, so perhaps that's to blame.

It could be that one cylinder is just weaker than the rest. Maybe a stuck or burned valve, bad rings, or a leaky head gasket, valve spring, something like that. A sticky valve seems sort of likely to me, maybe the heat soak of it sitting loosened it up?

Just wondering what else might be to blame that I could investigate or if any of you guys have seen this behavior before.
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andy caprice



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PostSubject: Re: chugga chugga   Mon Aug 03, 2015 6:47 am

Just to make sure I understand correctly (not being an english speaker); you mean a jerking forward motion, not the engine shaking the car? Mine did the former when it had a faulty egr valve. It was a new one. I don't know, if it was really faulty, or of wrong kind.
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phantom 309

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PostSubject: Re: chugga chugga   Mon Aug 03, 2015 9:36 am

I had this problem way back with the old gold run of the mill rusty roadie,.

In my case i traced it to a bad temp sensor on the water pump,. the ecm thought it was colder than it was so it was stuffing too much fuel in. Here in the east with the sensor stuck out front like that its not uncommon for the wires to get attacked by the green monster,. playing havoc with readings for the ecm,..
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lamune

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PostSubject: Re: chugga chugga   Mon Aug 03, 2015 10:32 pm

Ok, thanks guys, I'll try and consolidate the response here.

Andy - yes, the jerking is forward (or reverse, if in R) motion, not an engine vibration. I thought the EGR only works when the car is in closed-loop mode, is that right? If that's the case it shouldn't do it when cold. It does do it when cold.

Nick - The sensor itself is new, but I can check the wiring. And also the resistance of the sensor against the temperature chart.

Dan, yeah, I know, but I have yet to figure out how to physically do a compression test on this motor when it's still in the car. You're supposed to do it with a hot motor, and given how hard it is to get the plugs in and out when the thing is cold, I certainly have very little desire to try and thread a compression tester in there.

I'm not sure what it'd do except validate that there's a problem with compression. And if there is, it pretty much means tearing off the heads and investigating anyway, regardless of what the actual issue is. I suppose the only thing it would tell me is if I need to pull the heads off or not, which is definitely a good thing. Smile

EGR is an interesting suggestion to check. I haven't done anything with EGR on the car except fix a broken solenoid when I first got it.
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lamune

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PostSubject: Re: chugga chugga   Thu Aug 06, 2015 3:00 pm

Just out of curiosity, if I were to disconnect the EGR solenoid connector, would the car run fine otherwise? (obviously less efficiently)

That would eliminate the #2 bogeyman on this motor, with #1 bogeyman being the Opti of course.

Edit: also if you guys have any first-hand tips or tricks to get a compression test done on this motor without getting burned or requiring a chiropractor afterward, please do chime in. Smile
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PostSubject: Re: chugga chugga   Sat Aug 08, 2015 9:26 pm

atlantadan wrote:
From experience, I know you can have the exhaust manifolds off in about 20 minutes each side (taking it easy), provided nothing is frozen or locked up. Pull them off and running a compression test is easy

Definitely posted by someone NOT living in the rust belt,... Rolling Eyes
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PostSubject: Re: chugga chugga   Sat Aug 08, 2015 10:40 pm

As suggest above...check your EGR valve and the EGR solenoid. I just changed an Opti and a bunch of other maintenance stuff chasing what sounds like the problem your describing on a 96 RMW. In the end it was the EGR valve. Good luck and keep us posted.
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lamune

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PostSubject: Re: chugga chugga   Fri Aug 14, 2015 12:44 am

Well, it looks like the front-of-engine oil leak I've been chasing is actually the intake manifold leaking, so I'll be tearing that all apart which should make investigation of the EGR passages and system a bit easier.
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PostSubject: Re: chugga chugga   Fri Aug 14, 2015 12:38 pm

felpro 1284 is the gasket you want.

Is it leaking at the back too ? That's generally where it starts. Stick a vacuum gauge on it and check to see what your readings are before and after.
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lamune

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PostSubject: Re: chugga chugga   Fri Aug 14, 2015 11:20 pm

Rock Auto has the Felpro MS95580 set on closeout for cheap, so I got that set. It doesn't look any different 1284 in the pictures except the MS95580 comes with more gaskets. Are they different aside from that?

