My Fd seems to idle fine when it's cold. Once it warms up, the tach starts to bounce from 0-wherever it wants and then it goes out. I just changed the coils and that didn't solve the problem. Could a faulty igniter be the trouble?

 

A vacuum leak after the throttle-body will make for a goofy idle as you've described. Starts out somewhat normal and starts hunting 1000 - 1500 rpm as it warms-up. They can be a little hard to find. I'd start with a little mechanic's mirror and a small flashlight looking at all the vacuum ports on the upper intake manifold and the associated hoses. Of course there's also the gaskets to consider. Some have used propane and listened for changes to pin-point the leaks.

 

The part that I can't understand is that the tachometer stops working as soon as it sounds like it's missing. The whole car shakes like its a really bad misfire too

 

I misunderstood your first post then. Any other gauges involved?
I'm not certain what the symptoms of a bad ignitor are. I don't think they go bad very often. I believe they are self-grounding thru the mounting bolts.

 

Only the tachometer is acting up. All the other gauges work fine. The coils are brand new. I'm wondering if the crank sensors are going bad

 

I don't think CAS themselves go bad very often either. But the wires are shielded and those can get messed up. And, IIRC, cause some weird things.
I'll bump this up and maybe one of the big brains can chime in.

 

Wait. What exactly is the IIRC?

 

IIRC is internet slang for "If I recall correctly"....if I recall correctly :P

 

Sorry, yes. The CAS wires are shielded and I've heard of the shielding getting messed up and causing strange issues.

 

What kind of problem can the shields cause?