Tach Jump & Misfire

[Regor]

I am experiencing an issue with my tach and a misfire. The tach will jump all over the place and rest on zero while driving the car. Most of the time the tach will return to normal when sitting in park or at a light. Any light to heavy acceleration will produce a misfire that is definatly felt along with a tach that jumps all over the place. I does not matter if the engine is hot or cold.

I have quadruple checked the wires/routing. I have replaced the coil packs and ICM. None of the cables going to the ICM are shorting out on anything. Ground straps are good.

I am thinking it might be either the crank sensor in the side of the block or the one behind the harmonic balancer. Which one sends the signal to the ICM for the tach? How can I test either one to see if they are bad? The plugs only have a few k on them.

Specs:
Car - 1998 Malibu LS
Engine - 3100 - M

Thanks for any help!!

[bszopi]

Sensor in the block (rear side) is the one that goes to the ICM. That would be the next thing I would try.

[geoffinbc]

Agreed. I was going to say ICM first but I see you replaced that. The sensor can crack but still work although not properly. Pull it out and you should find your problem.

[jmgtp]

I had that happen once where the engine would miss and the tach would cut out - mine was bad wiring from CPS. But if you check that, I agree with the others and would go after the CPS itself next.

[Regor]

I'll check the crank sensor in the side of the block this weekend. Thanks!!