I'm doing a re-check, I cleaned the block face again under the intake, and I'll let the car sit and I'll see if any oil seeps and wets the block.
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phantom 309

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PostSubject: Re: chugga chugga   Sat Aug 15, 2015 7:26 am

If you clean the intake gap at the front rail with some aerosol parts cleaner,. you can actually caulk it up using your finger,.just like you'd caulk a bath tub or sink,.

It;s only splash oil under the intake no pressure,. if you use black rtv,. it bleds in well and you don't see it,..

If you take the intake off,. put a generous bead of rtv along the front and rear rails rail and a dab in each corner ontop of the port gaskets,.best results is to have patience and let the silicone setup for at least 20 mins,..lt1 is a dry intake so its easier to set into place,..pcv needs to be working well, good hose good etc,.
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lamune

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PostSubject: Re: chugga chugga   Sat Aug 15, 2015 12:53 pm

Nick, that's an awesome idea! Smile
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PostSubject: Re: chugga chugga   Sat Aug 15, 2015 1:41 pm

lamune wrote:
Rock Auto has the Felpro MS95580 set on closeout for cheap, so I got that set. It doesn't look any different 1284 in the pictures except the MS95580 comes with more gaskets. Are they different aside from that?

The actual gaskets are different, but the MS95580 will be perfectly fine for your application if you can get it cheap.

you can see the difference between the Felpro 1284 and the MS95580 [You must be registered and logged in to see this link.]
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lamune

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PostSubject: Re: chugga chugga   Sun Aug 16, 2015 3:39 pm

Thanks Jason, that explains it very nicely.
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lamune

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PostSubject: Re: chugga chugga   Mon Aug 24, 2015 10:55 pm

So anyway, update on this. I replaced some parts in the "usual suspects" list that would explain this chugging issue. I got a new EGR solenoid and motor. The passages in the back were pretty well clogged up with greasy carbon goo. Cleaned it up and installed the new parts.

Seems to actually run worse now. Yay! Very Happy
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phantom 309

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PostSubject: Re: chugga chugga   Tue Aug 25, 2015 6:18 pm

lamune wrote:
So anyway, update on this. I replaced some parts in the "usual suspects" list that would explain this chugging issue. I got a new EGR solenoid and motor. The passages in the back were pretty well clogged up with greasy carbon goo. Cleaned it up and installed the new parts.

Seems to actually run worse now. Yay! Very Happy

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lamune

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PostSubject: Re: chugga chugga   Tue Aug 25, 2015 11:30 pm

Nick: Very Happy

It's ok, I'll figure it out. But I really need a Tech 1 or more likely, a Tech 1 equivalent. I need some real data. I don't mind changing parts as a great learning experience for how to change parts, but throwing parts at this motor doesn't seem to be getting anywhere. Except maybe in oil leak stoppage, I think I've got that under control finally.

With the new sensors and EGR parts, I can make the car backfire if I blip the throttle quickly from idle with the engine warm. Cold, it runs nice and smooth. I almost blew the hockey puck out with one particularly nasty pop!
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phantom 309

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PostSubject: Re: chugga chugga   Wed Aug 26, 2015 5:55 am

Too lean off idle,..

Tps not adjusted right,.

Vacuum leak,..
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PostSubject: Re: chugga chugga   Wed Aug 26, 2015 12:18 pm

lamune wrote:
Nick: Very Happy

It's ok, I'll figure it out. But I really need a Tech 1 or more likely, a Tech 1 equivalent. I need some real data. I don't mind changing parts as a great learning experience for how to change parts, but throwing parts at this motor doesn't seem to be getting anywhere. Except maybe in oil leak stoppage, I think I've got that under control finally.

With the new sensors and EGR parts, I can make the car backfire if I blip the throttle quickly from idle with the engine warm. Cold, it runs nice and smooth. I almost blew the hockey puck out with one particularly nasty pop!

did you do the intake manifold gasket ?

you don't need a tech1. You can get the stuff to scan your car from pcmperformance.com
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lamune

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PostSubject: Re: chugga chugga   Wed Aug 26, 2015 7:42 pm

Too lean off idle,..

Well, it seems temperature dependent. More like closed loop dependent- in open loop it runs much nicer. Too lean in closed loop seems like OXS possibly giving bad readings. Or one of the temperature sensors is off. Or something weird on the EGR side. I'm just trying to think what's different in open vs. closed loop operation. The vapor purge thing is also active only when hot, I believe.

