'88 L98 Idles Rough

[Bonnell]

Hi Everyone,

I need help again. This is a long post but it seems that you guys always want all of the details to use to make a judgement so please have patience. I have a 1988 Coupe with an L98 - no mods that I know of except for headers. I purchased it in November 0f 2002 and it has been my daily driver. It will continue to be my daily driver until I have finished paying for the last kids car and college (about two more years).

I really need to keep it running. I purchased it at 88K miles and it now has just under 120K. It has been a lot of fun. I've had brakes, a tuneup, and a new ECM. The tuneup is about 10K miles old.

The car runs too hot in the summer with the a/c on at a stop (I've seen it hit 235 before I turned the a/c off) and I need to find out why but my primary post here is concerning idling.

When I sit at a stop, the car does not run rough like it had a fouled plug or bad plug wire where it misses at each firing, it just sputters every couple of seconds. Sometimes the RPM will very briefly fall from 600 to 500 (6 to 5 as we see it). I am concerned that it will die at some point so I often press the accerator to take the RPMs up a little. Sometimes when I am at a stop, it does not do this but it does more often than not.

I understand a little about the engine but I have only the experience that I have gained making small repairs based on the advice that I have recieved here.

Anyone out there have any ideas? I'm operating on a shoe string budget so I'd rather try and fix it myself.

Thanks,

J

 

[cntrhub]

Sounds like an injector problem, or non-spec spark plugs? Do you have OEM plugs in the engine? If you just installed plugs, check the gap again? While the plugs are out (all of them that is) make a compression check of the cylinders. Usually a low compression cylinder will cause poor idle. Try Chevron gas for a few tank fills. If you are using low grade fuel, try using Chevron Techroline injector additive and read instructions for use. While the engine is running, take a screwdriver and ground the tip to the engine head. Move the shank close to the spark plug cap and see if there is a weak plug wire that will jump the gap from the cap to the screwdriver shank. . Do this for each wire and see which one might be suspect? Change that wire only. That's as much as a "shoe string budget" I can help you with.

 

[Bonnell]

cntrhub, thanks for the reply. The tuneup was done by a local Chevy dealer so the plugs and wires had better not be sub-par. I only use Shell premium gas - never anything less.

As far as compression tests, that is what I was referring to as operating on a shoe string budget - I don't have the test devices for fuel pressure, compression, etc. I will try the screwdriver test but I sure would expect those plug wires to last more than one year.

Would a bad plug or plug wire casue a sputter every couple of seconds? I would have thought that I'd have a more or less constant missing.

 

[ErnieN85]

sounds to me like you might have a dirty condition in the throttle body and or idle air passage.

it's one of the first things to look at with a high mileage car. take the rubberfitting off the front of the throttle body and open the blades see how much black gunk is in there! This is caused by blowby if its dirty it will help a lot to clean it up. the best way is to remove the throttle body from the car. remove the idle air motor and then clean things up with carb cleaner. after doing this you may need to reset the minimum air setting and TPS. I can't say how familiar you are with this or how much you want to do yourself. the procedure is covered in the factory shop manual. if you are going to be working on your car you need one the Helm book is the only way to go! Haynes just won't cut it. If you decide to do this yourself. feel free to ask for more help I'm sure thet are quit a few willing to help

 

[silver 80]

you guys don't think it could be the EGR?
I'm on an EGR kick lately after I found mine missing the vacuum hose assembly, and no it hasn't thrown a code at me lately.

 

[ErnieN85]

On an84-96 the egr is controlled by the ECM and is not active at idle. so no it shouldn't be that also the egr typicaly is ported on a carb engine and isn't active unlit the throttle blades pass th port (you step on it).

EGR can mess things up if it's stuck but the usuall symptom of this is it just won't idle at all.

 

[SurfnSun]

Mine car does the same thing...I think it has to do with the race gas Im running though, doesn't really do it when I run straight pump gas. My car will idle from 800rpm down to 3,4,500rpm and surge up and down. Doesn't do it all the time, it is annoying but one of those sacrifices I give for running race gas. FWIW though, my 85 used to do it also. I figured it was just a TPI plague.

 

[cntrhub]

Bonnell, Electricity takes the shortest path possible. So if say.... the plug wire was burnt and resting on metal, then the spark would immediately short to ground and never make the jump to the side-to-center electrode and spark the fuel. You can literally watch a spark jump to ground, if you popped the hood and looked at the engine bay in the dark. The misfire you hear, or feel is usually a cylinder not firing at all.

 

[vetteboy86]

Is the "Service Engine Soon light" on. If so you should retrieve the code from the computer. I could instruct you on how to do this. My brother has an 87 and his had a rough idle to it, and the culprit was the MAF, and Burnoff relay. Once that was fixed it was fine. My 86 went through a rough idle spell and i had to take it to a shop because i couldn't diagnose it because the Service engine light didn't come on. It turned out to be an engine relay.

good luck

 

[drags1998]

See if you can find someone with a scan tool? if you have no codes, checkVERY CAREFULLY for a vacume leak, Make sure all hose clamps in the air intake system are tight, if all of the above checks OK. Im not really sure????????????/SORRY!!!!!!!:confused

 

[Bonnell]

Thanks for the replies. The SES light is not on. I will probably buy a scan tool soon. I'm going to ask about that next.

Thanks again.