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Hello everyone, I am new to the forum and come with an interesting issue that I hope someone of you may have had experience with.
I am diagnosing an issue on a 1992 740 with the 230B engine, REX/REGINA control system. All of the ignition tune components are new and I have verified that they are in good working order. The engine starts and idles well. Then at 3,000rpm it cuts out as if it had a rev limiter. I would suspect overrun fuel cutoff except that the issue is there under load while driving, not just in a bay test. I connected a timing light and can see that spark timing is actually jumping back and forth as much as 30 degrees when this is occurring and I can hear the popping sound of firing in the exhaust. The problem gets worse with more throttle pressure.
I have disconnected the knock sensor to eliminate knock retard as a possible cause.
I've connected a lab scope to the crank sensor and to the timing control signal to the power stage from the REX module. I can see the timing anomaly in the timing control signal. The signal is a 5v square wave and I can see the spaces between pulses fluctuate and the pulses "on time" drop off randomly. The off pulses seem to coincide with the off time spark events. My early model lab scope has poor resolution(my kingdom for a PICO) and I have a hard time making out any matching fault in the crank sensors AC sine wave that might explain the odd firing.
The owner has made several attempts at fixing the issue himself, including swapping REX modules(there are two on the pass seat and another installed) he has also swapped out the power stage and coil. The crank sensor is the "improved version" with the white stripe. My thinking is that since the anomaly is present in the command signal from REX, it is not an issue with the power stage anyway. Sorry for the long post, thanks for any help you can give.
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