Tps not adjusted right,.

It doesn't have any adjustments Smile - ohmmeter test seems OK. Need to use some scan tool to look at the raw data.

Vacuum leak,..

Always a possibility. Though I would expect that to not care much about the temperature, unless something expands and opens up. Nothing obvious so far. Thankfully there aren't too many vacuum hoses to check, those should be eliminated pretty quickly.
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lamune

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PostSubject: Re: chugga chugga   Wed Aug 26, 2015 7:46 pm

jasonlachapelle wrote:


did you do the intake manifold gasket ?

you don't need a tech1. You can get the stuff to scan your car from pcmperformance.com

Not yet. There could be a vacuum leak there, I would expect.

I've gotten hold of an OBD1 USB cable, I'll see if I can get it to do something useful. Any recommended software packages?
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PostSubject: Re: chugga chugga   Wed Aug 26, 2015 9:02 pm

does it run better if you unplug the maf,.?

do you have a simple vacuum gauge,.?

all this wanting to look at raw data stuff,. is all good and well IF you understand that the raw data gets so skewed none of it will make sense till you fix the problem(s)

O2's will not cause an off idle lean condition,.

Have you verified your iac is working,. ?

How is wot performance?


DON'T throw more new parts at it till you actually determine what the problem is,.
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lamune

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PostSubject: Re: chugga chugga   Thu Aug 27, 2015 12:46 am

Hi Nick, I didn't test the MAF, so here's what I did. Started with a cold motor, started it up. Runs fine. As it warms up you can hear the idle quality decline- gets a bit rougher, a little missing. Chokes when you blip the throttle. Once it warmed up I unplugged the MAF and the motor instantly died. Started it back up and it didn't seem to run much differently, but when you opened the throttle it really stumbled badly- much worse than before. I'm not sure if that's what you wanted me to do, so if I should test that a different way let me know and I will certainly do that. Smile

I don't have a proper vacuum gauge. I do have a hand vacuum pump with a gauge. I'll go pick up a proper one tomorrow if I have time.

WOT is fine, motor really screams once it spins up. Those Flowmasters and other uh- "exhaust mods" certainly contribute to that.

I didn't test the IAC. I took it out and cleaned it and its bore, that's all.
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lamune

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PostSubject: Re: chugga chugga   Fri Aug 28, 2015 11:03 pm

Ok, I've got a vacuum gauge in my hands now. So far what I've been able to determine is that I can suck about a 5 and blow about a 2, so I suck more than I blow, which should be a surprise to no one.

Nick, what do you want me to test? Smile

Oh, so here's a video. I couldn't get it to backfire during the video, but I got a few good bumps before I shot it.

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PostSubject: Re: chugga chugga   Sun Aug 30, 2015 11:59 pm

atlantadan wrote:
The monitoring program I use is free and dead simple. Its 'Scan9495'. You can read about it, and download it [You must be registered and logged in to see this link.].

Thanks Dan! I have it running just fine. I only just gave it a quick run with the car off, but the TPS works normally and the static sensor readings all look good.

Edit: Yes, I mean "I got the scanning program running fine". The car still runs the same. Smile


Last edited by lamune on Mon Aug 31, 2015 10:43 pm; edited 1 time in total
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PostSubject: Re: chugga chugga   Mon Aug 31, 2015 2:43 am

lamune wrote:
Thanks Dan! I have it running just fine.
Guessing you mean the programme, not your car. Do you think you might have a clogged cat ?

The reason Nick wanted you to unplug the MAF is that it forces the car to run in open loop mode, i.e. it takes the O2 sensors out of the equation.

If you're getting backfires through the intake, it is possible you have a plugged cat.
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PostSubject: Re: chugga chugga   Mon Aug 31, 2015 7:03 am

jasonlachapelle wrote:


If you're getting backfires through the intake, it is possible you have a plugged cat.

This is definitely not the problem with this particular car,. Wink

I read all the ignition components have been replaced twice,. What a Face

Is it possible you have #5 and #7 plug wires crossed,.?

