• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

mystery exhaust leak

1996XJ40

NAXJA Forum User
Location
usa
Have a troubling exhaust leak I can't seem to figure out...

Some history.. I have a 96 XJ that I replaced the typical cracked exhaust manifold on a while back.. at the time it seemed to still have a small leak that I couldn't find, well it doesn't get driven much so Ignored it.

Well fast forward to now; getting crap mpg, and the cat went. so after replacing the cat I decided to replace the manifold gasket again in hopes of finding/solving the leak. new felpro gasket, copper RTV, and cleaned and checked all surfaces excessively.
Also replaced the down-pipe donut, and checked for warping, new cracks etc.. no dice, still have leak somewhere.

Only seems noticeable when cold or when the engine is revved/under load..wtf am I missing? Should also mention there were no obvious signs of a leak on the old gasket either...the only thing i can figure is maybe the doorman manifold I put on there is leak prone...

TLDR version: replaced exhaust manifold gasket twice, still leaks.. wtf
 
Get a can of Seafoam. Start the engine and s-s-slowly pour about 1/2 can down the throttle body (very warm engine), then shut it off and wait about 15-30 minutes.

On restart, the engine exhaust will belch out white smoke like CRAZY. Any exhaust leak
will then be VERY noticeable. You may get a complaint from your neighbors about all
of the smoke, too, so you might want to do this away from where people live.

My guess (...and it's only a guess...) is that the exhaust manifold flange may be binding
against the intake manifold flange and is not seating fully or squarely.

I replaced the exhaust manifold on my own '96 XJ last spring and I had to grind a
few hundredths of an inch off of the edges of the exhaust flange where it butts up
against the intake manifold flange in order to get it to seat properly.
 
Get a can of Seafoam. Start the engine and s-s-slowly pour about 1/2 can down the throttle body (very warm engine), then shut it off and wait about 15-30 minutes.

On restart, the engine exhaust will belch out white smoke like CRAZY. Any exhaust leak
will then be VERY noticeable. You may get a complaint from your neighbors about all
of the smoke, too, so you might want to do this away from where people live.

My guess (...and it's only a guess...) is that the exhaust manifold flange may be binding
against the intake manifold flange and is not seating fully or squarely.

I replaced the exhaust manifold on my own '96 XJ last spring and I had to grind a
few hundredths of an inch off of the edges of the exhaust flange where it butts up
against the intake manifold flange in order to get it to seat properly.


I did run some seafoam through it this weekend, and while there was a little bit smoke in the engine bay (confirms there is a leak, however small) it wasn't enough to pinpoint the exact location. but sounds like it's at the rear of the engine..

I figured I would find obvious signs of a leak when I pulled the manifold the second time, but I didn't see anything obvious... I'm thinking you may be right about the manifold being the root cause though.. I checked the head with a straight edge and it was rail straight..

At this point I could just replace the manifold again, but I'd be throwing parts at it, and if it still leaks after a third tear down I think might just set the whole thing on fire.. :flamemad:
 
While you have the manifold off, check all the welds with a magnifying glass and a strong flashlight. Look for any spotty or poor welding during manufacture. This
includes looking inside the collector where the two downpipes meet and are welded
together.

When I did my exhaust manifold swap, I test-fitted how the intake and exhaust
manifolds would fit together by mounting them on a workbench.

I C-clamped the intake manifold, by the flange, onto a workbench. When I tried to fit up the exhaust manifold along with the intake manifold, I could clearly see where the exhaust manifold flange was binding against the intake manifold flange. In my case,
the edges of the #4 & #5 exhaust flanges were binding against the #4 & #5 intake
manifold flanges.

A couple of seconds with a 6" grinder fixed all. I took the metal off of the exhaust manifold flange edges. Some other owners decide to take the metal off of the intake
manifold.

After that, the exhaust manifold slid right in without any leaks or worries.
 
So it appears the flanges on the aftermarket manifold are a good bit taller than the intake. Didn't even notice this before, but I'm guessing it could the part of, if not the cause of the leak...


IMG_1097.jpg
 
yeah, I torqued to spec to start and re-torqued to slightly above spec after a full heat/cool down cycle. no change..

So I just pulled the whole thing apart again..and no sign of a leak on the gasket anywhere...double checked the exhaust manifold again and no cracks that I can see.. :confused:

anyone know of a good way to pressure test the manifold, make sure I'm not missing any tiny cracks?
 
With the engine running you can spray brake cleaner or starter fluid (carefully) at suspected leak location. You'll get a fluctuation in idle. I was able to narrow down a manifold leak that so far just needs a re torque every once in a while.
 
for the retorque, you need to do that while the engine is still warm, not after its cooled down. And are you hearing a tick while the engine is running, or smelling exhaust fumes, or?

I've always heard to wait till it's cold, I'll try it warm next time.

But it seems the manifold is ever so slightly warped, although I'm not sure it's enough to cause a leak.
I loose fit it on the head sans gasket and hand tighten the center bolt and one end, and there is a 1/8 gap on the other end..

 
Back together with a new gasket, and it's still leaking..

Pretty sure it's the manifold at this point. Aside from being slightly warped, it didn't seem to line up with the gasket holes perfectly either.

I think I'm going to just get a new manifold and hope for the best..
 
Well just to update, I replaced the manifold with a new Dorman unit (same as what was on it before) and no more leak , yay!

Still no idea where/how the old one was leaking..bad weld maybe.. :dunno:
 
Back
Top