Dan Alger

When the engine gets hot (oil temp above 190 F) the engine runs very poorly. The idle is very rough and the RPM won’t go above 5000. The problem is not isolated to a single cylinder. This condition only occurs on the racetrack on a warm to hot day and abates within 30 minutes after a race. When the car is in this condition the tachometer also indicates erratically. This leads me to believe that there is something wrong with the speed or reference circuit. In my research I have found nothing that indicates what part of these circuits are temperature sensitive. Has anyone ever had or heard of this problem?

Gage

Instinctively, I would be doing some fuel system diagnostics first, then secondary ignition source.
Check system pressure to rule out pump and supply issues and check injector pulse from the DME (injector drivers).
Check the coil.

Dan Alger

Thanks for the reply Gage! Those were my initial thoughts also. I changed the fuel filter and installed a dash mounted fuel pressure gauge. I replaced the coil, cap, and rotor. I have also checked the spark at the plugs while the problem was occurring and they seem strong and consistently timed. I have also checked the resistance and static voltage of the speed and reference sensors when the engine is cold, but not when it is hot. When I noticed the erratic indication of the tachometer the last time I had it on track I began to suspect the speed and reference sensor circuits, but I did not have the tools with me to check voltage and resistance. I do not own an oscilloscope to check the dynamic inputs or outputs to the ECM. I was hopping someone could tell me they have seen this and what it turned out to be so I can try to fix it before I take it to the track again at the end of this month. The car is not street legal, registered, or insured, so testing on the road would be a bad idea. I would just replace both sensors and hope it's not the DME, but it seem that adjusting the sensors without removing the engine is somewhat tenuous.