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  #1  
Old January 31st, 2017, 17:46
poomero poomero is offline
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93 4.0 exhaust running red hot

What would cause the exhaust on a 93 XJ 4.0 to run red hot. motor has 130K new exhaust manifold and no leaks anywhere.
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  #2  
Old January 31st, 2017, 18:03
Jeep Driver Jeep Driver is offline
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Re: 93 4.0 exhaust running red hot

Clogged cat.............other than that....IDK
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  #3  
Old January 31st, 2017, 19:32
Dragonlich1961 Dragonlich1961 is offline
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Re: 93 4.0 exhaust running red hot

like he said a clogged cat, or you are running stupidly pig rich. either way, don't drive it more than absolutely necessary, that cat is a fire waiting to happen, and it will probably take your truck with it if not careful.
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  #4  
Old January 31st, 2017, 21:41
poomero poomero is offline
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Re: 93 4.0 exhaust running red hot

Quote:
Originally Posted by Jeep Driver View Post
Clogged cat.............other than that....IDK
Quote:
Originally Posted by Dragonlich1961 View Post
like he said a clogged cat, or you are running stupidly pig rich. either way, don't drive it more than absolutely necessary, that cat is a fire waiting to happen, and it will probably take your truck with it if not careful.
Should have said the cat in new. Just put on today
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  #5  
Old January 31st, 2017, 22:50
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clydefrog clydefrog is offline
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Re: 93 4.0 exhaust running red hot

Where is it hot? The whole exhaust or just a section. More info would help, although it sounds like you have a clog somewhere. Why did it need a cat? Maybe there is another problem that took out the first cat (running rich).
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  #6  
Old February 1st, 2017, 05:09
jeepingben jeepingben is offline
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Re: 93 4.0 exhaust running red hot

A clogged injector caused this on my 95 YJ.
You say new manifold? Any leaks? If air gets in, the O2 sensor sees it as lean and the PCM will add fuel. The extra fuel will make the cat work harder.
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  #7  
Old February 1st, 2017, 10:36
poomero poomero is offline
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Re: 93 4.0 exhaust running red hot

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Originally Posted by clydefrog View Post
Where is it hot? The whole exhaust or just a section. More info would help, although it sounds like you have a clog somewhere. Why did it need a cat? Maybe there is another problem that took out the first cat (running rich).
Needed a CAT so the shop would work on it. I dont know if that was true or not but they put one on it. The CAT is running red nothing else.
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Old February 1st, 2017, 11:19
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ParadiseXJ ParadiseXJ is offline
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Re: 93 4.0 exhaust running red hot

The cat is likely the hottest spot on the whole vehicle, which it is supposed to be, to burn off excess fuel/emissions that got by the engine. On all the cats I've installed there is an arrow to indicate flow direction. Don't know if that makes a difference, but may be worth looking under there.

If it didn't have a cat, the shop either can't or won't work on it to avoid liability (or they may just say that) as it's illegal to remove a cat if it came with one from the factory (federal law) and/or do work on an illegal vehicle. Just sayin'
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  #9  
Old February 1st, 2017, 18:07
Jeep Driver Jeep Driver is offline
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Re: 93 4.0 exhaust running red hot

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Originally Posted by poomero View Post
Needed a CAT so the shop would work on it. I dont know if that was true or not but they put one on it. The CAT is running red nothing else.
Get that POS off there........if what you are saying is true? red hot? Get rid of it.

Find a different shop.
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  #10  
Old February 1st, 2017, 19:05
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ehall ehall is offline
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Re: 93 4.0 exhaust running red hot

exhaust leak will cause air to get sucked in, making o2 sensor think the engine is running lean, resulting in the computer dumping fuel to balance it out. The excess fuel burns off in the exhaust, making it super hot
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Old February 1st, 2017, 19:19
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four_shot four_shot is offline
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Re: 93 4.0 exhaust running red hot

If it's just the cat getting red hot, I'd lean towards the fuel system being extremely rich. If its the manifold, then I'd suspect a lean condition,(vacuum leak, clogged injector, bad o2.)

1)Check tune up first, excessive spark gaps can ship unburnt fuel down stream.
2)Check O2 sensor, they can skew lean over time and cause excess fuel delivery
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  #12  
Old February 1st, 2017, 19:49
poomero poomero is offline
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Re: 93 4.0 exhaust running red hot

Figured it out. Intake leak caused it to run super rich. All is good now
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  #13  
Old February 1st, 2017, 19:51
Jeep Driver Jeep Driver is offline
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Re: 93 4.0 exhaust running red hot

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Figured it out. Intake leak caused it to run super rich. All is good now
Lean = hot. Rich = cool.


How is it you had such a leak and not figured that out immediately?
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  #14  
Old February 1st, 2017, 20:36
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ehall ehall is offline
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Re: 93 4.0 exhaust running red hot

Lean will make the engine hot, rich will make the exhaust hot
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  #15  
Old February 1st, 2017, 22:57
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Re: 93 4.0 exhaust running red hot

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Originally Posted by poomero View Post
Figured it out. Intake leak caused it to run super rich. All is good now
Good job!

Back in the '90s, I used a company truck that the carburetor started dumping excessive fuel down the intake. I did not notice any problem until one night I saw an orange glow under the truck. That excess fuel was burning off in the muffler.
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