Hey guys, I'm having some issues with my 95 Z28 that my fiance' uses as her DD.

Mods:

Lingenfelter Intake
Gutted Cat
Cat Back
Mad Z-28 tune

Problem:

A few weeks ago my girl told me that the car wasn't running smooth and it seemed like there was a miss present.

I finally took it to work (cold start-up), and it ran perfect. When I drove it home later in the day, it ran OK, but i noticed a slight miss when trying to maintain speed. Heavy acceleration, and normal driving seemed fine, but on the highway, it would miss under load at lower RPMS.

SO I swapped over my Coil (new MSD) and ICM (stock) from my 95 T/A and she ended up gettting stranded at work. I had to go out there and get the car running.

The car started right up in the morning that she got stranded, and would just crank and crank later in the day, but not fire up. For some reason it seemed as if the car wouldn't fire while cranking over, but would try to fire after i would let off the key. Once it was running, it ran OK, but the same miss under partial throttle still exists. After I got it running and the car was warm, it will start right up everytime. After it sits overnight, no fire.

After she got stranded, I changed out the plugs and wires (nightmare w/ stock manifolds), and the car wouldn't fire up, it kept trying to fire when I let off the key just like before. VATS IS DISABLED IN MY TUNE. I did get it to run, and it ran great in the driveway.

Heres what I have done so far:

New Fuel Pump (went out prior to this problem, and has been running fine with new pump for months) VERIFIED PRESSURE - PRESSURE IS CORRECT(38 at idle, 46 @ WOT)

New Spark Plugs (gapped to .45) and wires. NGK TR55 and NAPA wires that I ohmed out

Coil and ICM that I swapped over from my 95 T/A (was working perfect on my Trans Am before)

New DELCO Oxygen sensors

coated most electrical connectors with a small amount of dielectric grease

What's left:

OPTI
WATER TEMP SENSOR
IGNITION SWITCH
ICM (maybe the one from my T/A is no good from sitting for 2 years)

The car runs good without a miss until it is warm, but now that I have this start up issue. I am leaning towards the ignition switch ( I already have a new one ready to install), but I suppose it could be the other 2 things also? I had to replace the ignition switch on my T/A a few years ago, and the car wouldn't fire at all, not like this. But I also heard that the Ign Switch can cause drivability issues if it has a short.

Ideas?

 

Oh yeah, forgot to mention that I have not had a check engine light at all during this whole debacle.

 

Could it be an injector issue?

 

Injectors are far less likely than the opti or icm. Are there any cel's ?

 

Get a $4.00 can of starting fluid and see what happens. If it fires right away you have some sort of injector issue where the pcm is telling them not to open. The key thing reminds me of an issue I had with my ignition switch. I had to replace them on both of my LT cars. As you crank the car try to also wiggle the key as you do so.

 

Tried Carb Cleaner and starting fluid - didn't help. Smells like the fuel was getting into the cylinders, but no spark.

I hear that the ignition switch is a very popular problem with these cars.

 

If you take one of them apart you'll see why, lol. Basically as you turn the key there are spring loaded contacts that slide along some metal pieces connecting each specific circuit. As these contacts slide they begin to wear down into the metal pieces. Now multiple that times 50,000 times that you've done it and eventually the grooves get deep enough that the contact can wobble around and in my case the contact slipped right out of the track. You have to drop the column to access it and the top bolt is a pain in the *** to get to.

 

no CEL's.

 

When you swapped over the ICM, did you use new thermal compound on the heatsink? If not it could be cooked.

 

Yes, a couple good size globs of Dielectic Silicone grease.

It still could be the ICM, but at this point I'm leaning towards the ignition switch.

What are some common ways to test the IGN Switch assy?

 

Yeah, that's not the stuff to use. Get some thermal transfer compound from a computer parts store or supplier.

 

Although "heat sink paste" is the best, dielectic grease will work. They are both silicone based pastes, and I don't think i could have fried the other ICM by using the wrong grease in 30 miles of light driving.

 

I have the same issue with my 97 LT1. The thing you said and mine is exactly the same is it only sputters and misses when warmed up. I am not sure an ignition switch would do that.

I have a new Opti replaced both front o2s, cleaned the MAF, new plugs, and just got an ICM and plug wires to install. While I do that I am going to space the coil away from the head per shbox.

I read this issue a lot on ths site and never really hear a cure.

Good luck I hope you can fix yours.

 

Using dielectric grease instead of thermal paste is like using petroleum jelly instead of dielectric grease. There's a right tool/chemical for the job. Use it!

 

No, it isn't.

Can we get back on topic?

I really doubt that is the problem with the car.

 

It's not the problem.

I have been in the electronics field for over 24 years and I hold a tube of ENVI-RO-TECH 16 Heat sink compound we use at work. It is silicone free. We use this on audio amps., PA finals or even some regulators that require it.

If you find you compound is dried out (it will be over time) you need new. Remember more isn't better in this case.

Back to why these cars have stumble at low RPMs when warmed up. If and when I find this problem I will come back and share and would appreciate any first hand knowlege of a cure in the meantime.

Thanks.

 

An ignition switch is an off and on kind of problem. I've been through 3 of these myself. All the ignition switch problems I've seen result from the thing just giving up one day after plenty of use. It rarely causes the problems that you listed.

Like I said, the icm and opti are the most prone parts for fai

 

not same car, but last week my friend with a zx3 was having same issues, would not start in parking lots etc after being warm, cold start was fine. Turned out to be a idle control valve. Not sure if the LT1s have this or not. I would say coil or opti maybe intake manifold gasket?

 

Remember, he's saying he's having trouble with cold starts and it fires fine once it's warmed up.

 

Correct but before he changed out the coil and ICM it was a different problem of missing running rough after warm up.

I do not know if changing out the ICM and coil contributed to his wife being stranded or know if the no start issue was present before this change.