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PostSubject: Re: chugga chugga   Mon Aug 31, 2015 12:09 pm

phantom 309 wrote:

This is definitely not the problem with this particular car,. Wink
I read all the ignition components have been replaced twice,. What a Face
Is it possible you have #5 and #7 plug wires crossed,.?

I just watched his vid now.  Looks like you are correct, needle movement doesn't suggest plugged cat.  Pretty good idle vac though.

I had to get some export pipes made for my brother's wagon which had plugged cats and was exhibiting very similar symptoms.
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PostSubject: Re: chugga chugga   Mon Aug 31, 2015 1:20 pm

I've been researching this problem, and have found that in some cases the #7 plug wire will rub against throttle bracket, or the the #3 wire rubbing against the motor mount on the driverside. Causing some driveability issues.

Also with car running check to see if you have vaccum at the PCV tube from intake, if you do then intake gasket is an issue. Just info I have found.
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lamune

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PostSubject: Re: chugga chugga   Tue Sep 01, 2015 12:47 am

Ok, yeah, as Nick alluded to, clogged cats are definitely not the problem here.

My wires aren't run through the stock channels under the exhaust manifold, half of them are gone anyway. So the wires, particularly on the driver side, are run around the outside of the manifold. They are new, so I doubt they're bad or worn through already. I did check them and they all are going to the right place.

I missed the PCV check, I can try that tomorrow.

I ran the scan9495 tool tonight and got two logs. First I started the car cold and just let it idle until it got warm. The car idled pretty well tonight, it was quite smooth, the vacuum gauge held really steady. Still a few light missing here and there. That's the first set of data and graph.

Then, the more interesting part. I started a new scan with the car idling and warm. I then opened the throttle and got some pretty good backfiring. The second data set has definite backfiring in it.

Here's a link to the data, let me know if there's a problem accessing it.

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PostSubject: Re: chugga chugga   Tue Sep 01, 2015 11:09 am

If someone has clogged cats,you better get to the vet right meow! (Cue rim shot.) What a Face
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PostSubject: Re: chugga chugga   Wed Sep 02, 2015 12:59 am

Andebe wrote:
If someone has clogged cats,you better get to the vet right meow! (Cue rim shot.) What a Face

I once had to deal with a clogged cat. It was not pleasant for me or the cat.

I did notice something odd lately, no idea if it's related, but there's a nice big splattered stain on the floor at the exhaust pipes. Rusty and black. Neither is unusual by itself, but I don't know why it shows up now and not before.

Oh I wanted to add that I started the motor up cold while I was spin-testing a new set of brake drums on the car. With the engine cold, blipping the throttle like I was in that video results in as-expected response. No delay, no choking, no backfire. Why this only happens when warm is really the puzzle in my mind. It at least makes physical problems like a blocked exhaust less likely. I suppose something mechanical like a valve sticking or not could be temperature dependent, if it's not some sort of open loop vs. closed loop scenario. I can definitely do more testing, so if anyone has any suggestions go ahead and post them.
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PostSubject: Re: chugga chugga   Thu Sep 03, 2015 2:28 pm

I can't open your file to check your O2 mV, but you're 100% sure that when warm, and you run the car with the MAF unplugged, it still runs as poorly ?
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PostSubject: Re: chugga chugga   Fri Sep 04, 2015 9:09 pm


Did anyone else actually look at the data?

When the throttle is opening  (TPS voltage rising), at above about 30% open and with MAP greater than 90 kPa, spark advance jumps to 252 - 255 degrees.

It would appear that it is maxing out the advance to the extent of the available numbers ( 2E+8 ).

It would tend to make one think that that data is corrupted, but the knock retard kicks in at the same time, implying that it r eally is adding some significant spark advance.

This was bounced off another person and he had never seen this before either.

The conclusion was a horrible tune, the result of a corrupted flash, or a bad PCM.

Is the PCM original, and if so, has it been reprogrammed?

That isn't much to go on, but perhaps it is a start.

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lamune

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PostSubject: Re: chugga chugga   Fri Sep 04, 2015 10:33 pm

Jay - one of my buddies at work noticed the same thing, but we had no idea if that was normal or not.

I have no idea if anyone tinkered with the program. It wouldn't surprise me.

I coincidentally pulled a PCM from a 94 Roadmaster sedan today. I could swap that in and see what happens.

