DPC said:
I think it is a sensor and I am hoping you guys have the magic answer. When I fire it up cold, all is normal. When it is warm at idle it seems to be missing and irratic but not any higher on the idle speed. Drives the same either way down the road
Any thoughts
Your '90 is a Renix (non-HO), the same as my '88. So we would generally follow a similar trouble-shooting routine.
I hope you're at least slightly aware of how your injection system works. During warm-up, the ECU ignores sensor feedback and operates in a default, fuel-rich mode according to a pre-set fuel map. This mode is also called "open-loop" because there's no feedback.
Once the engine warms up, the system switches over to "closed loop" operation and uses data from the oxygen sensor, coolant temperature sensor (not the one for the dash gauge), throttle position sensor, and I think a couple of others to control both air/fuel mix and ignition timing. The O2 sensor is a "consumable" -- it requires periodic replacement. The recommended interval is 80,000 miles, but I assume they are like light bulbs -- the rated life is an average, and some will last longer while others will fail sooner.
The last time the O2 sensor went bad on my '88 I noticed it at the point where the system switched over from open-loop to closed-loop. This was typically about 10 minutes down the road from a cold start. I have a 5-speed, and I began to notice a little "Pop" sort of manifold backfire each time I shifted. Then I noticed some occasional stumbling when running at partial throttle. Finally got smart and realized that the O2 sensor was dying, and when I let off the gas pedal (which results in an instantaneous rich mix because the throttle plate shuts down), the O2 sensor wasn't reacting quick enough to shut down the injectors. As a result I was dumping a little bit of unburned fuel down the exhaust, and it was backfiring up the manifold when it hit the catalytic converter.
A new O2 sensor removed all those symptoms. Note that I caught it fairly early. If I had an automatic rather than a 5-speed I doubt I would have noticed anything, because with an auto you don't let off the pedal for upshifts. My symptoms were very mild and didn't affect driveability very noticeably. I had not seen any drop in gas mileage yet. Dunno how long I could have gone before the symptoms because more evident.
Some of what you describe sounds similar to the effects I'd expect from faulty sensor feedback. It runs fine when cold because it ignores all sensor input, then when it gets into closed-loop mode the problems show up. So I think you should be looking at the sensors. O2 is the one that requires periodic replacement, that's why I asked how many miles on it. Coolant temp sensor is the one on the driver's side of the block. I doubt that's the problem, but that's the one that tells the ECU when the engine is "warm." TPS is a possibility, especially with a crappy idle, and the factory service manual has a good discussion on adjusting and trouble-shooting that puppy.
Hope this helps.
