LT1 performance issues

Ok just trying to get other opinions before I start tearing anything apart.

I've had an odd performance issue with my 94 rmw, the issue only happens after startup after sitting overnight or if I let the car sit for awhile after it's been running. the issue is usually just lack of power though sometimes it will back fire or nearly stall but only under moderate to heavy throttle.

The issue only lasts for a couple hundred feet though, also I hooked up an air mattress pump to the distributor vent harness on the car so that the pump was pulling air through the system as though the engine was running, I let it go for about 10 to 15 minutes before I disconnected the pump hooked the vent harness back up to the intake, when I started to car and got on the road there was no performance issue at all. Also with it getting colder out the issue seems to be less now then it was when it was warmer.

My one mechanic thinks it's more then likely an issue with the opti based on what I've posted here, also the car does have 138,768 miles under unknown driving conditions, though I have been a bit hard on the car myself but the issue has been there since I bought the car but was most noticeable this spring and summer.

a bit of an update here, as the temps got colder here in Wisconsin the problem for the most part went away. But when I did the thermostat the first time I made no effort to keep any collant from going down onto the opti spark, a couple days later when I went to take the car to work for the first time since the thermostat replacement the car ran like crap, it was surging sputtering and backfiring, and I was barly able to get her up to probably 1500 rpms, any higher and she would start chugging, and could barely get up to 50 MPH.

hoping the issue would clear up I continued to drive trying to take it as easy as I could, but after 3 miles I gave up turned around and headed back home, right as I'm getting back to the house the car suddenly starts running fine, but I still switch to another car and head the work.

The next day I give the RMW a second chance, but at first its' the same thing spitting sputtering chugging, and backfiring if I give too much throttle or the RPMs go over a certain amount, but this time I keep on driving and at about 4 miles she's fine and no more problems since.

also there was never a CEL illumination and no codes at all.

with this I'm fairly certain the seals on the existing opti are going out if not already completely shot, but wondering if you guys have any suggestions on anything else to check before I tear the front of the engine apart to replace the opti.

Also I have replaced the coil, so I know that's not an issue.

I don't believe you need a new Opti. Service the one you have instead of rolling the dice because most options end in failure.

Everyone dumps on the Opti even though the majority of folks never have an issue with them. They are typically blamed for every issue that people have. Especially when talking with the clowns on social media.

That said, it sounds like you may have a sealing issue. Start the engine and pull the nipple out of the throttle body boot. Put a Vacuum gauge on it and see if it reads anything. Can also put a small piece of paper over it and see if it holds it but I've had that fail even though it pulls strong Vac on a gauge.

If that fails you have bad Opti seals or a harness issue. The harness can be rebuilt very easily using parts off the shelf from any parts store.

The Opti itself can be serviced the same as every other Small Block Chevy distributor. Take it apart, clean it all up and replace the cap/rotor. Plenty of threads on the job over at ISSF.

Check your plugs and wires also because once mine was running like you describe so I tore it all down change everything and found this....

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You can just barely see the black dot towards the boot where it arcing to a manifold stud.

Also pull the throttle body and clean it thouroughly. The 94 I had was full of green snot under the cover and in all the passages, Cleaning it all out cleared up my problems.

Fuel pump.

fuel pump has been replaced multiple times already but I'm able to get up to the full 47 PSI.

if I went with a new Opti I had planned on getting an MSD, as they have the harness hardwired, so you don't have a plug right at the base of the opti, but they also added an extra mounting for the cap so it has five screws holding the cap to the base rather then the standard four.

I've done plugs, havn't done wires yet, as previous owner did summit racing 8.5mm and supposedly had cap and rotor done at that time, but I question that as when I did the fan clutch the belt for the fan was shot but the main drive belt looks almost brand new.

Once it warms up outside for more then a single day though I'll check it out, along with probably doing all the seals on the timing cover. Hopefully also have my parents garage cleaned out so I can work inside.

Also the pic of the front of your engine looks a lot cleaner then mine.

goldwolfnhn wrote:

Also the pic of the front of your engine looks a lot cleaner then mine.

