redlined when turned on after header install

1999 4.0 aw4 np231 - Searched but couldn't find this exact instance

I installed a new OEM style header today and when I turned the jeep on for the first time it immediately revved up heading towards redline. I shut it off instantly and inspected for anything disconnected. Everything looked fine so I restarted and it began revving again, but I shut it down before it could get above 3k.

From what I've read, high idle can be attributed to a vacuum leak. My issue was as if the throttle was 100% open.

Anyone?:helpme:

Yep, that's almost certainly a vacuum leak. Probably the intake manifold gasket. Check all the bolts and look really closely at the seam between the intake manifold & the head. Could be throttle body gasket, I suppose, though you may not have removed it from the intake.
Youtube / Google how to make a smoke machine at home, then plumb it in to the intake manifold. You'll see smoke pumping out of the problem.

#1 cause of a massive vac leak after header or intake install is the intake not seated properly on the dowel pins sticking out of the head. there are two, one front and one rear.

take an awl or a thin tipped flathead screwdriver and make sure this didn't happen:

Nice illustration, Grimm!!

And, OP, when the manifold gets hung up like that, a guy will assume he's got all the bolts tight, which he does, but everything is not seated.

OP here, checked the front dowel pin and it was of several mm. I loosened all of the bolts and was able to jiggle the intake manifold around and feel where it connected to the dowels. Put everything back together and she fired up and idled fine.

Thanks again guys, easy thing to over look.

For future viewers, you cannot see the rear dowel pin. Once you have all of the bolts in the head loosely, you can feel where the intake manifold needs to be to line up on the pins. Continually check its alignment as you tighten the bolts.

Those dowels are a booger. I put a new head and forgot to remove my old ones before I turned in the core. It was a PITA getting the right sized ones in there correctly. I also read some people use wood ones. Not sure I'd do that. Glad you got it worked out.

excellent, glad you got it worked out. the little stuff is the easiest to overlook, especially when you can barely get your hands in there to tighten it all up as it is.

:thumbup: