Quote:
Originally Posted by Chuck.H
If the tach drops rapidly while the car is in gear and in motion, then the speed sensor is bad IMHO.
This happened to me once, and what stuck in my mind is that the engine can't drop RPM that fast without depressing the clutch... try it, just take your foot off the gas at 4k rpm, it will just drop slowly as the car slows down.
If you replaced these recently, then maybe they just aren't spaced correcty.
HTH,
Chuck.H
'89 TurboLookTarga, 342k miles
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Chuck has hit the crux of it. The only way the tach can drop like that with the drivetrain engaged is if the DME thinks the engine has slowed that much, the DME is defective, or there is a major electrical fault. The DME provides the tach signal so it is saying that it stopped spinning which is the problem.
If you disconnect the AFM or remove the distributor, the DME will still show the speed the engine is spinning.
I'd look at the gaps on the reference sensors and their connectors. Because the engine is consistently running with very intermittent misses I'm not sure if testing the sensors with an oscilloscope would show the problem unless their output pulse is marginal. (Should be 2.5v)
The other things I would check is all the grounds. The ground on the left rear of the intake and the ground from the front of the trans to the body are the first that come to mind.