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Discussion Starter #1
Hi, let me start by saying that I am not new to automotive work (I have been working on 2wd trucks and cars for years). I am however new to 4wd, and jeeps.

I just bought a 94 YJ soft top. Overall it seems to be decent shape. It runs and drives well, but I failed the emissions test here in colorado. I easily passed CO and Hydrocarbons, but I failed NOX. My guess is the cat is toast. There are some major exhaust leaks (the whole system is pretty rotten) that need to be dealt with so I figure I may as well at least just change every thing from the down pipe (from the manifold back.

I am looking at this for a cat back:

Summit Racing SUM-640003 - Summit Racing® Cat-Back Exhaust Systems - Overview - SummitRacing.com

This for a down pipe:

Amazon.com: Walker Exhaust 53479 Front Pipe: Automotive

And this for a cat:

Catco Catalytic Converters 4684 - Catco Catalytic Converters Direct Fit Catalytic Converters - Overview - SummitRacing.com

Will all of this bolt right up and clamp together?

The only problem I see is that the cat back is 2.5in, and the cat is 2.25 in. Since the down-pipe is a stock replacement is a stock replacement I suppose it is 2.25 in outlet?

Part of me thinks I may end up replacing the exhaust manifold anyways, and if that is the case, it may make sense to just get a header (probably pacesetter) and go 2.5 in from the header all the way back.

Anybody done this? What universal cat with bigger inlets and outlet did you get? What about gaskets and flanges? Will all of this just slip fit and clamp together?

I know the 4.0L jeeps have a doughnut gasket between the manifold and the down pipe, will this help on a 2.5L?

On paper it doesn't look much more expensive to go this route, but if it will create headaches during the install I don't want to mess with it. Basically, I just want an exhaust system that doesn't leak and doesn't cost too much. Any power gained would be a welcome bonus given the short comings of the 2.5L engine.

Sorry for all of the questions, but I need some opinions fast so I can start ordering parts and make this thing less annoying to hear.

Thanks!
 

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The Walker downpipe doesn't list pipe diameter or fitment and I don't know what engine you have so I couldn't say. That cat is for the 2.5L with 2-1/4" piping. The cat-back is for the 4.0L with 2-1/2" pipe. They won't mate up. But with the right sizing I think it should all be slip fit and clamp together. Sometimes even slip fit pipes don't fit the way they should and take some persuasion. A pipe spreader and shrinker come in handy.

Just be aware that the cat might not be your problem for NOX. Possibly a contributing factor, but the cat also kills HCs. If that's testing good I would look at the EGR system first. NOX comes from high combustion temps first and foremost.

Course if your exhaust is shot I'd replace it anyway. Or you could move to NY, we stopped dyno testing and sniffing pre-OBDII vehicles. Now we do a "low enhanced" emissions inspection which basically means a visual check to see that all emissions devices are there and untampered with. Lol, of course everything's always there, but nobody is going to spend the time to randomly test anything for functionality for a $20 inspection fee. It's all politics.
 

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I had a catback on my 2.5. I got tired of it after a month and went back to a totally stock exhaust. I love having a quiet Jeep. As far as making all the exhaust components mate up just get the inside or outside diameters of the pieces you are trying to put together and match them up, and then just clamp them together. You can grab small pieces at any auto store to make it all fit together, just get the correct sizes.
 

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Discussion Starter #4
I hear you on the sound thing. I don't want this thing to sound like a honda with a fart can. The sound issue is probably the main thing keeping me from doing a header.

Correct me if I am wrong, but isn't the cat back exhaust setup on the 4.0 and 2.5 identical?

As far as the EGR goes, I swear I have seen one underneath the jeep low near the passenger side front of the engine compartment. But there is nothing about it in the service manual I have, and I have read that the is a stretch of YJ years that don't have one?

At first I was thinking I had a lean condition since HC came in low, but I am throwing no codes. It also seem to get a lean pop when I down shift, but that may just be a result of the exhaust leak.
 

