What would cause a 91 Honda CRX HF to rev up and down while sitting still and no one pushing on gas pedal?

Answer 1

Most cars will rev up and down a little while still warming up. it is a vaccum that causes it to do this. it helps keep the engine last longer. if it still does it when the car is warmed up.....well buddy you have a problem. most likely it is your throttle position sensor. that has been the most common issue i have seen and heard of when a car is sitting at idle and revs up and down for no apparent reason. its a simple parts change and i'd be willing to bet it's the problem. If the car goes from about 800rpm to about 2500, then immediately drops back down to 800, then revs back up to around 2500 over and over again, this is called Idle Hunting. It's caused by one of two things: your throttle position sensor's screwed up, or you've got an air leak somewhere between the throttle butterfly and the head. When you've got a leak, your O2 sensor will see there's too much air in the fuel mix and add more gas to fix it; eventually your black box will think, "throttle position is x, RPM is y, RPM y can't be achieved with throttle position x" and cuts off the gas until the RPM reaches a more believable reading. The root cause is normally, contrary to popular belief, not the TPS. Oh, don't get me wrong: if you put a new TPS in the car it WILL fix the problem...but the reason it really fixes it, is that while you're installing the new TPS you will also tighten all of the screws in the intake, and that actually fixes the problem. I had an idle hunter with a leak around a fuel injector; changing all the fuel injector seals solved the problem. So...start by tightening anything that even looks intake-related. Be sure to do the bottom clamp on the U-shaped tube that carries the air from the airbox to the throttle body. About half the time this is the culprit because it's hard to get to so no one ever looks at it...but it loosens up just like any other screw on the car.