(will a 94 PCM run in a 95? I believe the lockup torque converter behavior is different between 94 and 95, but I can just leave it in park for testing)


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PostSubject: Re: chugga chugga   Fri Sep 04, 2015 10:46 pm

jasonlachapelle wrote:
I can't open your file to check your O2 mV, but you're 100% sure that when warm, and you run the car with the MAF unplugged, it still runs as poorly ?

Yep, it runs worse actually.
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PostSubject: Re: chugga chugga   Sat Sep 05, 2015 12:35 am

Ok, my curiosity got the best of me, and since I couldn't find anything that said the 94 PCM would cause the car to explode or the universe to implode, I decided to try it. Result: no difference. Car still backfires.

But since science is better than a simple status report, I decided to be, well, scientific.

I added two more logs to the the data in my shared folder [You must be registered and logged in to see this link.] dated Sept 4.

The first log, I started the car from cold. I played with the throttle some, to get some data around how it behaves in open loop mode. I waited for the computer to flip over to closed loop mode, and the backfiring immediately starts as soon as we go closed loop. You don't need to watch it, by the way, you can hear it when it switches over. After gathering this log I swapped the PCM for the 94 one. That's the second log. The data looks basically the same to me.

So if it's not the PCM or the tune in the PCM, it seems like something is giving bad data to the PCM and making it pull the timing off so wildly that it's actually backfiring. But what?
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PostSubject: Re: chugga chugga   Sat Sep 05, 2015 11:58 am

start it stone cold,. unlug your temp sensor in the water pump,.

check engine light will come on,. both fans will run ( i can't remember if its a tow pack car or not)

blip the throttle every 15 secs or so,.. see if it still backfires when it warms up,..

if that makes no difference,. unplug both O2 sensors and run the car ,. blipping the throttle etc,..


Need to throw in a little freddy K advice here too,. clean all your grounds on the head by the coil/module,.

make sure they are all hooked up and not 1 broken off in the bundle,..
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lamune

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PostSubject: Re: chugga chugga   Sun Sep 06, 2015 5:23 pm

Nick, the grounds are good. I redid those when I got the car, much better than what was there!

So I did what you asked, but the opposite way. I disconnected the O2s and started it up. It took it longer to start backfiring, but it did. In fact it backfired so bad it blew the puck out this time. That's when I decided to stop the tests. I can try it later with both the O2s and the temp sensor unplugged and see what happens then. I put a log of that in the same place as the others just now. Also the car doesn't throw a check engine light with the O2 sensors completely disconnected? That's weird to me.

Anyway-

I also picked up a fuel pressure tester to see if maybe something is going on there. I get 36 PSI at idle, and if I open the throttle it pops up to about 45. I'm looking into that to see if that's normal or not.
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PostSubject: Re: chugga chugga   Sun Sep 06, 2015 10:06 pm

fuel pressure is good,.

unplug the temp sensor,.
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PostSubject: Re: chugga chugga   Sun Sep 06, 2015 10:16 pm

Uh, wow. The car reached room temperature, so I unplugged the ECT sensor (O2's are still unplugged)

It really didn't want to start, but it eventually did. From basically stone cold it runs really badly in this configuration, and I got backfiring almost immediately. The hockey puck got blown out of the engine bay with a big ball of blue fire, which was pretty amazing to see. The car died of course almost immediately after that. That dataset is now in the folder (DLOG-Save-2015Sep06-190407-1G4BR82P2SR409456)

This is what I get for changing stuff. I'm not sure what change may be to blame. Fuel injectors? Knock sensors?

Edit 1: I put everything back together and at least verified it runs the same. I pulled #1 and #3 plugs to inspect them since they're easy to get to. They look oil fouled to me, consistently black coating, looks a bit wet. I can pull the rest later and see. I wonder if fouled plugs may explain this.
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PostSubject: Re: chugga chugga   Mon Sep 07, 2015 2:01 pm

Ok, #1, 3, 5 and 7 plugs respectively. The evens are a lot harder to get to but I'll probably pull them later. Can you clean these iridium plugs?

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Adding #2,4,6,8. #4 is extra greasy because it fell on the frame rail.

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PostSubject: Re: chugga chugga   Mon Sep 07, 2015 3:19 pm

Hi Dan! Yes, they're black and varying degrees of wet. I guess short term I can clean them (nylon brush and solvent? Don't want to damage the iridium) and see if the backfiring goes away.

