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Old 12-10-2011, 02:55 AM   #1
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Question Found a rattle today. One of my upstream catalytic converters. (Detailed post)

So I pulled my cold (35°) Rubicon into my shop today after we closed and noticed a rattle coming from underneath that would slightly change pitch off idle. I got underneath with a stethoscope and found my rear upstream catalyst has broken loose inside its housing. The rattle does go away after the jeep gets to full operating temp. This puts me in a decision making position with my new favoritest thing in the world, the mean green jeep. It's a 2004 with 61,000 miles on it so, it is still covered under the federal emissions warranty of 8 years 80,000 miles. The only mods to the exhaust were when the 5.5" lift was installed, the muffler had to be replaced to clear the long arm lift system. I guess you could consider that as a cat back exhaust. Since I just found this today, I'll wait until Monday to speak with my local dealer about it. If they warranty it, I plan on getting the cats replaced asap. If they do not, I have routes to get factory converters wholesale although still not cheap. My question is this- if the dealer does not warranty my cats, is it worth installing a header and high flow cat? I am also EXTREMELY lucky to have a friend with his own full service shop that does excellent custom exhaust. Are the available headers for the 4.0 worth the money and effort of me doing a full new exhaust? I'd use 3" with a stainless braid flex section after the header, a magnaflow hi-flow converter, and some type of stainless muffler. What do you guys think? Is there any power or efficiency to be had with a full custom exhaust?

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Old 12-10-2011, 08:12 AM   #2
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So I pulled my cold (35°) Rubicon into my shop today after we closed and noticed a rattle coming from underneath that would slightly change pitch off idle. I got underneath with a stethoscope and found my rear upstream catalyst has broken loose inside its housing. The rattle does go away after the jeep gets to full operating temp. This puts me in a decision making position with my new favoritest thing in the world, the mean green jeep. It's a 2004 with 61,000 miles on it so, it is still covered under the federal emissions warranty of 8 years 80,000 miles. The only mods to the exhaust were when the 5.5" lift was installed, the muffler had to be replaced to clear the long arm lift system. I guess you could consider that as a cat back exhaust. Since I just found this today, I'll wait until Monday to speak with my local dealer about it. If they warranty it, I plan on getting the cats replaced asap. If they do not, I have routes to get factory converters wholesale although still not cheap. My question is this- if the dealer does not warranty my cats, is it worth installing a header and high flow cat? I am also EXTREMELY lucky to have a friend with his own full service shop that does excellent custom exhaust. Are the available headers for the 4.0 worth the money and effort of me doing a full new exhaust? I'd use 3" with a stainless braid flex section after the header, a magnaflow hi-flow converter, and some type of stainless muffler. What do you guys think? Is there any power or efficiency to be had with a full custom exhaust?
3" tubing is way too large for the 4.0L. 2.5" is more than plenty.

you may have noticed your jeep has 3 catalytic converters. two under the exhaust manifold and one at the skid. if you're going to replace one, you should replace all three. no sense in doing this three times as each cat slowly dies. replace the forward ones with factory spec cats, and there are plenty of lower cost aftermarket alternatives if the stealership doesnt cover it. the 3rd cat at the skid you can use a generic high-flow cat no problem, which should be cheaper than a factory cat.

and no, theres really no increase in power or efficiency to a full custom exhaust or headers unless you've done extensive engine mods with a piggyback tuner.

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Old 12-10-2011, 07:13 PM   #3
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Yes, i noticed the 2 front cats are one piece, then the big downstream cat.
Ive also been a custom car builder and mechanic for a long time, ive never heard someone say 3" piping is too large for a 4 liter engine! I regularly see 3" pipe from the factory on pretty much everything benz makes now. So a 5 liter V8 has a 3" pipe for EACH BANK. Thats a 3" for 2.5 liters of displacement.
I couldnt wait until monday to call the dealer, so i did today. They are comfortable replacing all 3 converters under the federal emissions warranty. But i think ive already got these new cats sold and will be doing my overkill 3" exhaust anyway.
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Old 12-10-2011, 08:05 PM   #4
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My 04 has 10k less than yours, LA lift, and a cat back system... VERY similar/same setup as you. The dealer replaced mine this past July, NO questions asked! But I did wait for it to throw the p0421 (I think) code. Have a copy of the fed warranty in hand if you take it to your dealer. Be prepared to leave it a day or two cause they usually do not keep that item in stock. They replaced all the cats! I did have to pay $200 just cause they had to mess with the RE skid... Which was fine with me, didn't want to mess with it myself anyway.

If you do it yourself... Rockauto has the Eastern Catalytic Converter system that is basically exactly the same as our OE setup. I would even be willing to bet Eastern makes the OE cats. Anyway last I checked you could buy the whole system for around or less than $330.

Just my .02... But I don't think you would gain anything in a jeep with high flow cats.
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Old 12-10-2011, 08:07 PM   #5
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Ive also been a custom car builder and mechanic for a long time
Unfortunately, that doesn't mean much. Logic goes a long way. Familiar with back pressure? Stick with 2.25-2.5" or else you'll be lacking in it.
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Old 12-10-2011, 10:12 PM   #6
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But i think ive already got these new cats sold and will be doing my overkill 3" exhaust anyway.
if you think you're going to gain anything by it you're sorely mistaken. it will only hurt your engines performance, and decrease the clearance you have around suspension parts. again, 2.5" is plenty. the 4.0L I-6 isn't a speed demon that operates in high RPM's, and its not a particularly efficient head design. In fact, the stock intake box, stock throttle body, intake manifold and measly 2.5"-->2.25" exhaust flows more air than the stock engine uses at WOT.
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Old 12-10-2011, 10:32 PM   #7
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you should see the size of your exhaust ports.. lawls.. they changed it up in 2000 (for the TJs) 99 for all other applications, for emissions BS.. I'm with everyone else saying 3" exhaust tubing will be BAD instead of good, especially for backpressure..

Heres a pre-00 head (this one is a 91-95 head)


Heres the exhaust ports on your head..

(photos are from a site showing the differences in heads)
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Old 12-11-2011, 07:48 PM   #8
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Exhaust back pressure myth - Mechanical Database

Lol nice! You guys wanna continue getting $hit fuel economy, go ahead. Most of you probably wont even read the article. If you do however read the article, it may open your mind a little to the possibilities of ANY engine. If my theories and beliefs where I
ncorrect, I wouldnt be in the automotive industry. I would like to see a newer 4.0 cylinder head though. I have a flow bench and am curious to see what its got going on. Or maybe you guys happen to know where i can find flow data on the newer head?
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Old 12-11-2011, 08:12 PM   #9
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everything after 00 (some 99's as explained before) had head casting 0331, (00-02 IIRC) were very prone to cracking, they revised the casting to help with the issue, but the head casting number stayed the same.

Here's an article i found with some #'s
Tech Specs
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Old 12-11-2011, 08:15 PM   #10
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what i'm saying is i wouldn't bother on your motor.. the head is crap.. they offset the poor performance of the exhaust ports by changing the cam profile and adding a better flowing intake manifold..
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Old 12-11-2011, 08:47 PM   #11
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I read the article you posted too.. you might be interested in this one as well..
Exhaust backpressure the myth - ThumperTalk
particularly concerning velocity of the exhaust.. this would be the better argument as to why you shouldn't bother throwing bigger pipes on..

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