6speedshifty

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

JCAIRE2

Sounds like the Idle air control valve. Do you have the factory manuals?

Sam Lam

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

AimHi

How much does a new IAC cost?

Midnite 88

Around 45-50 bucks. Is the '90 a MAF or Speed Density car? If it is a MAF car you can try unplugging the MAF and see if it runs better. I had a MAF die on me and had similar symptoms to yours. I have an '88.

6speedshifty

I don't believe the 90 has a MAF sensor. Thanks for the advice. I've been meaning to order the factory tech manual.

95Torch

Try cleaning your fuel injectors and air intake first. These are are both cheap quick fixes that I would start with. I had that problem a few months ago while shopping for cheap gas prices.

65Z01

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.

6speedshifty

How do you set base idle?

chriswtx

Clean the throttle body and the AIC valve...There is an article in the C4 techtips section(under the "other" button) on how to set the base idle...

9ineLives

i don't know if tune port has throttle pos sensor if it does it may also be bad

chriswtx

Yes it has a tps sensor but if it is bad it will usually throw a code...

Dads90

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.