Leaky exhaust system weld

[JHZR2]

Not too long ago, I got mixed reviews on a repair from a local muffler shop, for my 2008 ML320:

Thread 'Thoughts on this exhaust repair?'

Sep 22, 2025

My 2008 ML320 had a cracked flange clamp. It was a female-male flange.

I changed the clamp but it didn’t matter. The one side of the pipe was cracking at least 180 degrees around and so it needed a repair.

I took it to a local shop that I’ve been pretty happy with for other small jobs. They did this:

Three issues I see. One is that the OD of the repair piece the not stretched seems smaller than the OE pipe. It’s only a few inches, but I suspect the flange is also smaller. Is this enough to be concerned with?

The second is the use of the red sealant...

• JHZR2
• Replies: 39
• Forum: Mechanical/Maintenance Forum

• 

The shop has always been friendly and helpful. Well, a year prior, I needed a flex pipe replacement on my 1991 350SD, because it had gotten a hole in it and was leaking. I didn’t test it much, and I also know that there’s a slight manifold leak, so I didn’t think to look closer.

Today after work, I set up my smoke machine to reverify leaks. I had used a shop vac and soapy water before. Smoke is so much better.

Smoke didn’t come out as readily at the manifold. It came out lots at a weld.

This one:

I put my hand on it and condensed clear baby oil was wet at the leak point. I could hear the leak.

So not so great a repair here.

What should I do to repair it? I am tempted to take it back, they probably would fix it, but I’m also hesitant to cut the OE system more or anything like that.

I doubt that chemical patches will work. I don’t weld but it seems like I should learn.

Might it be smart to lightly grind the entire weld, and then try to solder it or apply something else?

Or is a specific type of welding the only approach? Can they weld and seal it over top of the existing weld? Should I maybe try to paint over it before (to attempt to seal) or after (to attempt to protect)?

The car sees little rain and no salt.

 

[slacktide_bitog]

Time for a new exhaust with 304 tubing and a Borla muffler

 

[Silverado12]

Permatex Exhaust Putty I would try this. I don't have experience with it, though, but have used their gas tank repair putty and it worked long term.

 

[Trav]

That is a poor welding job, find a good welder, he can grind that down and properly tig weld through it, that mig weld was too cold, incorrect wire speed, etc, he just welded more crap on top of crap that didn't penetrate well.

 

[Dave Hess]

That is a poor welding job, find a good welder, he can grind that down and properly tig weld through it, that mig weld was too cold, incorrect wire speed, etc, he just welded more crap on top of crap that didn't penetrate well.
View attachment 308434View attachment 308435

That’s what I wanted to type but didn’t want to offend JHZR2.

Not everyone at a muffler shop can do quality welding.

 

[50yr Street Mech]

Second vote for Permatex exhaust putty. Just clean up the weld and a little near it, works fine. When I was a kid, just starting to mess around with cars, I would just use a tin can with a bunch of worm clamps. Worked for me at the time

 

[2strokeNorthstar]

I did a shop vac and soapy water test on my F150 when trying to pinpoint a manifold leak and even found some factory connections with small leaks. I dont think exhaust systems are always 100% air tight under pressure. If it's obvious, sure but a few bubbles doesn't matter. Muffler shop welds are often function over form and have some leakage because they are welding in place, not removing the entire section and welding on a bench in ideal conditions. That would be too expensive and most customers would go to a cheaper shop.

 

[JHZR2]

I did a shop vac and soapy water test on my F150 when trying to pinpoint a manifold leak and even found some factory connections with small leaks. I dont think exhaust systems are always 100% air tight under pressure. If it's obvious, sure but a few bubbles doesn't matter. Muffler shop welds are often function over form and have some leakage because they are welding in place, not removing the entire section and welding on a bench in ideal conditions. That would be too expensive and most customers would go to a cheaper shop.

That’s fair, but I smell diesel exhaust in some conditions, despite the sealed hood and false firewall setup of these cars.

Curiously the exhaust manifold to head didn’t leak any smoke despite the shop vac test making bubbles. I’m afraid the leak at the weld is so large that that small flow capability of the smoke machine couldn’t push to both leaks!

