anyone..
Fluctuating Idle when warmed up.
Thread Starter
Pro
Joined: Jul 2004
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From: 1985 L98, Auto. O'FALLON, IL
Fluctuating Idle when warmed up.
Here is were I am at. I have a 85 with an automatic transmission. I have had the car for less than 1000 miles. When I bought in July it had 75000 on it. When the car warms up the Idle fluctuates from 500 to 900 RPM. I have done a complete tune up, plugs, plug wires, cap and rotor, oil and oil filter, airfilter, pcv, fuel filter. over the weekend I changed the intake manifold gaskets upper and lower due to a small amount of fluid on top of the intake, I also changed the valve cover gaskets at the same time while I was at it. The last thing I changed was the IAC. I cleaned the plenum and throttle body along with everything else while I was at the intake gaskets. Tonight I adjusted the TPS voltage IAW the instructions in the tech section. still the same thing. I am not sure if the EGR could cause this? I am using a Haynes manual for reference and am not sure how this component works. I am open to any Ideals as to what I can check next, I am starting to become frustrated with this problem.
Melting Slicks
Joined: Jul 2004
Posts: 2,695
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From: Chicago Illinois
You can change everything on that car until you finally find your problem. From my experience with my 93 that ran like crap and a flunctuating idle. Its a guessing game, some times you get lucky. From now on I take it to the professionals and let them diagnose it. My problem ended up being the opti distributor, but I already changed every sensor on the car. Good luck
Melting Slicks
Joined: Jul 2004
Posts: 2,695
Likes: 0
From: Chicago Illinois
It is very possible it could be the O2s. They would cause the symtems you have on your car. Something is sending a signal to your computer because you said it only happens when car is warmed up or in the closed loop. Like I said its a guessing game.
Drifting
Joined: Aug 2004
Posts: 1,448
Likes: 1
From: Tracy CA
I'm going through the same thing with my '89 with L98 engine. When it gets warm, it starts "loping". I've done the ECM, IAC, TPS. I'm running out of TLA's
. Took it to a neighborhood shop... they suggested injectors.This winter I'm gonna take it to a 'vette shop and let them have at it.
Or... maybe I'll just sell it and get an LT4...
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Drifting
Joined: Aug 2004
Posts: 1,448
Likes: 1
From: Tracy CA
Originally Posted by onetrackmind
EPIC, when they suggested injectors, was there a reason or were they just taking a guess?
Anyway, they didn't do a pressure test, to my knowlege. I kinda got the impression that:
a) They were just shooting in the dark, and/or
b) They didn't want to take the time to properly diagnose the problem.
Wish I lived closer to Cupertino/SJ area, or could find a good wrench here in Fremont/Hayward
...Anyone???...
Melting Slicks
Joined: Jul 2004
Posts: 2,695
Likes: 0
From: Chicago Illinois
The injectors work fine until car is warmed up which eliminates that idea. O2 are cheap. Maybe give it a shot see what happens. If you do take your car to someone for fixing go to Chevy or a good vette shop. Sometimes with the Corvette you have to pay the piper. Most other shops are in the guessing game with you.
Thread Starter
Pro
Joined: Jul 2004
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From: 1985 L98, Auto. O'FALLON, IL
I am frustrated , but not enough to take the car to someone else. Part of the reason I wanted the car is to wrench on it. Hopefully I can learn something from this besides how not to save a buck.
Burning Brakes
Joined: Nov 2002
Posts: 807
Likes: 21
From: Los Altos CA
Try disconnecting the oxygen sensor and see what happens. If it idles smoothly then, it is the system hunting for the correct mixture. There could be several causes but the injectors are a high probability. I would replace the oxygen sensor before replacing the injectors since that is simple and cheap but it probably is not the problem. You can get new injectors from five-0 motorsports for under $300. Check ebay. He also has some almost new injectors for under $200.
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
Have you checked for vacumn leaks? And when you said you replaced IAC, did you actually replace it or just clean it? The IAC has what's known as a pintle, if I remember right the pintle screws in or out-but in the end run it should be adjusted to 28mm, from the end of the pintle to the surface-not the gasket. The IAC-is sometimes called a carbon magnet-when you remove it-it's important to clean all the holes, cause that's where carbon builds up. NOT saying this is your problem-but it's a main source of idle problems on our C4s. Good luck and let us know. 
Drifting
Joined: Aug 2004
Posts: 1,448
Likes: 1
From: Tracy CA
My IAC is new, but I did it before I found this forum. Gonna try to pull it again this weekend and double-check the pintle measurement.
It ran fine when I first got the car. The "wonkiness" started kinda all-of-the-sudden. One day fine, next day lobing.
It ran fine when I first got the car. The "wonkiness" started kinda all-of-the-sudden. One day fine, next day lobing.
Advanced
Joined: Jul 2004
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I agree with Rick that vacuum is a good place to look when idle is lopey but the engine runs good at speed. You can spray water around suspected vacuum leaks (like intake manifold gaskets) while idling and the engine will smooth out if you spray where the hole is. I had a similar problem with my 91 but it turned out to be injectors. Idle was hunting so I cleaned IAC, throttle plate, etc. and replaced plugs, wires, cap & rotor, but it ran worse each day. Sam Lamb posted here that injector resistance should be 16-17 ohms (he recommends checking hot & cold); mine were all over the map with 5 bad and 3 good. Just pop the electrical connectors off the injectors and check resistance across the two terminals. Jfb tells me the best way to check injector integrity is with an impedance bridge, but a resistance check is quick, easy and might spot your problem.
Thread Starter
Pro
Joined: Jul 2004
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From: 1985 L98, Auto. O'FALLON, IL
Ok. thanks to everyone for the advice. I got home today and measured each injector, they were all at 17 ohms while the car was hot. I then disconnected the O2 sensor and started the car and It idled right around 650. I think this is a good sign and I will try to replace it as soon as I can. I am going out of town and will not have a chance this week. Thanks again for all the input I really enjoy the brain storming power of this forum.
Thread Starter
Pro
Joined: Jul 2004
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From: 1985 L98, Auto. O'FALLON, IL
Epic, when I installed the new IAC the instructions do not say to set the length to 28mm they say it should not exceed 28 mm or it will be damaged. they lenght will reset itself when you start the car for 10 seconds turn it off and start it again.