1990 rough running/idle
Thread Starter
Intermediate
Joined: Aug 2004
Posts: 37
Likes: 0
From: California
1990 rough running/idle
Hello, I have a question regarding my 1990 L98 6-speed. Over the last several weeks it has devoloped a rough, throbbing idle and has died several times in parking lots at low speeds. I've replaced the oxygen sensor, PCV valve and fuel filter but it is still doing it. I'm wondering if it is an ignition problem or possibly a fuel pump.
It runs fine when it is cold but after 10 minutes of driving I will stop at a red light and the idle begins to flucuate and throb between about 300 and 700 RPM. When it dips down low to the 300 RPM point it rattles a little like a mild case of spark knock. Sometimes if I rev it a little it will even out again for a while. Once I'm moving down the street it seems to run ok. Sometimes it bucks a little in 2nd and 3rd gear at constant speed. Does anyone have any suggestions before I try replacing the fuel pump or taking it to a mechanic? The computer doesn't seem to be spitting out any error codes. Any suggestions or advice would be appreciated. Thanks.
Mike
It runs fine when it is cold but after 10 minutes of driving I will stop at a red light and the idle begins to flucuate and throb between about 300 and 700 RPM. When it dips down low to the 300 RPM point it rattles a little like a mild case of spark knock. Sometimes if I rev it a little it will even out again for a while. Once I'm moving down the street it seems to run ok. Sometimes it bucks a little in 2nd and 3rd gear at constant speed. Does anyone have any suggestions before I try replacing the fuel pump or taking it to a mechanic? The computer doesn't seem to be spitting out any error codes. Any suggestions or advice would be appreciated. Thanks.
Mike
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
I assume you have stock injectors in the 90 Vette. Check the resistance of each injector by removing the connector and placing the probes on the two injector pin connections. Check them all when the engine is cold and check them all with a hot engine. The readings should ALL be in the 16 to 17 ohm range. Anything below the above range indicates a shorted injector coil. I had three shorted injectors on my 89. The 89 to 91 Vettes come stock with GM Multec injectors which have a high failure rate due to shorted coils. This is an easy test to make! Happy New Year.
SAM
SAM
Drifting
Joined: Nov 2000
Posts: 1,428
Likes: 1
From: Jacksonville FL
Originally Posted by AimHi
How much does a new IAC cost?
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Your '90 L98 has a speed density (SD) system instead of a MAF.
I agree that a fluctuating idle is likely an IAC issue. You can try cleaning it before replacement. In that case clean the TB as well as the IAC pintel and it's bore under the TB.
Then you will need to set base idle, my method is on my site. But ignore the TPS adjustment of my method as your TPS is not adjustable.
I agree that a fluctuating idle is likely an IAC issue. You can try cleaning it before replacement. In that case clean the TB as well as the IAC pintel and it's bore under the TB.
Then you will need to set base idle, my method is on my site. But ignore the TPS adjustment of my method as your TPS is not adjustable.
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Burning Brakes
Joined: Oct 2004
Posts: 898
Likes: 1
From: Pearland Texas
I had similar symptons. Turned out to be carbon build up on a plug. I took it in and they did get a code though. It was probally the cheapest repair my mechanic charged me. I would not haved guessed it was a plug because it acted so scarey when it idled. Sounded like a internal engine part was about to shatter. good luck and let us know what it turns to be.
