racing engine after manifold install

my engine races at or near red line after installing a new OEM exh manifold. i have reconnected everything properly and there is not any issue with the throttle linkage or TPS, or any of the sensors. i did add a bored out 62mm TB at the same time while i had it apart.

i found this thread when i searched, but the guy never resolved or posted a resolution to, his problem. but mine sounds like the exact same issue. http://www.naxja.org/forum/showthread.php?t=96274&highlight=engine+racing+header

i have tried retorqueing every bolt, reseating every connection, everything short of taking the manifolds back off and on again, which i dont think would help at this point.


any help would be appreciated.

It might be a really bad vacuum leak. Maybe your intake/exhaust gasket blew.

But I can't imagine the engine revving to redline with the throttle closed (it would be way too restrictive to allow the engine to make enough horsepower to rev that high), even with vacuum leaks.

Id check your throttle cables. Its a good possibility that the throttle isn't closing all the way with your foot off the gas.

i have reconnected everything properly and there is not any issue with the throttle linkage or TPS, or any of the sensors. i did add a bored out 62mm TB at the same time while i had it apart.

Click to expand...

Not trying to be wise ass or anything but there has to be.

Just a thought .. would this be a symptom if the 62mm TB did not have a properly milled butterfly installed?

i have been back under there took a look and there is a very small gap where the intake manifold sits on the gasket. I was able to slide a business card in between the manifold and the gasket at 4 5 and 6, while the exhaust header is sitting flush all around.

The flanges on the header i got from autozone are about 1/16 thicker than the intake manifold flanges and its causing a gap all the way around the intake. since they share mounting bolts.

i cant think of a good way to fix this except to pull it all out and get a different exhaust manifold. i thought of shimming the bolts on the one side in some way but the bolts are near impossible to get at normally, so this would probably not work.

what if i took off just the intake manifold and applied a high temp rtv all around would that work or??

lilredwagn said:

Just a thought .. would this be a symptom if the 62mm TB did not have a properly milled butterfly installed?

Click to expand...

it's not leaking there. its doing the same thing now with the old TB back on it.

RTV is not the answer for a gap of that nature.

here you can see how the mounting flanges are different on the intake/exhaust...



the gap. and "the shim" that didn't help.



new TB...

im probably going to just pull the exhaust manifold and return it to autozone. get the banks or borla and be done. I can't figure out why the OEM replacement would be that far off.

I'm really regretting going with the OEM, but thought I could ride today if Ijust went ahead with it *since it was in stock locally*.

Oh well, live and learn.

Thats something I've never seen before and something to remember as a possiblity for future reference. Thats a stock OEM jeep exhaust manifold ?

If it were me, I'd measure the flange thickness on the exh and int for several (if not all) cyl to see if it's consistent. Then take both back to AZ in case they don't believe it. Have them mic up the new one if you choose to replace it with another from them.

Well, this just happen to a friend and I with a Dorman exhuast manifold for an 89 purchased at NAPA (stock style). In our case, I don't think its a flange issue (although thicknesses are about 1/32" difference). We had to "persuade" the two manifolds together with a prybar to get 'em to fit. Bad move, cause now it leaks (only about 3 minutes on engine). Guess we'll be pulling to figure out where interference comes from. Wish we'd done that B4 we put in the engine. I suspect one of th flanges on the new manifld is a tad oversized. Good thing we have a spare block to test-fit.

I would pull the bolts and use a grinder to make the exhast flange level with the intake, tighten and see if you can get the gap closed. U sure nothing is stuck underneath?

OK, I've had some time to delve into the Dorman manifold issue a bit, now that I've blown my second manifold gasket.

After removing the header and taking to my NAPA dealer, we threw a straight edge across the manifold and discovered it warped by about 1/16 inch, with the high spot in the middle. Even worse, the "ears" on the end pipes were warped inward such that when torque was applied to the outside bolts, the inner bolts on #1 and #6 flanges wewre lifting and preventing contact with the gasket. It was much the same problem as shown in the photos here: http://www.naxja.org/forum/showthread.php?t=98813 (post #8). In my case, the gasket was creeping from the exhaust back pressure and tearing at the intake manifold. My 1st gasket lasted about 200 miles until I put on a legal exhaust (work in progress). Then, I'd get one or two warm ups and blow a gasket.

The NAPA dealer ws great and took it back no questions asked (Kudos to Reibe's in Grass Valley). One advantage of customer loyalty. Here's where it gets intersting: We looked at two more manifolds with a straight edge right from the box and both of them were warped out of spec as well (spec is .017") in completely different ways. They ordered two more in from their other stores and we'll pick the best of 5.

Looks to me like Dorman has some QC issues, so if you have a recently-purchased Dorman header, and are experiencing vacuum leak symtoms, you may want to check your manifold for warping, particularly if you see the tell-tale offset intake/maniflod flanges.

I've not had to get into this yet [knock on wood] but I really never liked the jeep design on either the 2.5 or the 4.0. My 97TJ with the 2.5 has a leak, probably from a cracked header so that may become an issue this spring, we will see.
My 98's exhaust header was cracked, found out about it when the engine came out for the jasper replacement. Drill two holes at either end of the crack and then welded it, hopefully those two holes will take some of the stress off that area. So far it's nice and quiet.