If a cold compression test will tell me anything useful, I can probably do that now. The problem with the compression test is that it's not easy to determine what the cause of low compression is, is it? It could be any number of things.

If the problem is bad valve seats, that's pretty easy to do. I can pull the heads and have them redone, but that only makes sense if the rings are still in good shape.
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phantom 309

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PostSubject: Re: chugga chugga   Mon Sep 07, 2015 5:19 pm

tangent best describes this thread,. the car will run with out 02's hooked up,. knock sensors hooked up,..

will you calm down with the wanting to change stuff all the time,..

now i,m thinking you have fuel injector plug mixed up,..??

didn't this engine run fine till you changed everything around??

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lamune

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PostSubject: Re: chugga chugga   Mon Sep 07, 2015 7:27 pm

Nick, bear with me as I work through this one. I'm hoping I'm on to something here. Smile

Yeah it could be another tangent. Anyway this is in the general discussion area, thankfully. It's getting pretty general.

The wires out of the harness going to the injectors come out in very obvious spots, so I don't think there's a mix-up problem there. Sadly the wiring colors in the FSM do not match the wire colors coming out of the harness except for #1 and #2 injectors, and I checked both the regular book and the supplement too.
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phantom 309

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PostSubject: Re: chugga chugga   Mon Sep 07, 2015 9:43 pm

did you pull the intake manifold?

does it actually run SMOOTHLY on all 8 cylinders at any time?

all the internet evidence seems to point to a big vacuum leak,..or plug wires crossed,or injectors wires crossed,.

So maybe vacuum lines hooked into the wrong port,.

Thats why its semi happy running pig rich when its cold,. as it trims itself out it is too lean,..(inmho)

Same question,...
did this car run fine till you "fixed" everything?

because its seems to me that we are assuming you are doing everything right,..?

there was a certain other car that was posting weird stuff,. changing programs to fix it,..adding huge percentage of fuel across the board,.
all kinds of weird and wonderful internet suggestions to check the most obscure things,..
when i saw the car in person,.within 10 mins,. it was one vacuum line hooked into the wrong spot causing all the grief,.
then the owner had to go back and undo all the "tangents" he had gone and added to cover up the symptoms,..

Another wagon owner swore up and down the firing order was right,.  when it finally was right the car ran perfectly,.18436572

so often the problems get posted when the owner has not "fixed" something correctly,.or done a poor re-install etc,..


if this engine runs smoothly on all 8 cylinders at any time now,.
it DOESN't need a compression test,. or its valves done,.  bad valve )s) will give you a dead miss,.
do you know if it has a dead miiss>?
can YOU tell if its running on all 8 cylinders?

enquireing minds need to know now,..

Lastly,.. have you carefully looked over the wiring harness? to see if its compromised from vermin,.or rubbed thru,. or pinched anywhere?

It;s coming to the point where its going to be cheaper for you to fly me there,.
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lamune

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PostSubject: Re: chugga chugga   Tue Sep 08, 2015 12:52 am

Nick - man I would love for you to take a look at this. I agree that it's probably something stupid simple. So you have a number of good questions and I'll see if I can answer them. I'm really curious as to what's going on here myself.

The general running condition of this motor hasn't changed much since I first got it. It generally runs "OK". It definitely improved when I did the ignition tune up work, but generally it's been the same. It idles pretty well, with an occasional random miss you can hear in the exhaust and when you've got your head under the hood, and you can feel it if you've got your hand on the motor. It's something I've never been able to eliminate. So the answer to the other question as far as between when I got it and right now after I've fixed a bunch of stuff that didn't need fixing (and some things that did) it runs better now than it did before, overall.

In terms of this particular hesitation and backfire- it always hesitated. I didn't notice a backfire. I'd say in terms of this behavior the problem is worse than when I started.

The last major change I made was replacing the fuel injectors. I could undo that as a test.

Can I tell if it's running on all 8 cylinders. Well, I suppose I can't say for sure, but I see no evidence that I've got a dead cylinder. All the plugs are very similar looking. The engine (slowly) revs up smooth and feels OK when driving. I had two wires reversed at one point and the result was rather dramatic, so I know what that feels like. I don't believe that's the problem.

