95 Rough Idle at higher temp.

[bnhynum]

Here is the scenario. My drive home from work takes about 30 minutes and contains highway and stop-n-go traffic. By the time I pull into my garage the car is idling rough. The air temp has been 80+ and the car coolant temp is between 220-230. I let the car cool for a couple of hours and it will run fine again for another 30 minutes. (Lather-rinse-repeat). With some advice in this forum and my neighbor a seasoned corvette mechanic I have completed the following repairs and maintenance checks:



- replaced plugs and cables.

- cleaned TB and AIC valve.

- replaced EGR Valve.

- checked all fluids levels and for leaks (oil, trans, coolant). All check out ok.

- checked for computer codes. There were none.



The car seems to run fine until it reaches the higher temps. The car has 98k miles but I have done regular maintenance as long as I have had it.



I am not sure what to try next. Any help would be appreciated.

Thanks
bnhynum

 

[Ken]

Ever since it happened to me, and the symptoms sound the same, I always suggest checking the catalytic converters.

 

[SG4206]

My '95 acted the same and then got worse! There are 3 oxgen sensors. Had to replace the 2 on the right bank.

 

[bnhynum]

Update

Replaced Intake Manifold Gasket. Seems to have corrected an oil leak and oil pressure is more stable but I still have the rough idle and stumble. Starting to appear at lower temps now. Going to check charcoal filter cannister and catalytic converters for damage. O2 sensors were replaced about 5 months ago.

 

[bnhynum]

Replaced the PCV and fuel filter and checked the charcoal filter cannister, it looks fine, no loose particles. I would like to test the catalytic converters, but how? Can it be done with them on the car or do they need to be removed? And if so, what do I look for? I plan on having the injectors and fuel pressure tested also.

 

[redc4corvette]

Have some one rev up the engine and put your hand at the back of the exaust on the right side and do the same on the other side .
If it blows your hand away with the same force on eather side the cats probably are not plugged.
not very scientific but it works.

 

[bnhynum]

Took care of the cats this weekend and replaced the O2 sensors while I was there. The car keeps running better at peek operating temps but still has a rough idle and the magical 225 degree sputter and choke. Next is fuel pump\injectors and opti-spark I guess. sigh....

 

[WhalePirot]

The manual speaks to your symptoms, but not exclusively related to temperature. You might have a sensor that 'breaks down' when hot; an air leak at the MAF at temperature or a number of other issues. Can anyone you know monitor the ECM block learn while it is acting up?

Check vacuum lines carefully, especially where they get extra hot, for small cracks and brittleness. Check wires the same.

If it's losing coolant, check for water vapor at the hot exhaust, then pull your plugs, checking for any rust, if there's vapor. Those temps might be opening small cracks, in any number of places, allowing either vacuum(changing mixture) or water into the cylinders.

I don't see why the opti or the injectors would act up only when hot, unless hot water drips on it only when the engine is at those temps. Doubtful, that it would cool enough after only 30 minutes to evaporate such a leak.

My symptoms of a plugged CAT were lack of power at higher RPMs. I doubt such would be heat related, though. Eventually, it would barely rev much above 4000. Running rich, can be a *****. In my old engine, the mods I did exceeded the ECM's ability to trim out fuel, causing the richness and eventual plugging.

 

[Vettelt193]

sounds like the opti to me.

I had a problem with my '93 similar to yours. What I think was happening is the opti was 'stuck' in one position, which was too advanced for really hot temps. It would run fantastic up to a point, then start to run like crap after that point (say, 215+, and would run worse and worse with each degree of heat higher).

If it were me, I would check that first.

 

[SSTibet]

I had a very similar problem with Bnhynum combined with SES fault light. Changed all three O2 sensors, cleaned the EGR and Idle Air valves, fueld from a well known station and added some injector cleaner. Runs like a butterfly and fault codes went away. I would start with a diagnostic test if I were in your shoes.

 

[bnhynum]

Looks like it was the Opti-Spark distributor. Replaced it and the waterpump and so far so good. Unfortunately in my zest to clean the timing cover I must have blown the gasket out. Nice oil leak going now. Oh well, no time to let the knuckles heal.