Life long Central Florida car. Spent a hard life moving a big family around that lived on a farm. They took care of general maintenance but nothing else.

As for MSD parts, folks love their Cap/Rotor, the entire Opti not so much.

Ok, you didn't say the fuel pump was done, nor that you have tested pressure.

94... my next guesses, in order of experience with a couple 94s.

- coil wire (especially if those 8.5s were just the plug wires)
- coil
- ICM

well hopefully tomorrow afternoon I should be able to pick up my cousins old Snap On scan tool that he has the gm obd1 adaptor for so that I can have that hooked up next time I drive the RMW and hopefully get something on what's causing the problem with more certainty.

jayoldschool wrote:

Ok, you didn't say the fuel pump was done, nor that you have tested pressure.

94... my next guesses, in order of experience with a couple 94s.

- coil wire (especially if those 8.5s were just the plug wires)
- coil
- ICM

Sure he has. You are just lost because he has multiple threads about the issue his car has.

Also, he has stated in this very thread that he has a new coil installed already.

lol, perhaps one of the twenty moderators we have could merge the threads.

the other issues didn't seem related to this one which was why I made different threads for them

well had the scanner hooked up toady on the way to work and on the way home. I kind of wish that the scanner was a new model though.

But the only thing I really noticed was that the o2 sensor readings where all over the place all the time going from 80 then jumping up to 900 something mv continuously while driving and sitting idle both closed loop operation and open loop

I also wish my cousin had been available to explain some of the different readings and what the abbreviations mean like, left, right BLM, left, right bpw, or even left, right int, as the numbers for those where off a couple points from each other

Left and right mean just that.

BLM is Block Learn Multiplier.

Go here if you really want to get into it and do some reading if you really want to understand.  http://www.gearhead-efi.com/Fuel-Injection/forum.php

For proper scanning use a Laptop and an ALDL Cable. There are a few programs for scanning, tuning, etc out there for the LT1. I use EEHACK to read codes, scan, upload tunes, etc. Tuner Pro RT is used for the tuning.

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SCAN 94/95 is another code onee to have.

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goldwolfnhn wrote:

well had the scanner hooked up toady on the way to work and on the way home. I kind of wish that the scanner was a new model though.

But the only thing I really noticed was that the o2 sensor readings where all over the place all the time going from 80 then jumping up to 900 something mv continuously while driving and sitting idle both closed loop operation and open loop

I also wish my cousin had been available to explain some of the different readings and what the abbreviations mean like, left, right BLM, left, right bpw, or even left, right int,  as the numbers for those where off a couple points from each other  

I just went through months and months of my car running like crap. Short verision was the ICM was no good.
I replaced, fuel pump, EGR, Coil, wires, temp sensors, and it would run fine, then like crap.
I had data pulls where the O2s were crazy, then temps , and a bunch of other weird things.

My car only has 70K and I"ve had at least 7 or 8 other LT1 cars and never had an ICM go bad, but found out it's a wear part.
If you haven't I'm thinking that may be the culprit.

I still have three spare ICMs in my toolbox for the day I need one. The blue wagon taught me to be prepared.

well apparently according to several youtube vidoes the upstream o2 sensors, (which is all my wagon has) are supposed to be jumping all over like that as long as they don't go higher then what I was getting or zero out at all.

other thing is, is there a way of testing the ICM, as right now the only good warranty one I can find (lifetime warranty) is around 100 dollars but I could pick it up immediately, but I don't want to pay that much only to find out that that's not whats wrong.

O2 sensors like that (single wire ones) constantly bounce back/forth between rich and lean. Some scanners have a variable called "Cross Counts" which is the number of times that it crosses back and forth. The ECU constantly oscillates between rich and lean to maintain the correct air fuel ratio. If you accelerate cross counts will typically drop or o2 voltage readings will go higher due to being richer (at least temporarily - this all happens pretty fast). When you back out of the throttle and coast it will go low until it catches up.

Don't know what to tell you about the ICM (sorry)