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I have a thrush muffler or some generic muffler unsure but it is definitely not stock from the PO of my Jeep, has a throaty sound but definitely not rice burner, sounds like a 4 banger diesel tractor engine when it idles. Get compliments on it all the time. So before you go all out on high performance exhaust sometimes cheap sounds better and has the same gains. Oh yeah the 2.5 in mine will do 70+ on the highway no problem. I could post a video for you if you'd like me to tomorrow or something.
 

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Oh you're right there's no EGR, I never even noticed. I would say retard ignition timing a little to richen it up slightly, at the expense of CO and HCs to get a fair balance but Alldata advises that base timing is non adjustable. So I'd say you have a slightly lean mix. Either track the cause of that, or maybe a fresh cat will bring you enough into spec to pass.
 

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Discussion Starter #8
I was figuring I was a little lean. I plan on changing out the O2 (looks very rusty as the rest of the exhaust does) when I do the whole system. I also plan on testing my fuel pressure, and changing my fuel filter.

I sure wish I believed that fuel injector cleaner actually did something, because I would imagine that my injectors are really in need of cleaning.
 

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94YJ Turd said:
As far as the EGR goes, I swear I have seen one underneath the jeep low near the passenger side front of the engine compartment. But there is nothing about it in the service manual I have, and I have read that the is a stretch of YJ years that don't have one?
That is your charcoal canister for the evaporative emissions control system. It absorbs fuel vapor from the tank while parked and purges to the intake while driving.
 

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Discussion Starter #10
That is your charcoal canister for the evaporative emissions control system. It absorbs fuel vapor from the tank while parked and purges to the intake while driving.
isnt the charcoal canister located near the brake master cylinder? At least what looks like every charcoal canister i have ever seen seems to be under there.
 

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My down pipe was good but I went on autoanything.com and found a high flow magnaflow cat for $104 and a flowmaster 44 series muffles for $75. Then I went to napa and bought the tail pipe for $35. Oh and I got a catalytic converter flange gasket for $3 at autozone.

Not sure if you're looking for sound, but this setup sounds great on my 4.0 and I'm sure it'd sound good on a 2.5. If you plan on putting a high flow intake on it some day, I suggest putting a high flow exhaust on it too like what I did on mine. If not, the stock setup will do you just fine :)
 

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Discussion Starter #13
Move to Alabama and do away with that CAT all together....no inspections here:whistling:
:dance:
No offense, but the emissions test is a small price to pay to live in the mountains, and not have to live through the humidity you got down there. I used to live in southern Indiana, and that almost killed me. I would imagine Alabama is worse.

By the way, I ended up getting the dynomax super turbo catback, a walker high flow cat, walker down pipe, and a new doorman exhaust manifold. It bolted right up and no more exhaust leak! But I think I found at least a contributor to the failed NOX test. A LEAKY INJECTOR! So now I gotta change those too!. Hopefully, after that, I can fix the issues in my steering column, buff the SOB, and then just drive.
 

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Discussion Starter #16
It is my understanding that I cant manually adjust the timing:confused: I would have pulled out the timing light a while ago if this weren't the case.
 

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No I don't think you can. Looks like the distributor can only be installed one way and can't be rotated. Alldata shows it as non-adjustable so I figure that's correct.
 

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Timing has no affect on mixture... Advance or retard it all you can, all you are affecting is the firing point in relation to piston position.

The only way to change the timing on our engines is to slot the crank sensor mounting holes, and thats a crapshoot, you might only get 3*
 

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for my 93 Wrangler with 4.0 I went with new magnaflow direct fit cat (found on ebay for 90 or so) and for the muffler and tail pipe i went with an 18" magnaflow muffler with tail pipe. The muffler/tail pipe came from company called performance curve. Tail pipe MANDREL bent to stock specs. Muffler/tail pipe shipped to my door within 2 days was like 135. Online performance and OE car parts site Performance-Curve.com - Home they have several muffler options, i wanted something that would flow well and sound deeper under throttle. I am very happy with my end results. Local shop wanted 400-500 for one installed. This one cost me around 250 and couple hours work. You'll need, i believe 1 extra clamp unless you're going to weld it up. I have not replace my manifold or my down pipe as of yet.
 
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