Hello All, I just replaced the exhaust manifold on my 99 xj and now it redlines at idle. what have I done wrong?

 

Vacuum leak. The intake manifold isn't seated against the block properly. Either you left some bolts loose or the new exhaust manifold isn't letting the intake manifold seat properly. Inspect for interference between the two.

 

Dang it, was afraid of that. I was kind of hoping I just left something off. I have been reading about port matching gaskets could this be an issue for me?

 

Nope, port matching gaskets won't have anything to do with it. That's a "performance" mod you can do. Probably not worth the time though.

It seems to be pretty common for people to have huge air leaks when they replace the exhaust manifold if you aren't careful. I had to shave some high spots on the intake manifold casting when I installed a Dorman replacement manifold or else it would not seat flush.

 

My buddy dropped the intake, could it be a sensor that is causing this?

 

NO!!!!!!!!!!!

The intake manifold is not seated over the locating dowels.