so here is my problem, i have a 2001 ranger 4.0 4x4 and my check engine like came on. well what happens is when my engine is idling, it revs to 1000 rpms then back to 500 rpm and it keeps repeating that, it doesnt do it when i drive just when im stopped, what is wrong?
thanks
devin

 

Are there any recent mods you just did?
A couple of things that come to mind.
1. Make sure the egr tube is connected. It's a black tube that goes from the air intake tube to the valve cover. It could be this or another vacuum line pulled. There's one that goes to the brake booster also.
2. Throttle position sensor. You can hook a voltmeter up to the wires and open and close the throttle body. The meter should move and stay within a certain limit. A manual can give more detailed specifics.

 

IAC (idle air controller) is causing it.

Mine did this but it was worse it went from like 200 rpm to 2000 and back agian.

I would try cleaning it or getting it replaced.

 

Sounds like the IAC to me. I have cleaned mine before with non-chlorinated brake cleaner...no issues so far