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Old October 8th, 2014, 11:06 PM   #1
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Default Exhaust getting HOT.....why?

I'm putting my Jeep back together after a long build. It's a 98 TJ, 4.0L, AX-15, NP231 and has sat for 3.5 years without running. I got it running tonight and after the initial smoke and rough running, it cleared up pretty well. After having it idle for a while, then revving it some, and then back to idle this was going on:

The catalytic converter was getting HOT. I let it cool and started it up several more times. Even just letting it idle for 5 minutes this would happen.

For now i's a factory manifold and crossover pipe, flex pipe, then a brand new(though probably 6-7 years old) Magnaflow cat in the stock location, a small muffler, and a turndown. Minimal backpressure and pretty loud. Eventually the turndown will get replaced with a full tailpipe, but that's beside the point.



I know catalytic converters get hot, but I can't imagine it's normal for it to get THAT hot, especially with such a free flowing exhaust. What would cause this? Running lean? Maybe an injector stuck closed from sitting so long? I've got another whole fuel rail I can throw on if need be. Any other likely culprits to investigate? It's all stock 4.0L powertrain, computer, fuel pump, etc.

I need this thing running in tip top shape ASAP so any help is appreciated!
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Old October 9th, 2014, 03:05 AM   #2
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Usually it's combustion happening in the catalytic converter. Got an exhaust leak and having it run way too rich? Does it go away when it goes closed loop?

Is the monolith plugged?
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Old October 9th, 2014, 03:09 AM   #3
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Timing?
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Old October 9th, 2014, 04:32 AM   #4
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Remove cat problem solved
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Old October 9th, 2014, 06:06 AM   #5
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Quote:
Originally Posted by wrath View Post
Usually it's combustion happening in the catalytic converter. Got an exhaust leak and having it run way too rich? Does it go away when it goes closed loop?

Is the monolith plugged?
Nope, no leaks as far as I could tell. I doubt it's plugged, it's brand new. I didn't see it go away, but maybe I'll run it longer tonight and see if it does.

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Timing?
Timing has not been changed from last time it ran.

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Remove cat problem solved
If that's what it takes to fix it I'm not opposed, but I'd rather fix the problem than the symptom.
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Old October 9th, 2014, 06:25 AM   #6
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Step 1: remove cat
Step 2: install turbo
Step 3: profit








Step 4: replace blown engine
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Old October 9th, 2014, 07:03 AM   #7
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Cats overheat from working hard to remove unburnt hydrocarbons.
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Old October 9th, 2014, 07:22 AM   #8
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Cats overheat from working hard to remove unburnt hydrocarbons.
Makes sense. The more I think about it the more the lean scenario doesn't make sense; I would expect that to make the manifold glow, not the cat. So I see 1 of 3 scenarios being the case:
1: Cat is being overworked by all the gunk in the engine being burnt off.
2: Cat is being overworked by excess fuel being dumped in by a stuck injector
3: Secondary combustion is happening within the cat due to excess fuel being dumped in by a stuck injector.

No codes are being thrown so I doubt it's O2 sensor related, but I'm open to ideas/suggestions.
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Old October 9th, 2014, 07:36 AM   #9
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I would think you would blow a code for a stuck injector but don't know.
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Old October 9th, 2014, 07:37 AM   #10
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Cats overheat from working hard to remove unburnt hydrocarbons.
x2.

Do you have an OBD II monitor that you can watch the live data as the engine runs? You may not be so far out of bounds so to speak that you'll throw codes yet but you might be able to see if something is amiss.
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Old October 9th, 2014, 07:39 AM   #11
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I don't have one, I can pull codes but not monitor, but I may try and borrow one. I suspect that an injector is just partially stuck, otherwise it'd run worse. Swapping the fuel rail is easy so I'll probably start there and cross my fingers. Like I said, any other ideas are more than welcome as well.
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Old October 9th, 2014, 07:47 AM   #12
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Thats going to happen as it burns up fuel. Misfires also cause raw fuel into the exhaust. Usually would be a blinking CEL though.

Do you have a laser thermometer? Check the temp of each exhaust runner right out of the head.

Also, pull your plugs and look for evidence of incomplete combustion.
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Old October 9th, 2014, 09:53 AM   #13
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Was the gas changed since it was parked? If not then is is problably due to the junk fuel being burned in a not so nice fashion. My sister's Benz did the same thing after sitting for a number of years. Her goofball husband changed the oil every 6 months but never started the engine. Then he wonders why we call him Jerry Lewis, the nutty prof.
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Old October 9th, 2014, 09:56 AM   #14
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Yes, fresh gas in a new fuel cell
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Old October 9th, 2014, 11:12 AM   #15
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Is it possible you had a mouse build a nest in the exhaust ad it set along time. My brother had a mustang that's at why he was in the navy. When he came home it was way down on power. Muffler was packed full.
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Old October 9th, 2014, 12:18 PM   #16
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Possibly in the manifold but doubtful. The rest of the exhaust is all new.
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Old October 9th, 2014, 01:07 PM   #17
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Harbor freight sells super cheap temp guns. I picked one up with a coupon for I think $25. I don't think it's highly accurate, but I figure it's better than the drip water on it test.

You're welcome to borrow it for a few days if you want.
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Old October 9th, 2014, 01:19 PM   #18
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Thanks, I appreciate it. If the new fuel rail/injectors don't take care of it tonight then I may take you up on that.
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Old October 9th, 2014, 02:09 PM   #19
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Not much help, same setup, started up first time after 6 years and I had flames shooting out my custom made header dumps. Used a bunch of extra gas first 4 hours of running. Something wasn't right, but it fixed itself. I'm going to watch and see what your prob ends up being.
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Old October 9th, 2014, 02:13 PM   #20
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Hopefully the same thing happens here! I did throw some small flames and sparks when i was revving it last night, both of which make sense for running rich.

I just remembered I've got some performance Ford injectors out of my buddy's LS1 sitting in my toolbox. If they're the 4 hole model and the plugs match up I'm gonna throw those in instead of the spare stockers I have.
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