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exhaust leak on exhaust manifold

latenights

NAXJA Forum User
Just a quick question after trying to search for it. theres a pretty big crack in the exhaust manifold where it all collects at the end which is pretty common from what i've heard. thats not my problem, i already have new ones waiting to go in when the weather gets warmer. But i am gettin 11-12 MPG city on a stock height 96 XJ 4.0 with 30" BFG A/T. Im just wondering if that could be causing an o2 sensor to be off and making my gas mileage suck. I'm not concerned about this, im just wondering.
 
Yep. That can cause it!
 
Ok, thanks. I figured that's what it was but since most people on here are a lot smarted then I am I'd figure I'd ask. I got some new stainless ones with the expansion chambers for $100 at a swap meet so hopefully they'll be going in this weekend.
 
Check your motor mounts and the transmission mount- that is supposed to be one of the contributing factors to cracked manifolds- even my APN header has a little warning on the install sheet that worn mounts likely caused the stock piece to fail, and would wreck the header as well.
 
Engine/tranny move around too much on worn mounts, the collector starts cracking as it can't move with the engine, exhaust escapes before the O2 sensor through the cracks, sensor sees a lean mixture, tells the PCM to dump more fuel in, tanking your mileage. That seems to be the prevailing theory- one thing leads to another. Not sure on the bellows, but if you can fix the root cause, it should be fine, even if they don't necessarily help.
 
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