This has been happening a lot lately, after a usual drive to work or wherever, I put the Jeep in park and the engine revs up to around 1500 RPM's. If I put it back in drive, the RPM's go back to normal. I usually just turn it off with the engine revving at 1500.

I have a crack in my header, could this somehow be the cause? Or could it be something else?

Thanks for the help

 

I'd definitely attend to that crack, but high idle is usually linked to a IAC on it's way to IAC heaven.

 

your header crack will just leak exhaust. you need more air in your engine to send more out, so it is either bad intake/exhaust gasket, your TPS (throttle position sensor) or a sticking throttle linkage/throttle body. start your engine and try to force your throttle body closed, if it budges a little, then you have a problem with your throttle spring or your linkage. i know i bent my linkage once and it would stay at 2300 rpm till i went into gear. good luck

 

does that crack really effect things?

 

yes the crack will cause exhaust vapors and loss of torque/horsepower. also it could crack more and you will get ****ty gas mileage. but for the most part unless you are missing pieces of your manifold, it shouldn't effect the way your engine runs that dramatically. it will also cause you to fail smog if you have to do it.

 

On the same note it's normal for your engine to rev differently in Park as apposed to being in drive, this is the case with all cars. I haven't been in a single car where the RPM's don't move a little when you pop it into Park....