Can't pass the tailpipe test...

Welp....

Went for inspection today and it's all good other than the dreaded tailpipe test...

Long story short, results say it needs a tune up

I just did plugs, wires, rotor, coil pack, and the cap was already new when I bought it...

Tonight i plan on changing the oil and fuel filter...

Any one got any clues??

Where did the test fail?

Try a long drive in second gear to keep the revs up and get the convertor real hot.Try to arrange the trip so that you can pull into the inspection location with the convertor as hot as possible.
The hotter it is the cleaner it test usually unless your convertor is toast.Best of luck!

Have you done any mods to the motor or exhaust system?

I have a dyno max 17734 muffler and no resonator.

I failed at idle, state of PA your allowed 130 ppm.

I was at 318 ppm, which I take it was running rich.

Now I do have the caddy 4.9 TB mod as well...

Thanks guys...

318 ppm of what? There are 2 gases that are measured in ppm, HC and NOx. If the HCs are high, there is a condition causing it to run rich. If the NOx reading are high there is likely something wrong with the EGR system. But, if the test is only run with the vehicle stationary the EGR is likely not an issue, unless it is being commanded ON during the test.

Keep this in mind though, an EGR valve that is not completely seated will leak EGR gases into the intake and cause a rich condition. The reason this happens is because EGR gases decrease the manifold vacuum at idle and the MAP sensor sees this and reports it to the ECM as a load being placed on the engine. The ECMs response is to add more fuel for the increased engine load. Phewww... that was mouthful!

There is also the possibility the Cat has degraded enough where it is not doing its job completely.

But as Flasheroo has mentioned get that cat as hot as you can before attempting another run at the test and it may pass.

Does the car burn much oil? That will raise HC levels in the exhaust. Changing the oil may help if it's half-way to the next scheduled change. Changing it to one of those "high-mileage engine" formulations designed to reduce oil consumption may also help.

If the mix is too lean, misfires will raise HC. If the mix is too rich, incomplete combustion will raise HC too. How's the O2 sensor and wiring? Similarly, spark plug issues can cause misfires too.

The reduced backpressure of the modified exhaust may not be allowing the cat converter to run as hot, reducing efficiency. If the backpressure is adequate, a larger cat converter may help.

How much room did you have on the NOx level? The two are usually inversely proportional -- a hot engine will produce more NOx but less HC and vice-versa. Maybe the car had been sitting and cooled down before the test?

The EGR reduces NOx by injecting exhaust gas into the intake and cooling the combustion. Could it be leaking at idle? EDIT: I see GM tech already addressed this issue while I was typing.

Loosening the valves lash by 2 -4 thou (0.002" -0.004") effectively reduces the valve overlap, giving a bit more time for combustion of the HCs. (but at the cost of higher NOx, of course).

Retarding the timing will reduce HC (of course, that means re-programming the PCM). Similarly, raising the idle may help (as long as it's below the maximum allowed for the test)

Finally, I have heard (but never tried) that running the tank down to almost empty and adding a couple of gallons of white gas (AKA Naptha, campstove fuel) and a gallon of methyl alcohol (AKA methanol, wood alcohol, wood spirits) will make the car pass. Top off the tank with real gasoline asap after the test though.

buickwagon wrote:

Retarding the timing will reduce HC (of course, that means re-programming the PCM). Similarly, raising the idle may help (as long as it's below the maximum allowed for the test)

If I am guessing this correctly, the car in question is a 1993 TBI engine and uses a conventional distributor. Timing for this engine requires timing to be set at 0 degrees TDC, with the bypass connector disconnected, and this is referred to as base timing. Once the bypass is reconnected the ECM takes control and advances the timing by 20 degrees at an idle.

So, if base timing is retarded by -2 degrees, using the above method, now when the ECM takes over, it will still advance to 20 degrees, but the effective timing will now be 18 BTDC without having to reprogram. The opposite would be true also.

Increasing TPS voltage by as little as 0.1V will raise the idle speed. Retarding the timing will cause a decrease idle speed slightly.

Holy smokes guys, alot of input to process through my brain lol...

Well I will get a new air filter and re-install the factory airbox tomorrow, as well as double check all lines...

I did realize the pcv valve was filthy which i cleaned it up, as well as i did change the oil this evening. The oil was indeed dark and dirty...

