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This problem has been vexing me for ~3 months now, but I was too busy with school to care until now. I will recap the symptoms and treatments applied thus far:
Patient: '93 940, B230F with REX/Regina EFI, 24x,000 miles
Symptoms: mysteriously starts 'missing' above 4500RPM. Reducing load(throttle) coaxes a few hundred more RPM out of it before it starts again. Behavior appears to occur at lower RPM (~3500) under greater load (accelerating at highway speeds uphill, for instance.) Behavior is reproducible in 'Park' sitting in the garage.
Treatments: Replaced the usual suspects - RPM sensor, coil/powerstage unit, fuel filter, cap & rotor, FPR, and even the fuel pump & sock. The only changes noted were a tiny rise in the RPM threshold where the behavior began when the coil was replaced, and a tiny lowering of the RPM threshold when the fuel pump was replaced.
Plug wires and plugs < 1yr. old.
Removed the knock sensor from the block (but kept it electrically connected) - no change in behavior.
Took out the plugs today and regapped them from 0.032 to OEM 0.028. Noted all plugs smelled a little on the rich side, plug 3 especially. Pulled injector 3 electrical connections and ran motor - same behavior. Duplicated this test on each individual injector with same results.
Verified timing belt is synched - distributor drive pulley did not skip a tooth. I purposely skipped it one tooth and did not show any change in behavior.
Car passed CT emissions test (rolling dyno type) last week - did not get into the affected RPM region.
Pulled a little hair out...
I'm really getting stumped on this one. My latest theory is that maybe the engine is getting too rich at high RPM, but what could cause this behavior? The only two things that seem to be left are the ECU and the O2 sensor. Having passed the emissions test, I tend to think the O2 is good. I have a Regina-equipped buddy that I can swap ECU's with if necessary. Anybody got any other ideas? The car just drives so well under all other conditions... this is really puzzling.
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