Rough Idle- Where to start looking

[RonnieH]

Recently, the car started idling roughly ('90 Coupe - 32,000 miles). When first started and idling in "Park", idles smoothly. After reaching operating temp, idles very rough in "D" - the RPM's vary +/- 50-100 RPM. Rough idling doesn't seem to be caused by a miss and the car runs great accelerating and at speed. Problem first appeared intermittently just before I put it away for the winter last year and got worse when I started driving it in the spring. There are no codes that I have seen. I had a non-related problem about two years ago and replaced the ECM for that problem. I had a similiar problem on one of my other cars a couple of years ago and I fixed that by replacing the oxygen sensor, but I would think that would give me a code.

I haven't started troubleshooting yet as I'm not quite sure where to start. Hoping some of you folks have had similiar experience and can advise a good action plan to try and narrow down the cause. Thanks in advance for any ideas.

 

[JimBobC4]

Recently, the car started idling roughly ('90 Coupe - 32,000 miles). When first started and idling in "Park", idles smoothly. After reaching operating temp, idles very rough in "D" - the RPM's vary +/- 50-100 RPM.for any ideas.

I would check for a vacuum leak someplace or a clogged EGR valve. I'm not real familiar with an L98 however. Maybe an IAC valve is sticking or dirty ? Either way I think your getting a foreign air flow into the system, causing the system to "hunt" for the correct settings.

 

[gw962]

The O2 sensor may be sluggish and not throwing a code.
The injectors may also be a problem. Use a Volt Meter to ohm the injectors when hot. These should be around 16.5 +/- .5.