I will find out what exactly it failed in the morning, I know it did pass everything but just sitting there at idle...

gmtech wrote:

If I am guessing this correctly, the car in question is a 1993 TBI engine and uses a conventional distributor.

Doh! It's right there in his name too. you are, of course, correct: the 93 has a conventional distributor.

I think the decrease in rpm from altering the base timing should still be temporary though. If I recall correctly, the system will try to maintain the desired idle speed by adjusting the IAC pintle position. Also, I'm not sure that you can raise the idle setting mechanically.

Even if you adjust the throttle stop (by drilling out the plug in the throttle body to access the screw or by adjusting the position of the TPS) the system will read the current TPS value at the next key-on event and use that as the new 0° throttle angle, and again, adjust the idle to the desired speed by adjusting the pintle position.

The ECM should raise the idle speed if it detects the AC compressor is running, but I don't know if the original test was done with the AC on or off or if there is a specification for that as part of the emission test. It's too bad their isn't one of those solenoids they used to use on carburetted cars to kick up the idle with the AC on -- it could have been wired to a 30 second delay timer to kick on after the engine starts.

Here is what i failed at...

I was gonna have this BG treatment ran through it and see what happens... I was also gonna take alook at my injectors see if its dumping too much fuel...

What else do they measure? EG: if they look at NOx, and you have lots of room, then you may be able to tweak things so the HC goes down even though that tweak drives the NOx up a bit. It might be helpful if you told us all the results.

Sorry... these are my papers...

I look closer and realized the the inspector put that I had a 6 cylinder... could that made a difference in the reading?

I don't think the number of cylinders matters when it comes to the PA standards for Hydrocarbons. According to PACode.com the maximum limit is based on model year and weight/type of vehicle. Not to mention engine type gas vs. diesel. Look at section 177.204 basis for failure. It states that for 1993 and newer the maximum HC limit is 130 for a car weighing less than 6000lbs GVWR. I believe the number of cylinders is only to determine what the maximum RPM's can be during idle.

All that being said I'm not from PA and just did a simple search on the web for PA emission standards to find this information. So. . .I could be wrong.

Quickest & cheapest way might to just bribe the guy & get out.

Or move to Florida where I don't have to worry about stuff like this.

I know you have a tbi, but when I put a CAI on my LT1, my fuel matrix went from a little rich, to way rich and cost me about 3 MPG. I swapped back to the stock airbox and it went back where it was and I hooked everything up right....just sayin'

Here we go huh? Lol...

I most certainly gonna put the stock air box on as well as a few other maintenance things

Boomhauers_93Wagon wrote:

Here we go huh? Lol...

I most certainly gonna put the stock air box on as well as a few other maintenance things

Is that stuff even safe for use with a catalytic convertor?

buickwagon wrote:

Doh! It's right there in his name too.

I amaze myself sometimes, this is where I assumed the vehicle in question. If he is like most of us, he likely has more than one vehicle, I have 4 vehicles.

Also, I am not talking about removing or drilling out any plugs for an idle adjustment. Enlarge the holes in the TPS mounting holes and remove the locating tab on the sensor itself. This will allow an ever so slight adjustment to be made by rotating the sensor, in the tune of about 0.1v.

Boomhauers_93Wagon wrote:

Here is what i failed at...

I was gonna have this BG treatment ran through it and see what happens... I was also gonna take alook at my injectors see if its dumping too much fuel...

Create a small vacuum leak and I do mean small, you may need to experiment with this, in the intake manifold i.e. vacuum line. Appears your problem only exists at idle and not at the 2500 rpm threshold. At 2500 RPM the small vacuum leak will "go away" due to its size and will become insignificant.

Obviously the correct way to do things would be to find the cause and fix it. What is being suggesting here in these posts are merely a Band-aid.

Maybe this article will shed some light.

Thats pretty sneaky!I like it

gmtech wrote:

Enlarge the holes in the TPS mounting holes and remove the locating tab on the sensor itself. This will allow an ever so slight adjustment to be made by rotating the sensor, in the tune of about 0.1v.

Yes, but doesn't the ECM "auto-zero" the TPS reading at each key-on? Any adjustment will only raise the idle until the engine is restarted. Then the ECM will read the new TPS value, take that as the new zero and set the idle accordingly.

It may auto-zero as 0%, but it still looks at the voltage value and makes its decision. Think about this, if the 5V reference dropped to 4V or increased to 6V and you did nothing else. What is the computer going to do at this point? It's going to look at the signal coming back to the ECM, set the TPS value as 0% and read an incorrect voltage value. The ECM knows nothing more than what it sees, garbage in, garbage out (GIGO in computer speak) and this is what it responds to.