And yes the assumption that I'm doing everything right is just that- an assumption. I could very well have done something wrong. The vacuum lines, if I touched them, went back to where they came from. I did replace them since most of them were disintegrating. However that also assumes they were in the right place to begin with. There aren't too many on this car. The lines to the PCV, breather, and brake booster are pretty hard to get wrong. There's the line to the EGR solenoid, the purge solenoid, the fuel pressure regulator and the distributor. I should have a diagram in the FSM I can reference to check that possibility of the wrong hose in the wrong port.

The harness looks OK, but obviously most of it is hidden in corrugated plastic wrap. Nothing is obviously damaged. An internally broken wire to a fuel injector comes to mind as a potential gotcha.

I don't think it's a bad valve or something mechanical. Such a problem would probably manifest itself all the time. It may be worth explaining that the hesitation and backfire I'm talking about only occurs when you snap the throttle open quickly, like if you're at a stoplight at idle and then floor it. If you build RPM slowly, it runs just about as fine as I've ever seen it.

It reminds me of when the accelerator pump in a carb gets clogged or stops working.

I haven't yet pulled the intake, but since I've checked just about everything else (and will do the homework you've suggested) I'm certainly willing to do it.

Now to add some more info. Since I had the plugs out and there was a sale at the parts store, I got some Delco #14 platinum plugs and installed them. Wow, it really smoothed out the idle quite a bit. From cold, the engine revved freely and I was really beginning to think I may have fixed it, but alas as soon as it got a little warmer, the backfire on hard throttle came back.

And since I had the plugs out to Dan's suggestion, I did a compression test cold with all the plugs out. Here's my numbers:

#1: 225
#2: 220
#3: 215
#4: 215
#5: 225
#6: 215
#7: 195 (this was the worst looking plug, too)
#8: 220

I'm not sure what a good cold compression number for this motor is. The difference between the highest and lowest is 15% which I believe is the more telling thing.

Anyway, that's where we are now. If the next move is to redo the intake manifold gaskets, I'm ready for that.
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PostSubject: Re: chugga chugga   Tue Sep 08, 2015 6:00 am

Well,. you have my sympathy,. its much more frustrating for you actually doing the work,.

It would be easier if we could all drop by and have a beer in your garage and then put our collective heads together and fix this,...

Clean plugs it runs better...,so its pointing towards weak ignition or lean fuel,..inmho.

number 7 isn't firing, which points to my first guess the 5+7 wires are crossed,.
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Compression numbers are fine,.

Things are contrary tho,..  slow throttle opening and very light throttle easy driving to keep it running smooth, generally indicate lack of fuel volume,.i.e fuel filter, bad pump,. pinched fuel line,. etc,.  You say that wot runs are just fine,..

The same symptoms can also point towards really weak spark,..

More backfiring with the ect disconnected,. points again towards wrong spark timing,.

it's getting plenty of fuel,.but it's getting fired at the wrong time, hence the backfires up thru the intake,.

perhaps you could remove a plug wire at the spark plug end when its idling, just slip your pinky in the end till its contacting the metal part inside the boot,  and record how bright your eyes light up??  What a Face
and whether or not your hair stands up, and maybe if you are able to measure the amounts of leaking body fluids,..that might help determine if the spark is good,.?   lol

I,m just about out of idea's,. clever ones , and not so clever ones,..

This problem seems to be pfm,..

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PostSubject: Re: chugga chugga   Tue Sep 08, 2015 6:00 pm

I don't have but a simple TBI but I can attest to fireballs shooting out of the intake because the timing being off. I had the dizzy 180 out of wack and boy did she burp some fire. If it were me I would quadruple check the plug wires. I know it's frustrating I hope you get it figured out.
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lamune

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PostSubject: Re: chugga chugga   Wed Sep 09, 2015 1:28 am

Hah, yes, I will definitely re-check the wire routing and vacuum hoses.

A buddy at work who is an F-body guy and knows LT1's pretty well took a look at the data, and some of the readings are outside of what he's used to seeing. The IAC numbers look weird to him, for example. The BLM values should be 0-15 and I'm getting 18. O2's indicate lean mixture on throttle application, etc.

I didn't have much time to play with it today, but I wanted to add that on the suggestion of multiple people I did let the motor warm up with the knock sensors disconnected, and the behavior did not change.
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