Nasty leak at the manifold (I think) after installing new cats and O2 sensors

[fizzlebottom]

I installed the Flowmaster cats + all new NTK O2 sensors.. Bought all new fasteners (extruded u-nuts + SS bolts) for the manifold-to-pipe flange connections, and SS nuts & bolts for the downstream cat-to-muffler flange.

Got it all buttoned up with appropriate torque settings:

• 21ish ft.lbs. for the manifold-to-precat flange bolts.
• 25ish ft.lbs. for the downstream cat-to-muffler bolts.

Went to start it up. No O2 codes, but it is super farty. There's clearly an exhaust leak somewhere. It sounds like it is coming from the motor, so I'm wondering what the possibilities are.

• Can the flanged connection be misaligned or uneven? Everything went together pretty smoothly?
• The new extruded U-nuts did seem a bit tight at the end when I first test-fitted them to the bolts. Could they be causing false torque readings?
• Is my brain tricking me in thinking that the leak is at the manifold when it could really be the muffler flange?

I really did this by the book, minus replacing the super rusted pressed-in muffler flange bolts with SS nuts & bolts.

Any insight will help. Worst case scenario, I'll be taking the TJ to a local muffler shop to replace the ~15yo muffler, and could have them find & fix the issue. But I wouldn't mind being able to drive it there without sounding like an angry can of bees.

 

[CharlesHS]

I would do an inspection of the exhaust system with the vehicle running a give it a listen.
Could be a misaligned flange, missing gasket or loose bracket.
Usually you can look for carbon tracking, but since you did this recently you may not be able to see anything yet.
Just recheck the pipe connection points that you recently connected.

 

[AjRagno]

Your toque specs on the flange bolts are correct. You can loosen the bolts realign everything and then make sure each side is tightened evenly. If you still have a leak, I’d say it’s a manufacturing issue. Your ‘05 does not use a flange gasket and as far as I can tell there is not one available

I have Eastern cats on my LJ and they look identical to Flowmaster. I’m not impressed and looking at alternatives. Mopar and Magnaflow cost around $1300, vs $300-$400 for most others. I think ultimately you get what you pay.

 

[KingCarGuyZ]

this happened to me, see My Build Thread

for me it wound up being a bum cat, if the angles are slightly off, the CAT won't meet with the header and it will sound horrible, you can check this with an angle gauge on the ground, and no there shouldn't be gaskets. sorry for the hasty post but I'm in a rush rn, ill try and explain more later if no-one else does.

 

[Chris]

I would do an inspection of the exhaust system with the vehicle running a give it a listen.
Could be a misaligned flange, missing gasket or loose bracket.
Usually you can look for carbon tracking, but since you did this recently you may not be able to see anything yet.
Just recheck the pipe connection points that you recently connected.

Yep, this is where you start.

Start the vehicle, let it run, get underneath it, and look / listen for the leak and where it's coming from. You probably think you buttoned it up 100%, but you're going to find someplace where it's leaking. It shouldn't be that difficult either.

 

[SvtLdr]

I would start with a leak test on the exhaust system. Grab your shop vac, a bottle of soapy water and go to town. If you have any leaks, they'll show up immediately. I was able to find some cracks in my manifold, a bad donut gasket, and a leaky muffler inlet in about 5 minutes.

 

[SvtLdr]

What's nice about this test is you don't need the vehicle to be running while you're underneath...

 

[MutantNinjaTJ]

Stainless steel hardware is problematic in that it will gall the threads under tension making removal, re-use and proper bolt tension hard to accomplish. While it sounds like a good idea, stainless hardware is a pain in the ass to use correctly. sometimes anti-seize helps but I would just go with OME hardware. Just sayin!

 

[fizzlebottom]

this happened to me, see My Build Thread

for me it wound up being a bum cat, if the angles are slightly off, the CAT won't meet with the header and it will sound horrible, you can check this with an angle gauge on the ground, and no there shouldn't be gaskets. sorry for the hasty post but I'm in a rush rn, ill try and explain more later if no-one else does.

I looked at your build thread. The Magnaflow cats definitely do look way nicer than these Flowmasters. The two big differences I can see are how much room you have above the cats to fit a socket, and that yours have nice tidy heat shields vs my bulky spacious heat shields. I've noticed that the 2004-2006 cats are different than 2000-2003 models, so unfortunately I couldn't get the same as you for the $372 on Magnaflow's site. Mine would cost upwards of $500.

Think I'm gonna get this truck over to the local muffler shop and see if they can get me up and running.

 

[fizzlebottom]

I didn't get a chance to try again today, but I did take one of the original rusted bolts and a manifold nut to check something. It seems that the extruded nuts are tapered on the top of the threaded hole and the original bolts have a tapered, unthreaded tip. They do not behave like typical nuts & bolts; they only fasten together according to depth of the bolt in the nut, and stop pretty much as soon as the bolt tip is flush with the end of the nut.

The problem is that in my infinite wisdom, I bought slightly longer SS bolts to go along with the brand new extruded nuts. I think what is happening is that when I torqued them, the torque wrench registered the tightness of the flat bolt end against the tapered nut end. They never even get the chance to squeeze anything inbetween.

I'm going to head back to the parts counter tomorrow and buy 4 new (pricey) Mopar bolts (p/n 6505754AA). Gonna try this madness again. If it doesn't pan out, the exhaust shop that I'm going to have replace the muffler already has my vote of confidence that they can help solve the problem.

 

[fizzlebottom]

I know you were all eagerly waiting for an update. I fixed the problem(s).

• As I mentioned, the SS bolts I bought as replacements were not appropriate for the extruded U-nuts. I bought OEM hardware at the parts counter.
• The flared pipe ends were not lining up easily with the manifold at all. There was a lot of fiddling to get things meshing properly.

I took it all apart, patiently aligned and progressively tightened the flange bolts til they were properly torqued. Got everything else together, started it up, and no noticeable leaks!

I'm going to have the ageing muffler replaced at the shop this weekend.

Thanks for all the insight folks. I think if I were to do this again I'd have grabbed the Magnaflow cats instead of Flowmaster, just because of how much cleaner they look and how much lower profile the heat shields are. But if this setup lasts me 5 years then I'll be happy.