Im from.pa go to a different countyy pa does not have emmisions everywhere the lil smaltowns also im in pittsburgh till sunday maybe we could get some coffee

gmtech wrote:

Create a small vacuum leak and I do mean small, you may need to experiment with this, in the intake manifold i.e. vacuum line.

I can see that working in open loop, at least initially. I'm not sure if that will work in closed loop. The O2 sensor should adjust the integrator (ie: short term) almost immediately and that will alter idle block learn (ie long-term) fuel trim values. It should be around 128, give or take 6 or 8, which is the centre of the adjustment range. So there's lots of room for compensation and once the limit is approached, it should trigger the CEL (which I think is an automatic failure for most testing programs).

Quote :

It may auto-zero as 0%, but it still looks at the voltage value and makes its decision. Think about this, if the 5V reference dropped to 4V or increased to 6V and you did nothing else. What is the computer going to do at this point? It's going to look at the signal coming back to the ECM, set the TPS value as 0% and read an incorrect voltage value. The ECM knows nothing more than what it sees, garbage in, garbage out (GIGO in computer speak) and this is what it responds to.

The 93 may be different from the 92, so my experience may not apply. In the 92, the ECM will accept anything from .2 to .95v as 0% TA. 5v is read as WOT. At 0%, the ECM will presume the throttle is closed, and therefore regulate the engine speed to the desired idle rpm through the IAC circuit.

I have tried to do exactly what you suggest on a 91 K1500 5.7 (needed to up the idle a bit for improved idle alternator output on a plow truck). Yes, adjusting the TPS would affect the idle while the engine was running, but as soon as the key was cycled, the idle returned to the normal desired idle rpm. That was when I drilled out the plug and tried adjusting the actual stop. Again, it would work wheil the engine was running, but after cycling the key the idle returned and it was clear from the scan tool values that the IAC pintle had found a new home.

I think we may have hi-jacked this thread, so I will make this my last post on this.

That is correct, the TPS may reset to 0% on some vehicles and the ECM will "reset" and use that as the baseline 0% voltage. In the non-auto zeroing type system this is not true. For me to point out what systems are and are not, auto-zeroing, I would have to look at them on a case by case basis. I had a TBI system that did not work using the auto-zero feature and adjusting it helped.

On a typical system when the ECM sees an increase in voltage from the TPS it views this as a requested input from the driver, as throttle movement. In anticipation, the ECM commands the IAC to increase a few steps, therefore raising the by-pass airflow a specific amount. The IAC is used to maintain an ideal idle, but in the case where the throttle is opened during normal driving it is there to prevent a deceleration stall when the throttle is quickly and fully released.

Now as far as the vacuum leak, again this is all experimental and yes, when in closed loop, the ECM may overcome the leak via the STFT and, if required, LTFT adjustments (that is if the STFT adjustments get too far out of whack). The LTFT would have to get way out of order in order to set the SES light, typically in the tune of about 15% (or whatever the threshold is set to for the given application). Or in the case of block learn and integrator about 18 points +/-, again, speculating the actual value here.

Who knows, maybe the O2 sensor is biased lean causing a rich tailpipe because it is cooling off at idle and going into open loop. Which brings me to this point, most single wire O2 sensors, that I am aware of, go into open loop at idle because the exhaust stream cannot maintain the proper light-off temperature of the O2 sensor for it to remain effective to correctly report the oxygen content in the exhaust stream. With that said, the vacuum leak ideal could work.

Again as previously stated, we've been looking at ways to put a Band-aid on this issue. Let's get to the root cause and fix it correctly.

I have thoroughly enjoyed the conversation buickwagon. If you'd like, send me a PM.

Paul

Paul,for those not in the know would you please explain the following
STFT
LTFT
SES
Thank You

Flasheroo wrote:

Paul,for those not in the know would you please explain the following
STFT
LTFT
SES
Thank You

STFT = Short Term Fuel Trim
LTFT = Long Term Fuel Trim
SES = Service Engine Soon

...and here a video to help further explain this terminology. Enjoy

Boomhauers_93Wagon wrote:

Holy smokes guys, alot of input to process through my brain lol...

Well I will get a new air filter and re-install the factory airbox tomorrow, as well as double check all lines...

