Got a 93 prelude. Remote start works fine and Tach is fine when first drive. Have to drive interstate partly to work. When I get off, the first stoplight I stop at my Tach is reving up almost 1000 rpms up and down. Kinda feel a pull when I slow down to take a turn as well.
What is causing this? Like I said, it only happens when its really cold outside. It was -1 degrees this morning.
Thank you for any info.

 

You could have a dirty Idle Air Control valve that freezes on the highway and when you slow down enough to engage the idle circuit, the frozen valve has trouble moving.

The throttle body can also ice up and prevent the throttle plate from properly closing and causing high idle.

 

The tach itself is malfunctioning? Or your engine is revving?

The throttle body has it's own coolant line so this doesn't happen.

 

The engine is revving up as well.

 

It can still happen under the right conditions. It's not because you have never seen it that it is not possible.