 

[Mainia]

That is a poor welding job, find a good welder, he can grind that down and properly tig weld through it, that mig weld was too cold, incorrect wire speed, etc, he just welded more crap on top of crap that didn't penetrate well.
View attachment 308434View attachment 308435

That is a job for MIG, not TIG. TIG is HYPER sensitive to impurities. Even grinding down, if you have pin hole rust TIG will puddle blow on you. The top photo has horrid welding, you don't want that guy again. The flex pipe fix is a good weld. Tell them you want them to fix it and you want it fixed by their best welder and MIG welded. As far as the sealant, the guy did a hack job. I have ZERO issues with high temp sealants on flange application like that as it helps it stay a "non leaker". It is just your job looks like a hack job, from lack of caring to do a professional job, to an actual hack non skilled employee doing your job. Go back and ask "The A Team" to please fix the "D-Teams " work. But of course be more diplomatic then I am on this thread.

 

[JHZR2]

That is a job for MIG, not TIG. TIG is HYPER sensitive to impurities. Even grinding down, if you have pin hole rust TIG will puddle blow on you. The top photo has horrid welding, you don't want that guy again. The flex pipe fix is a good weld. Tell them you want them to fix it and you want it fixed by their best welder and MIG welded. As far as the sealant, the guy did a hack job. I have ZERO issues with high temp sealants on flange application like that as it helps it stay a "non leaker". It is just your job looks like a hack job, from lack of caring to do a professional job, to an actual hack non skilled employee doing your job. Go back and ask "The A Team" to please fix the "D-Teams " work. But of course be more diplomatic then I am on this thread.

As I recall, they cut the old corrugated flex pipe at the bottom, removed the entire upper section of the downpipe at the turbo flange, and then bench welded all the rest.

The bad weld is the last one made after bolting up, next to the transmission, in a blind spot.

Not making excuses, and lessons learned.

 

[RooflessVW]

All the acronyms...

My exhaust guy bent and torch welded an entire system header-back for my Trooper in about an hour and a half.

Same guy would heat that up and add a bit of filler - have you on your way in 15 minutes.

 

[Mainia]

Blinds spots are a bear, and on thin tubing even worse, you can't see your puddle. It is hard to get a industrial MIG wand up in a tunnel if not impossible. The problem is, your throwing more heat into it for a "fix", as the guy should done a fast micro weave weld in the first place if he did not want more heat/amps. Not all places have a TIG, but to get the top at the time, some guys go back to Oxy Acetylene or TIG the top if they cant get to it with a MIG. We can critique the heck out of this, but it won't help the issue. I would go back and ask them to fix it. It happens all the time to places that don't test for leaks, so they are good at fixing it. The sooner the better as far a corrosion at the weld points.

 

[JHZR2]

Blinds spots are a bear, and on thin tubing even worse, you can't see your puddle. It is hard to get an industrial MIG wand up in a tunnel if not impossible. The problem is, your throwing more heat into it for a "fix", as the guy should done a fast micro weave weld in the first place if he did not want more heat/amps. Not all places have a TIG, but to get the top at the time, some guys go back to Oxy Acetylene or TIG the top if they cant get to it with a MIG. We can critique the heck out of this, but it won't help the issue. I would go back and ask them to fix it. It happens all the time to places that don't test for leaks, so they are good at fixing it. The sooner the better as far a corrosion at the weld points.

Speaking of corrosion at weld points - shall I clean it in acetone and then paint it with high temp exhaust paint or something?

 

[Mainia]

Not until it is fixed.

 

[JHZR2]

Not until it is fixed.

Of course I mean after

 

[Bailey28]

After welding scrub the area with a stiff wire brush then hit it with high heat BBQ grille paint.

 

[Arc]

Stove sealer works pretty good 2700 degrees but everything except replacing the pipe and welds will only be temporary.

 

[JHZR2]

Stove sealer works pretty good 2700 degrees but everything except replacing the pipe and welds will only be temporary.

Define temporary. The car doesn’t see much water, and no salt. I’d drive them anywhere in an instant, but recognize that older cars were never as resistant, and I don’t have a need to expose these cars to it…

 

[Arc]

Define temporary. The car doesn’t see much water, and no salt. I’d drive them anywhere in an instant, but recognize that older cars were never as resistant, and I don’t have a need to expose these cars to it…

Temporary as in all patches will crack and fall off. JB Weld, stove sealer or whatever you use will not stick to the pipe forever. Ive tried everything over the years.

 

[Hollowed_Mist65]

If I were you, I'd give JB Exhaust Weld Wrap a try. I haven't used it myself but looks like it may work well.

I hope you get it sorted.