Here's the problem.

I have a 93' speed density 396ci LT1 M6 with Ed Wright tuning.

The car starts fine, but idles around 1,200 RPM's and the tach (aftermarket autometer) jumps around from about 1,300 to 1,100 RPM's The speedometer needle jumps around at idle and when given throttle (it is sparadic, so it doesn't change with RPM or anything like that) The factory tach doesn't work. It acts like it has a rev limiter around 2,800 RPM's and cuts out really bad.

The car has been running for over a month now with no problems and then all of a sudden it wont rev past 2800rpms or idle like it did. I didn't change anything to cause this.

Here's what I have changed or tested etc.

New Opti
New cap and rotor
noid light on all injectors
new plugs, then checked all the spark plugs to be sure they were getting fuel
Checked for spark on all cylinders
ran a heat gun on the primaries (all temps were close to the same)
new coil and ignition module
swapped computer chips
put an MSD box on the car (runs and revs alot more consistent, idle doesn't fluctuate as much and it revs smoother but still cuts out at 2,800 rpm's)
Disconnected and reconnected the battery
checked for wires burned to headers etc.
car does not have any codes
car doesn't have EGR or Cats

I am going to swap computers tomorrow.....Any other suggestions??

Is there a Default on LT1 cars that if the speedo sensor goes out, it will take a reading off of another sensor?? I think the two may be related since it is jumping all over and the factory tach isn't working.

I put an automatic pcm on a 94' M6 car and the speedo works, but only reads 1/2 your actual mph. That's why I think it is able to take reading from somwhere else besides the speedo sensor. (this is not the car I'm having problems with)

 

My ignition switch under the dash melted. My speedo/tach etc jumped around. It only did that for a brief amount of time before completely going out...however its something to check.

 

I just installed a delteq ignition, and the instruction manual says the 93's have a tach filter. Maybe yours is bad? or your aftermaket tach is not happy running with filter? not sure about missing

 

What switch are you referring to? the one that the actual lock cylinder is attached to or what?? What does it look like?

Any Idea where the tach filter is located??

The car didn't miss with the MSD box attached that I noticed...maybe more of a stumble than a miss??

Thanks for the suggestions......I'm open for anything at this point!