dropping RPM counts

I had a powerlogger trace, but can't find it right now.

Anyways, I have replaced about every electrical component on the car, but once I'm cruising over 3k RPMs, I start losing RPM counts, and the car starts backfiring while the tach goes erratically between 3k and 0 RPM. Datalogger shows the RPM toggling between 3000 and 1500 so it looks like somehow I am missing some of the pulses from the crank sensor.

If I shift it into OD and drop the RPMS below 3,000, the problem goes away instantly, then when I shift back into 3rd and the RPMs are above 3,000 it starts losing tach counts and backfiring again.

I replaced crank sensor, ECM, and ignition module but the problem persists. I cleaned the contacts on the crank sensor and ignition module also.

What else could cause the ECM/ignition module to calculate the wrong RPM?
 
the components that feed RPM pulses are

crank sensor
ignition module
ECM.

The mounting nuts for the ignition module need to be tight, the correct nuts have built in star washers since the grounding of the module is critical.

A loose balancer bolt, or a loose crank sensor bracket?

electrical terminal in the crank sensor plug, or ignition module plug could be corroded or have lost tension from age.

Bob
 
your RPM reading drops by 1/2 and sometimes by 2/3

so the crank sensor is skipping one or 2 teeth at a time.

Is your balancer bolt loose?

Does your engine have a lot of crankshaft endplay?

I would look at mechanical issues.

Bob
 
I've seen displays like that. It turned out to be an improperly grounded ignition module.
 
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