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I took my 1999 GMC Sierra 1500 w/5.3l to a transmission shop to have it replaced. The process requires disconnecting the exhaust pipes going into the engine to get access to the transmission. When the shop put it back, the driver's side pipe connection "disintegrated" as the guy described it. It's in a cold state with lots of rock salt on the roads so there's a lot of rust. The pipe itself is fine, and the bolted "bracket" part that connects to the engine seems fine. It appears to be just the connection point, maybe a weld, almost like caulking (but not really caulking of course), was holding the pipe and engine together. The pipe is able to move upwards into the connection fine.

Questions:

  • Can I just put the pipe back in and do something to seal them together again? Or do I need to replace that section of the exhaust pipe or the entire exhaust pipe?

  • Should the transmission shop pay for the muffler repair since they broke it? Or not, since the parts were rusted before they started work?

  • I have lower-to-intermediate mechanic skills. Oil changes, filters, brakes, rotors, spark plugs, fuel filter -type of experience. Can I do this repair in my garage or should I take it to a muffler shop?

  • Any SWAG cost estimate? (SWAG = Scientific Wild-Ass Guess)

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5 Answers 5

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Can I just put the pipe back in and do something to seal them together again? Or do I need to replace that section of the exhaust pipe or the entire exhaust pipe?

You can attempt welding. It's not easy materials to weld, with thin walls and a lot of corrosion, and possibly high enough temperatures to burn some carbon out of the steel - but the downside is small, as the worst that happens is that you destroy it, and have to buy new. Exhaust parts are usually quite cheap.

Should the transmission shop pay for the muffler repair since they broke it? Or not, since the parts were rusted before they started work?

On a 24 year old car? Nah. It's in the category shit happens. Exhaust systems are exposed to corrosion, and failures is rather common.

I have lower-to-intermediate mechanic skills. Oil changes, filters, brakes, rotors, spark plugs, fuel filter -type of experience. Can I do this repair in my garage or should I take it to a muffler shop?

It's a fairly easy repair - remove the old and install the new parts. Bolts may be rusted, so you will need some hand tools, patience and penetrating oil. But it's fairly low skill, and the consequences of fuckups tends to be small. So I'd say go ahead.

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  • See, there you go. +1 for a decent answer. Feb 3, 2023 at 22:39
  • Thanks. I must add it's one of the most welcoming SE's so far :)
    – vidarlo
    Feb 3, 2023 at 22:45
  • Oh, I know where you're coming from there. We're glad you're here. Feb 3, 2023 at 22:55
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    "Bolts may be rusted, so you will need some hand tools, patience and penetrating oil" .. and a torch.
    – George
    Feb 4, 2023 at 3:54
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    It has astonishingly little corrosion for a 20 year old steel piece. I suspect it is stainless. It will be difficult to weld and the welding will ruin the stainless characteristics that made it last this long. So it will rot out in a few years. Feb 5, 2023 at 20:40
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If I were you, I'd try to take it back to the shop and tell them they busted the exhaust while replacing the transmission. It was working just fine before they touched it. You can see what they say, but more than likely, they are going to laugh at you (behind your back, of course).

That pipe you're looking at is the top end of the catalytic converter. You could possibly weld it back together, but to do it right, you need to replace the cat. If you don't get the weld done right, you'll end up getting an exhaust leak which will not only do the obvious (leak exhaust), it'll also mess with the O2 sensors, which will make the engine run wrong and most likely cause you a check engine light (CEL).

If I remember correctly, in order to get the transmission out, you have to take the exhaust off. Since it was removed (I believe) when the transmission was replaced, it should come off fairly easily. There are the studs on the manifold on the to end and a slip fit which should be under the transmission. This would have had to have come apart as well. Taking these apart should be relatively easy. You're lucky, because usually these are a REAL bugger to get off. You can dismantle this with a regular mechanic's tools (as long as you have metric). You'll need to remove the O2 sensor from the pipe as well. This may be a little bit harder to get done. This is a 7/8" hex on it. There is a special socket which can be used, but you should be able to get it off with a combination (spanner) wrench. A better option would be to just replace it along with the cat. More than likely it's going to be rust welded in place if it hasn't been replaced anytime in the past year.

As far as cost, that's something you'll have to figure out on your own. We don't do cost estimates on here because it all depends on where you're at and what you're working on/with. Prices vary wildly.

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I agree with the other answers in general, but would like to add to them.

This doesn’t look like rust to me, it it a brittle weld that has cracked and separated. Everything looks in very good condition and with a bit of cleaning would be suitable for re-welding.

I would push the pipe back into place, add a few tack welds if possible, then remove the exhaust to weld properly, before refitting. If it is not accessible to add the tack welds first, alignment should still be possible, since the cracked joint should still align well.

If this is indeed the Catalytic Converter, then it is likely made of stainless steel which is susceptible to cracking and would be an expensive part to replace, so worth repairing.

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    I can confirm, it is indeed a catalytic converter. The 99 5.3L LM7 engine GM put in these trucks (GMC and Chevrolet) had two cats just downstream of the exhaust manifold, which is what you see and has the studs in it. I agree with your assessment, as it, it should be easy. If you look at the last image, the "head" of the cat is just in the picture. Here's an image of an aftermarket one. The bottom section is the part you're seeing in the OPs image. +1 for a good answer. Feb 4, 2023 at 17:46
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First and foremost: Soak everything in penetrating oil. A LOT.

