Tach bouncing
 

The tachometer bounces like crazy on my '02 Mountaineer, or it just sits on zero and doesn't work until the truck is fully warmed up(heat also takes forever to warm up) and I've been driving for awhile, then the tach works fine.

What causes this?

Thanks, Bob

Well 1st, the heat and tach are not related. When you see the tach bounce, can you hear erratic engine idle keeping pace with the tach? And how big are the fluctuations in the these bounces?

Yes I realize the heat and tach are not related, just seems weird that the tach works fine when the truck is all warmed up. Maybe the tach sensor is mounted right on the engine and it isn't working properly til it warms up?
I really haven't had any time to look into it yet, it's been a rough winter in Ohio.

The engine RPM's do not match the erratic tach. The tach will either just sit on zero, or it will bounce from zero to 3000 or more while the engine is idling perfectly smooth.