My "91" Vette has always had a very minor less than perfect idle.

It's so, slight that most people wouldn't even notice it but, I could feel it intermittently in the steering wheel when idling in the garage. It's just not perfectly smooth like it should be.

The L98 engine (90,000 miles) runs great and just passed California's emissions test with flying colors. Gas mileage is great and the engine is well maintained.

Things I have checked or replaced.

MSD wires.
New AC Delco FR5LS spark plugs .035 gap.
New Cap & Rotor.
Timing set to 6*.
New fuel filter.
Air filter.
New EGR valve.
Seafoamed engine (hardly any smoke).
Mobile one full synthetic oil/oil filter change.
FIC Bosch III fuel injectors.
Cleaned throttle body and IAC.
Checked TPS voltage.
Checked minimum idle.
Checked fuel pressure 48lbs.
No vacuum leaks.
No plug wire arc's.
No codes set.

Things I'm considering.

Ignition Control module.
HEI Coil.
Fuel pump.
Fuel pressure regulator diaphram.

Since the engine runs so, well and efficiently, do any of the above have anything to do with the less than perfect idle? I've read numerous posts on idle problems but, they all refer to engine power loss or noticeable rpm drops which is not the case with my L98. The tack needle doesn't even shake at all.

Thank's for your help.

 

sounds like you just have a case of an old engine. you have to remember that this car is 17-18 years old, its old enough to drive its self! Also in 91 they didn't exactly make engines as well as they do now. Another thing is that it's a big v8 with a lot of moving parts and the sligtest discrepency in anything could make it idle a bit rough. However, i think you are on the right track with nipping this problem before it gets any worse, im just saying that you may find that nothing is wrong enough to diognose any problems yet. Good Luck!

 

just by chance did you check the iac pintle. It has a preset depth of no more thsn 28mm.. If this is out to far the iac will not perform properly. Just a thought since you did everything else. .

 

Yes, I cleaned the pintle and checked the pintle length which should be set to 1 1/8" from the IAC base. I also, jumped the A&B pins on the diagnostic port which should reset the IAC.

 

is the idle wandering at all or just a little rough, like cam lope?

 

The idle is just not perfectly smooth like my old "91" Camaro was. It just feels like a slight intermittent bump that I feel through the steering wheel.

From reading a lot of old posts on idle issues, I'm going to replace the Coil since it's cheap enough and has solved a lot of other members idle problems.

 

Don't take this the wrong way but....
Are you that bored???


I know it is YOUR car and if you want to obsess over something that is completely normal for a 90K mile 17 year old vehicle then rock on with your bad self.

P.S. - I hope you realize that this is all in good fun

 

I was thinking the same thing!

Seriously though, If you were to see what I was talking about I'd think you'd agree something was going on.

I only became more aware of it after my brother borrowed my Vette for the fourth time and said, "what's wrong with the idle". He drives a new Dodge Hemi truck.

 

It may be as good as it gets - post your emissions results or better yet, capture scan data at idle and post that. Otherwise, a cylinder that's producing less power than all the rest is about the only thing left. None of the items you're thinking about would effect just one cylinder and since you've allready replaced all the things that would, I'd try a compression test - particularly between 7 & 5 since 7 is the cylinder that usually goes on the 113 head Vettes. Those leaks start out fairly small so it can be really difficult to figure it out, but a less than perfect idle is telltale sign. Check for rust on the #7 plug threads and coolant that looks bad. If your emission results show a particularly lean engine - one with a higher than average NOX at idle, all the more reason to suspect a cylinder is down from all the others. You can confirm the lean condition with a scan, but a compression test is better - run it hot and cold (just leave the plugs a tad loose after checking it cold so that they're easy to pull back out once it's warmed up).

 

When I changed the spark plugs, they were all normal and the threads were all clean (no rust). Coolant looks great. Here are the Emission results:

 

if your going to replace the coil you may want to check your ICM as well since its under the distributor cap, if its the original it might be time for a new one, and at 40$ or so it would one less thing to worry about in the future. just my $.02

 

If the car runs strong and efficiently, like you state, I'd leave it alone. But, one thing that may smooth it out would be a new tb and iac. Hope you figure it out.

 

Reset the car's computer drive the car and let it re-learn....then see if you still have the "miss/idle blip" issue.

 

Did you do the IAC reset as set out in the FSM?
1. Slightly depress accelerator
2. Start and run engine for 5 seconds.
3. Shutoff engine for 10 seconds.
4. Restart engine.

I overlooked this part when I cleaned my TB, IAC, etc. Measured the IAC before install, but I still had the slight roughness. I found the reset procedure by accident and tried it immediately, with great results. Idle smoothed out but I'll still have a little bump every once in awhile, which I attribute to age.

 

Thanks, gw962 I tried the reset procedure and it did seem a little smoother!?

Tomorrow I'll change the Coil and post the results. I'm not going to change the ICM because I've read here that it's just a resistor that switches on and off and wouldn't really cause an idle issue.

 

Look at my thread in the scan/tune section the last reply from SUNCR makes sense.

If nothing else left to change (parts), then it might be on the "inside".

He gave me a tip about checking every cylinder wich is not in the FSM.

Might be worth to check.

I´m going to try it out and see what happens.

I have the same thing with my car and done a number of things to improve idle.

Perhaps our engines were made by the same "gang" that day/month.

My vin is 1G1YY2383M5100085.

Should be somewhere in sept/oct 1990 when they built our cars.

Could be same parts bin they used on our engines.

Dunno if engines were made in same factory, or if they only put the cars together in B G, Ky.

 

Your tailpipe emissions are close to perfect with the CO #'s showing just a tad of fuel being added (or left over), but well within design limits - so if there's something going on, the CAT is handling it quite nicely. Mine did with a weeping head gasket (it always passed) which may be why it took me so long to figure it out (like when it finally let loose altogether). I'd still check cylinder balance. Just know that the 113 headed L-98 is frought with head gasket failures (allmost always at #7) which GM blamed on misalignment of the intake manifold gaskets allowing dissimilar metals to come in contact with the coolant which led to galvanic corrosion which destroyed the head gasket. For '91, the heads were counterbored to accept gasket retainers to maintain intake gasket alignment and that seems to have cut down on the number of low mileage incidents (mine were probably leaking at 25,000 miles), but '91's do pop up around here with the same issue. I have some theories as to why they fail - change in gasket materials (no asbestos) - GM's use of Coolant Sealant Tabs to counteract their engineers' fear that these newer materials wouldn't make it through the warranty and those tabs seem to plug everything up - and the fact that the Vette L98 restricts coolant outflow (the F-bodies do not) at the rear of the heads which keeps coolant there longer which leads to boiling/pressure build up/not enough coolant (once flow is even more restricted) and that blows out the gasket - none of which I can prove, but it's certainly frustrating to see so many L98's with the same problem and the iron headed motors don't seem to suffer from the same problem or have the idle issues you see on these motors.