I have an 89 L98 with 69,000 miles on it, and the engine does not idle as smoothly as I would like. The engine seems to idle fine but, has a Very Slight roughness to it that I can't seem to correct.

So far I have installed a new cap, rotor, MSD plug wires, AC Delco rapidfire plugs .035" gap, set timing to 6* btc, K&N air filter , used Chevron Techron fuel injector cleaner and only Chevron Premium fuel.

I've been searching on this forum for awhile but can't find any similar situations. I would like to get suggestions from the pro's so I don't replace unnecessary parts.

I was thinking of changing the O2 sensor or cleaning the IAC motor next. Thanks



[Modified by GKK, 5:04 PM 2/29/2004]

 

Clean IAC, reset TPS

Check for codes. O2 sensor shouldnt cause it.

 

:iagree: :thumbs:

 

I had the exact same issues you mentioned and did the same things too. I also changed the wires, plugs, cap and rotor, the IAC and the oxygen sensor. On my '91 it turned out to be fuel injectors worn. 1 completely died and nailed down what it actually was. You can use an ohm meter to check resistance of the injectors. It should be about 17.5 ohms. I had 7 that were 17-18 ohms and 1 that was 3.5 ohms. At first when I would unplug the lower impedance injector the engine would die. Evidently it was running full open most of the time and thats what caused the rough idle. Eventually it completely died and when I would unplug it there was no change in engine idle. But the engine ran so rough I couldnt drive it by that time. I was quoted some outrageous prices to get them changed so I decided to do it myself. Another Forum member gave me this link:
http://slponline.com/view_product.asp?PARTNUMBER=23051

And I got a set of them, installed them, and they work *perfect*. However...I had a thread going in this forum while I was doing it and a couple of other guys ended up buying the same injectors and had some issues with them. They said they were too short and didnt fit their engine.
http://forums.corvetteforum.com/zerothread?id=743450
http://forums.corvetteforum.com/zerothread?id=738030
http://forums.corvetteforum.com/zerothread?id=670714
http://forums.corvetteforum.com/zerothread?id=671094

Anyway, thats most of the threads about my injectors while I was going thru this odyssey. Feel free to email me if theres anything else I can help with.

 

Im sure this is going to sound like a dumb question but how do you "reset TPS"?

 

I think I've found the problem. Each injector on the driver side measured 18.0 ohms and three injectors on the passenger side measured 16.0 ohms and one (closest to the firewall) measured 5.4 ohms.

I'm going to order the set of 8 Bosch injectors from SLP and clean the IAC while I'm at it. It doesn't look like too hard of a job. Thanks everyone.
:)

 

Yeah, that one with the way low impedance is definitely the culprit. Did you try unplugging it while the engine was running? On mine when it was half @ssed bad the engine would die when I unplugged the bad one. After the injector failed completely it made no diff when I unplugged it. While I had the plenum off I sprayed a ton of carb and choke cleaner all over inside it and the IAC hole to get it sparkling clean. If you run into any problems you can email me if you want to. I dipped all the injector ends in motor oil to lube the o-rings, same for the tubes that go side to side between the fuel rails. After I installed the new injectors and reassembled the fuel rails I pressure checked it by turning the key to ON to let the fuel pump pressurize the system. Oh, and I left the fuel rail locking clips OFF of the injectors. Youll see why when you go to reassemble everything, lol. Good luck!

 

I had the exact same thing on my '90 and it was the injectors. Like you, I had one that had a very low impedence value. It would idle fine at startup, but as soon as the ECM went into closed loop, the roughness would appear.