My father and I have been working on an 02 cherokee Laredo. The original task was a head gasket job due to overheating and exhaust in the coolant. That all went smoothly and the engine (4.0 I6) is now all back together. We fired it up and it starts instantly, however, the engine now wants to rev up almost like someone is flooring it until we shut it off. Any ideas? We thought it may be a vacuum leak in the manifold so we took it all back off, retorqued to spec and put it back on, however it is still doing it.

 

The intake manifold is easy to crack. Everything went together snoothly?

Triple check the manifold(intake) it's easy to get the lower Dowell pin out of whack and it can offset the manifold or crack it.

 

I had heard about that and we made sure everything lined up and was not cracked. Also had not heard of it going to such high rpms when that happened. Will check it again. Any other ideas?

 

We're gonna try that, suddenly got busy with work and stuff though so it's hard to find time

 

Stuck throttle plate?
Bad breathers / pcv system
Bad oil cap
Valve cover not sit well
Brake booster or it's check valve have up

 

Ok so quick update with a new problem. We got the jeep to run perfectly and idle. However, the fan will no longer come on when it should. Going to replace the PWM sensor just to be sure. Fuse is good. Any known problems that could affect it?

 

If you're talking about the fan relay, it's a good start. Have fun with that!