CO951

I need help diagnosing a problem. I had my stock '89 951 at the track a week ago (1st track day this year), and it was running rough/missing at WOT, above 3500 rpm. It didn't seem to occur, or not as badly, in 2nd gear as in 3rd. I hadn't noticed the problem before, but I rarely use WOT in 3rd on the street. It was easily over 100F on the track, although the problem existed before the temps got that high. I was using Colorado's premium fuel, which is 91 octane, and the track is at 4100 feet ASL, which is 1500' less than where I normally drive. Someone at the track thought maybe I got some bad gas, so I put a bottle of Heat in, which seemed to help slightly, but didn't resolve the problem. I’ve tried WOT in 2nd several times since getting back home (about 5600’) and getting fresh gas, but haven’t had the problem. I finally got a chance today to run WOT in 3rd, and didn’t have the problem, but the temp was in the 70s.

Does anyone have any ideas what the problem might be and/or how to diagnose it further?

sweanders

Bad gas seems likely, especially since you can't reprodruce the problem with fresh fuel. It is hard to troubleshoot a problem that isn't there..

dand86951

Since your car is stock there may be a few things to check. The car may have been missing in the hotter temps, due to a coil, plugs, plug wires, distributor rotor that were just a bit hotter than could be handled to get proper spark to the engine.

The air temp sensor may be giving incorrect reading causing it to possibly go too rich. Did you notice unusual amount of black smoke when on boost?

The air vane may not be sending the right voltage to the DME at the hotter temps.

CO951

When I first got the car last summer it was running VERY rich and it felt almost the same way as it did this time. I had black smoke last summer, but I didn't notice any this time. I had replaced the coil, plugs, wires, rotor and cap last summer and had it running pretty good. The plugs are Bosch WR7DC, whould a cooler plug help if it is a heat problem? How would I check the air temp sensor?

special tool

Plugs are not the place to start.
You need to know exactly what your boost level is using an aftermarket guage.