I did realize the pcv valve was filthy which i cleaned it up, as well as i did change the oil this evening. The oil was indeed dark and dirty...

I will find out what exactly it failed in the morning, I know it did pass everything but just sitting there at idle...

You may also do some reading here,very helpful stuff.

http://www.thirdgen.org/techboard/tbi/

When my wagon was acting up and failed numerous times at emissions,I read something on adding a heated O2 sensor and then it passed.This might not work for you,good luck.

gmtech wrote:

Flasheroo wrote:

Paul,for those not in the know would you please explain the following
STFT
LTFT
SES
Thank You

STFT = Short Term Fuel Trim
LTFT = Long Term Fuel Trim
SES = Service Engine Soon

...and here a video to help further explain this terminology. Enjoy

Thanks Paul and no I not going to watch a 50 minute training video!You must really love your work!And thats a good thing but thats
way longer than my attention span is even when I was young!

gmtech wrote:

I think we may have hi-jacked this thread, so I will make this my last post on this.

Fair enough. And yes, I have enjoyed this exercise as well.

I personally think that if one understands the basic operating principles behind a system, one is better equipped to diagnose and correct a problem -- which, as you said would be preferable in the long term to a band-aid solution. Flasheroo may not agree, but I enjoyed the video. Pretty basic but a good starting point for anyone who wants to learn about the system. And I learned something too -- the swing is actually necessary to feed the bed. I always figured it was just a necessary step of the approximation process. I guess it explains why GM never implemented the super-cruise mode.

I had a rich mixture on the Starship, and it turned out to be the ground at the thermostat housing. I cleaned it thoroughly including the stud, and reassembled it (only do this with the engine cold). The problem went away instantly. It did cost me a cat in the process. Running rich long term will clog your cat.

Makes sense -- if the O2 sensor ground is at a different potential than the ECM ground, it would act like an applied voltage bias -- eg: the O2 sensor is putting out .45v relative to its ground, but the ECM sees a voltage of .35v relative to its ground, so the ECM fattens up the mixture.

hate doing this emission nazi stuff,but thats a rant for later lol.. have you check the spray pattern from the injectors to make sure theres no dripping? maybe to a seaform head treatment wont hurt either.my 91 i had a cat back system and like a dummy was talked into a aftermarket cat, failed,tried to trick it out,no luck.got another cat passed no problems,my 92 377k on it all stock still passes.nothing like a good challenge

Fred Kiehl wrote:

I had a rich mixture on the Starship, and it turned out to be the ground at the thermostat housing. I cleaned it thoroughly including the stud, and reassembled it (only do this with the engine cold). The problem went away instantly. It did cost me a cat in the process. Running rich long term will clog your cat.

A short response. Grounds are VERY important with today's computer based vehicles. As mentioned, high resistance in a ground will rob a signal of the correct information to the ECM/PCM, think GIGO. Gremlins in the electrical system cause many weird things to happen.

Welp figured I would drop by and give an update, put the stock air box on just have been chasing things tryin to figure it out...

I did put that stuff in my gas that claims it will pass, we shall see what happens tomorrow as I'm going take it back for my re-test.

I did have another question, what is the piece the goes from the factory manifold to the air-box? Mine tore all to nothing...

It supplies hot air to the intake on cold start up. Five bucks at the parts store.

Boomhauers_93Wagon wrote:

Welp figured I would drop by and give an update, put the stock air box on just have been chasing things tryin to figure it out...

I did put that stuff in my gas that claims it will pass, we shall see what happens tomorrow as I'm going take it back for my re-test.

I did have another question, what is the piece the goes from the factory manifold to the air-box? Mine tore all to nothing...

"Stove pipe" ...and like jos said $5 @ the parts store.

so it really dont have nothing to do with emissions?

Helps the fuel vapourize better when cold. Could affect emissions at that time.

Welp, sitting here at the tail-pipe test station, waiting nervous as hell lol...

Let us know what happens!

I did not read every post here, but since I also live in PA, here is a very simple possible solution-go to another testing station. Over the years, occasionally I have had a car not pass, and the shop I use just told me what was wrong. If you are using a shop that does not have that ability, I would go to another shop! I can't believe that a licensed PA testing station can't tell you why your car is failing-it's their job. Years ago I managed a body/mechanical shop that did tons of inspections, and I can only remember one or two cars that were a real problem as to why it would not pass. Try somewhere else-just my 2 cents.