I just picked up a 98 LS and I've been having trouble with it's idle. The idle is a bit rough, and sometimes it weakens and drops very low. This is more of a problem if I have just come off the throttle, i.e. if I blip the throttle while idling or during coasting to a stop. Usually the car recovers from the weak idle, and then it will idle forever at 750 until I use the throttle again. That seems to "reset" whatever is wrong. About 10% of the time the car can't recover from the weak idle and dies. It's also worse if the car is cold but it happens even when the engine is warmed up.

- There is no CEL code
- There is good spark
- Engine timing is correct
- Ignition timing is correct *but* it fluctuates a bit between 14-18 BTDC on idle

I have tried cleaning some things and replacing some parts from my 96 GS-R, which idled fine (although it had a vacuum leak):

- Replaced the spark plug wires (GS-R)
- Replaced the spark plugs (new)
- Replaced the fuel filter (new on GS-R)
- Replaced the fuel injectors (GS-R)
- Replaced the IACV (GS-R)
- Cleaned the throttle plate
- Replaced the ECT sensor (GS-R)
- Replaced the valve cover ground (new on GS-R)

I have checked the PCV and I *think* it's working but I'm going to replace it to be sure. What else should I check? I need to get this solved ASAP.

-thanks

 

Sounds like a fuel problem. When your car is running good...where does it idle at?

 

It idles at 750, although it's not the strongest idle in the world

Edit: I also tried replacing the FPR, which didn't solve anything. Since I have the problem after trying a different fuel filter, fuel injectors, and FPR I don't know what's left in the fuel system to check.

 

Have you tried adjusting the idle on the throttle body so it idles at 1100?

 

I've turned the TB screw a full rotation in both directions with no effect on idle. That's been sort of a head-scratcher for me too. I was under the impression that it is pretty sensitive. But it's also hard to set because the car won't run with the IACV unplugged.

 

Ok, next thing is try a diff ecu

 

I've got a spare Neptune-chipped ECU sitting around I can try. I'll get an LS basemap on it and give it a shot. Thanks for your help so far.

 

I plugged in the OBD1 ECU and the car behaves much better, but I think that's just because of the higher idle on the chipped ECU. I was also able to datalog, and the MAP sensor is reading very high (470-500 mbar) at idle. Any idea what could be causing this? I figure it means that there is a vacuum leak or a bad seal in the throttle body. The sensor reads correct pressure with the engine off. The TPS signal appears correct (-1 at no throttle, 101 at full throttle), and the fuel pressure is optimal and stable.

 

Have you tried compression testing it?

 

The compression test was even so I thought it was good. Turned out that whoever rebuilt the head on this car before I got it set every single valve too tight. Not sure what the lash was but it was close to 0 because I had more than a full turn before each valve was correct. I just corrected the lash and she's running like a champ. Thanks everyone for your help.

 

I bought this car a few weeks ago, and the previous owner had some head work done. Whichever shop did it must've set the lash wrong and he'd been driving it that way ever since. Every single valve was wrong, and by the same amount.

Generally over time valves do loosen instead of tighten, but I've heard of them tightening on their own, too. I feel like I've had to loosen them sometimes doing valve adjustments.

GL with your idle problem. Hope it's an easy fix. Feel free to post here for troubleshooting and whatnot.

 

Nice to see you had better luck than me. I had that issue and it turned out to be a the piston in cylinder 3 with a cracked ring and ring land -____-