The right stuff to use is Aero Kroil which is a little hard to find. In a jam, Liquid Wrench will suffice. Do not use WD-40; it's not penetrating oil. Really. It's often misused as that, but we can't afford to fool around here. Those bolts need to come off without snapping.

You need to soak every bolt associated with this assembly. And this is a big assembly. Soak them as often as you can.

This is a huge assembly comprising both cats, 0-4 oxygen sensors and a lot of pipe.

The assembly connects to this exhaust manifold, the other side's exhaust manifold, and a flange at the back that may go to another catalytic converter or simply the exhaust pipe. Soak all these bolts.

Also, search the pipe for oxygen sensors and soak those too. It'll help a lot if the oxygen sensors can be removed intact, as they are a matched set.

There will be oxygen sensors both before the cat and after it. Since you have 2 cats, I presume 4 sensors. The pre-cat sensors may be on the exhaust manifold; if so disregard.

Best option: have a pro shop weld it.

The reason is this whole catalyic converter assembly is costly - about $300 for the part. (possibly a lot more in the 12 states that follow California emissions). But honestly, that's really not bad at all for that huge assembly with two whole cats. That reflects the popularity and huge volumes of parts sold for GM C/K families.

Back in the old days, exhaust pipes were the first thing to rot out on cars. You'd maybe get 30,000 miles on an exhaust pipe (in the snow belt). What changed is they started making them out of stainless steel. No, it's not shiny and looks corroded, but boy howdy they last a lot longer!

So this is not a thing you could possibly DIY-weld. Even if you succeeded, your welding would alter the steel so it isn't stainless anymore. And that means it would rot out quickly like in the good old days. So you'd be right back to buying another one.

However a shop has a reasonable chance of being kitted and skilled to do the correct stainless welding.

Since they would need to weld this on the car for fit, I recommend aggressively cleaning any leaky-oil mess from around the area. They will not want to weld around that.

Plan B: Unscrew those screws and replace the whole cat assembly.

Normally I send people to a Pick-n-Pull yard for spare parts. Cheap, and you get a lot of "free practice" doing the work on expendable cars. Unfortunately that won't work here, because the yards chop off the catalytic converters. They'll have the ends of the pipe, just a Sawzall slash where the cat proper should be.

You might be able to get the little chunk of pipe ahead of the cat, but really, any exhaust shop will stock that.

You might ask full-service (THEY-pull-it) wrecking yards if they can sell you a used cat assembly flange-to-flange. However the government discourages used-cat sales, both for clean-air and anti-theft reasons.

Your cats proper are probably fine if you're not getting a Check Engine light. 1996+ cars have oxygen sensors both ahead of and after the cat, so they can detect a failed cat. That's why I want you to keep the oxygen sensors as a matched set - they work together.

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  • Mark where the weld is on the exhaust pipe (by measuring from the end) and mark the angle how the pipe fits in the flange !
  • Grind of the old weld from the pipe. It doesn't look like rust
  • Remove the bolted flange from the engine (with little bit of wd40 on the bolts, or some other means undoing the little rust of the bolts)
  • Weld pipe to flange on the same position as before (that's why to mark were the original weld was)
  • Do a few SMALL spot weld tacks on the inside too (for what I can see on the picture there ain't no welding spots on the inside of the flange) but some spot welds on the inside make it way more firm. (not on the end of the pipe, else it won't fit anymore.)
  • Replace flange with attached exhaust with the bolts using a bit of coppergrease on the bolts. (Copper grease is high temperature grease, ideal for exhaust systems.
  • Put some zinc spray on the new weld (so it doesn't oxidise) (not necesary for stainless steel parts)

Cost estimate if one has the equipment to weld :

  • $2 for grinding disc,
  • $0.5 for wd40
  • $3-$10 for welding filler wire (depending on the material, standard metal or stainless steel)
  • $1 for copper grease,
  • $5 for zinc paint (not necessary of its stainless steel)

Should take no more then an hour time (excluding removing and replacing of the exhaust)

It could be there's an exhaust gasket between the 2 flanges, there's liquid gasket maker for exhausts (like gum gum paste for exhausts). (Saves you a gasket kit, most of the time gasket kits are completely engine gasket kits)... cost around : $10 for a tube liquid exhaust gasket

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  • How many mechanics have got both a mig setup and a spot welder? Or am I misreading that bit?
    – 2e0byo
    Feb 5, 2023 at 18:53
  • The GM C/K line has a huge installed base and huge parts demand, the economies of scale make parts widely available and ridiculously cheap. A few years ago I paid $60 for a rebuilt starter. $60! As such a $10 can of make-a-gasket doesn't make sense when an OEM quality exhaust gasket is $2 and stocked everywhere. Feb 5, 2023 at 20:54
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    @2e0byo maybe a difference in terms : with spot welding I mean mig/tig weld with a few weld ticks/spots. Not the spot welding one can do like on domestic batteries or sheet metal
    – HermDP
    Feb 6, 2023 at 2:13
  • @Harper-ReinstateMonica If parts are so cheap and wisely available, then indeed an OEM exhaust gasket is the way to go. For some cars/brands gaskets are only sold in complete motor kits (exhaust gasket included)
    – HermDP
    Feb 6, 2023 at 2:15
  • @HermDP ...and the pipe protrudes through the flange, so there's a surface to weld on. I somehow missed that and spot welding---which would be well-nigh impossible with stuff this thick anyhow---was all I could think of. However apparently it's stainless anyhow.
    – 2e0byo
    Feb 6, 2023 at 15:23

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