Hi. I have a 94 C4 that mostly runs great. The only time I have a problem is when the outside temp is warm and the engine temp (on the digital guage) is around 230 degrees. At that time if I come to a stop light and am sitting still the idle will stumble and surge. The car will actually buck a little at times. The car has 80k miles and is an automatic. At first I thought it was a heat issue so I replaced the water pump and radiator (the analog would almost show pegged). That didn't fix it. I replaced the IAC (inexpensive ) and no luck. Also replaced the coil wil a MSD (found on clearance for $20). As far as I know the car has never had a tune up. So after talking to a few friends, with mechanic backgrounds, I reluctantly agreed it may be the Opti. It wasn't. Also did plugs and wires at the time. Anyone have any thoughs. At this point I feel it may be the ecm. That when the under hood temp gets high it gets flaky. Any thoughts? Thanks.

 

Mine would do that and bog when Hit the gas put two new front o2's never ran better. Might look at the icm with new thermal grease

 

Thanks for the advice. The car runs and pulls strong. Only problem is stopped and idle. You think new grease for the connectors?

 

The grease is under the icm it can get old or removed not knowing about it.

 

Oh ok. Thanks. I picked some up after your suggestion. I will give it a try.

 

Someone posted this.

Look at vacuum check valve thingy by right side fuel rail...fixed like perfect for me for 4 bucks

 

I added the grease and will have to check.it tomorrow. It us too cool.out and rainy to test it now.

I looked at the check valve on the right side under the cover. The only one I see is for the vacuum that controls the vents for the heater/ac.

 

Good luck

 

Antfarmer2, I think I may have solved my problem. I had to wait for dry hot weather to really test it. Tested it last weekend and during the week but it was cloudy and cooler out but it ran fine with no issues. Drove it today, 94 degrees out and sunny. Sat it traffic and idle never got erratic or bounced. I'm keeping my fingers crossed and will see if it makes it through the weekend. If it does I will post up what I did.

 

What all did you do? Might want to look at the temp sender on top of the water pump also.

 

Yes, what did you do?

 

I seem to have same problem when hot out, has quit running but starts right back up, even after is sits for an hour or so.
only has happened when hot outside.
it seems if i run engine at 2000 RPM for a minute or two it runs better.
i was thinking fuel injectors but maybe not.
would like to know what you did if it fixed the ruff idle problem.
Thanks

 

Check the fuel pump. Low pressure might be causing this.

 

Ok I was going to wait until after the weekend to post so that I could test longer to verify that it may be fixed. My wife is convinced I fixed it, I am more hesitant. But since several others are having similar troubles I will post now and you can try what I did this weekend. Ok, grab a drink this will probably be long because I want you to know all my thoughts and trials. And you can decide if your issue is similar.

I am not a certified mechanic but have been working on cars since I was 12. I do all my own work on all of my vehicles unless they were under warranty. I have rebuilt and worked on my 2nd Gen Trans Am's, Jeeps, Chargers, Nisan, Toyota, Ford and Porsche. This is my first Vette.

First thanks to those who offered their thoughts and suggestions.

In summary I bought the car, 94 with LT1 and 79K miles in April 2014. Car ran great until late August. I am in Denver at a mile high altitude. And despite popular belief that it is always cold and snowy it actually is sunny (300 days a year) and gets hot (over very 100). Car ran great and pulled hard. Problem was engine temp would hit 230 or more on the digital and almost pegged on the analog. That is when stopped at a light or stop sign or traffic and in drive, idle would get erratic and stumble and car would lurch at times. Put car in neutral and it was better. Back into drive and symptoms return. But once in motion again all was good and ran perfect.

I didn't do anything with it except drive it during the winter (nice and cool out and never had an issue). So May 2015 I start to fix it.

The fuel pump was replaced by the previous owner shortly before I purchased the car. I found people reference the IAC as the problem. I figured the car is 20 years old and I have no idea how it was cared for so I spent $20 and put one in. Didn't help. I found a new MSD coil on clearance for $22 and figured good upgrade at least. I did not replace the ICM but did put in the new thermal grease that was supplied. Didn't help. I still felt it was a temperature issue. So I replaced the water pump, temp sensor on the pump, and radiator. Didn't fix it. Some to a friend who used to work as a GM mechanic in the 90's he said it sounded like the Opti spark. A friend who has run and built vette's as well as raced said it sounded like an issue with the distributor signal, aka opti. Just about everyone one the Web said opti. I didn't want to believe it or pull the water pump I just put on. So...

New MSD opti, new plugs (AC Delco) all gapped correctly, and new plug wires. Put it all back together. Didn't help. At this point I still think it is a heat related issue. I am thinking ECM.

So I post on here and Antfarmer2 suggested thermal grease on the ICM, thought I did that but why not. So got a tube and cleaned off the new/old grease and put it back on. But while I was at the parts store I also bought a wrap for the ECM. Keeps heat off of turbos and exhaust. I got one that was the perfect fit. Take the cat out and no problems. Hmm...ok drove it really hard. Outside temp about 85 but cloudy. I drive hard enough to get the engine to 234 on the digital. Still running great. No idle issue that would normally be present. Next couple of days are in the 70's and cloudy and raining so no testing.

Yesterday (7/23) 93 degrees and sunny. Ok if it is going to act up this will be the day. Drove to work (about 25 miles) no problem. I drive home in stop and go traffic and no problem. Engine temp never went over 221 on the digital. No erratic idle. Wife says I fixed it. She may be right (I can't let her read that).

So after hundreds of dollars and many cuts and bruises on my hands a $6 tube of grease and $32 dollar blanket may have fixed it. Well not really fixed. I think the ECM is bad when the under hood temp gets to high and it affects my idle at a stop. I think the thermal blanket reduced the heat the ECM sees and keeps the problem from occuring. I am going to remove the blanket this weekend and see if the trouble rerurns.

I will post the results Monday. Sorry this is so long but I don't want anyone possibly throwing money at a problem when they don't need to.

 

Have my fingers crossed for you!!!!!!! Try smacking the ecm and see what it does.

 

Thank you very much
I will check the fuel pump.

 

Problem solved.

Ok I'm a little late with my update. The problem is the ECM. With the blanket on to reduce the heat I run with no problem. When I remove it and it's hot the stumble at idle in drive returns. I suspect that when the under hood temp gets high something in the ECM gets flaky and the computer throws the timing out of whack.

So now it's time to get a new ECM.

 

good job

 

Thanks for info

 

You're welcome and thanks to everyone for their input and taking the time to read my long winded posts.