Intermittent High RPMs

[surfobx031]

I started my ‘06 X 4.0L last night and it immediately revved to 3000 RPM. Tried twice more and same issue. I did notice it didn't start right up the first try, kinda hiccuped then turned over. After cussing the motor for 5 minutes it started fine and I drove to store and back without a problem. Also started fine this morning. Had similar high rev issue last year & cleaned throttle body, IAC (which is only a few years old), and replaced TPS. The problem persisted until I finally found a missing bolt on top of intake manifold. But that was a consistent problem each time I attempted a fix, it never went away until I found missing bolt.

The air temp yesterday was mid 90’s, only drove maybe 6 miles or so around noon. When the high rev issue presented was just after a thunderstorm and 10 degree drop in air temp, approx. 6:30PM.

Any ideas?? I didn't have a chance to get at it last night and start poking around, so just wanted to throw this out there to see if anyone's experienced the same. If it's a vacuum leak again, I would suspect it would be a vaccum leak every time I started the motor, not just 3 times and all's well after.

 

[Jerry Bransford]

Missing bolt? Did you or someone else remove/replace the intake manifold at some point? If you did and you're still getting intermittent high rpms after replacing the missing bolt, my bet is you have a vacuum leak where the intake manifold seats to the head.

It's easy to create a situation where it's prone to leak if time and care isn't extended to the intake manifold seating process. If there's a leak there, the only fix is to remove the exhaust header and intake manifold and reseat it very carefully using the positioning studs to make sure it's in the correct position.