I have a 98 honda civic ex w/ 161000 miles. Been driving the car for about 2 weeks now. I have no check engine light. But when I put it in park or neutral the tachometer jumps up and down up to 3000rpm. Also when I'm driving it seems everything is always revving hard/loud/rough. When I come to a stop in drive the car wants to keep going and the tac is at 1500-1700. Figured it might be a thermostat issue from what I've read on here. But my operating temp is right on. And I have heat so maybe not the thermostat. Any help would be appreciated. Not an expert at all. Thank you!

 

Have you ever replaced the PCV valve? That can cause the symptoms you describe. Good luck.
Duane