1986 GP Tach & Oil pressure gauge readings are wack!

[Andy]

I have a 1986 GP w/ 2.8 and the tach acts like it's polarity is reversed? When cranking, the tach jumps up around ~2500 rpm, but as soon as the starter disengages and the engine is running, the needle drops down below 0 - the end of then needle is out of sight. When not running, the needle points back to 0. Also, the oil pressure reading goes max - pointing straight up, beyond all numbers. I have replaced the sensor, no help.

I think maybe the incorrect tach reading is causing the computer to provide incorrect fuel mix, which is also causing problems w/ the catalytic converter. I replaced the converter and O2 sensor about 6 months ago and had to replace the converter a couple weeks ago (Monroe replaced it no charge, not sure they'll do it again!). Does this sound right?

Any suggestions????

[GOT2B GM]

That must be an 88/89 Grand Prix, an 86 is a RWD G body. On a helpful note, maybe your ICM (ignition module underneath the coilpacks), or your crankshaft position sensor is causing your tach issue.

[Andy]

My bad, it is an 88 Grand Prix. And the crank position sensor is one of the few sensors left to replace. Thx! I'll let you know how that turns out.