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2000 ML55 AMG blown out cats FIXED!

7K views 5 replies 4 participants last post by  Mike_Huddleston 
#1 ·
This forum has been a tremendous help to me, so I'm going to post a description of my solution to my 2000 ML55's blown cats.

Last November while driving enthusiastically the AMG threw a flashing check engine light and dropped dramatically into limp home mode. I eased it off the highway and into a parking lot, and I had our favorite local dealer come get it for a diagnosis. They trailered it off and let me know that the front left cat was blown and that the front right had already been replaced by the previous owner's indy with a too-small unit spliced haphazardly into place. I'd been under the car quite a lot, but frankly hadn't noticed that because the front right cat is tucked up in the engine compartment.

The dealer wanted $3800 to replace both front down pipes and to replace two of the EGO sensors. But of course, it might be more...

Since that's approaching half what I paid in total for the best bargain AMG on the planet, I said "no thank you."

Exhaust parts for the ML55 are hard to find. The down pipe/cat assemblies aren't on AutohausAZ, which surprised me, but I began running into some Magnaflow components on ebay and Amazon. What I found, however, is that a large number of sellers would post the 49863 left side assembly but NOT the 49864 right side. Virtually all sellers would want $654 and change for whichever side they posted.

Why most sellers only advertise the left side is this: Magnaflow has a half dozen left side assemblies in stock, but the right side assemblies are now build-to-order only.

Why everyone asks $654 and change is this: that's the maximum contractually allowed discount off list allowed in advertising by Magnaflow authorized resellers.

So I found a guy in Florida -- Monster Distributing -- who would accept an offer for $1100 for both sets, delivered to Texas, one from stock, one in 3 to 4 weeks from Magnaflow.

When the 49864 arrived some weeks after the 49863, I jacked up the ML55 and started tearing out the old downpipes. The collectors are easy to get to when you pull the front wheels and the front fender liners and the one small additional heat shield, left side. The right post-cat EGO was crossthreaded and I destroyed it removing it. The right pre-cat EGO had a badly twisted pigtail and the insulation was coming apart. I used a sawzall with a short metal demolition blade to cut both sides between the cats after dropping the rear transmission crossmember. This made removing the old pipes quite easy. The hardest part was removing one of the clamps on the resonator -- the nut siezed up completely and the splines of the clamp's stud tore out of the wedge block. I had to weld the head of the stud to the clamp body to fix it in place, and then twist the siezed nut and half the stud off. The clamps are spot-welded to the resonator inlet sleeves but they break loose easily. Luckly when it broke loose it did so cleanly, neither leaving a nib on the inlet sleeve nor tearing out a hole.

The resonator sleeve clamps are Torca Stainless Accuseal 2-1/4" nominal units. You can find them on ebay for about $7 to $8 each if you look. List appears to be twice that. I got four delivered for $33. They're exactly the same part Magnaflow provides to join the front and back halves of their down pipe assemblies. Buying spares guarantees none of them run away and hide during assembly.

I looked all over for EGOs the evening after I started tear-down. I found some unspecified manufacture Chinese units on ebay that ran less than $30 each.... but I'd have to wait for them AND I really didn't like thinking about the quality. Looks like the original Bosch part number is 15202, itself a universal replacement for unique pre- and post-cat part numbers OEM ... but the best price I found on 15202 was Napa, $129 each. Autozone has a newer part, Bosch 13782, about $80 each, available in-store the next day from local distribution. I picked up two a day or two later while waiting for the new clamps. These are plug-in EGOs.

At first it appeared that the "guaranteed direct replacement fit" down pipes would plug right in.

It wasn't to be.

The Magnaflow pipes are each supplied in two parts for assembly into the trucklet. You still need to drop the transmission crossmember, but otherwise they just lift and wiggle into place. The tail end of the 49863, where it should slip into the factory resonator/crossover, was stretched substantially. Comparing it to the stock pipe showed that it was at least .050 too big to fit into the resonator clamp. The tail end of the 49864, on the other hand, wasn't expanded at all, and at a stock 2.25", wouldn't make a tight seal at about .050" undersize.

I might be able to expand the end that's too small, but there's no way I know to shrink the one that's too big. I tried slotting the 49863 -- the slots maximally misaligned with the resonator sleeve slots -- and jamming it into place, but there's just no way. The tail end of the 49864 is only about two inches long after a bend to clear a body crossmember .... no way to get an expander up into it far enough to stretch it out.

I called Magnaflow tech support. "Dave" told me to contact the vendor to arrange an exchange. I emailed and called Monster Distributing, but they never got back to me. I called Magnaflow twice more, each time being promised a return call. But that never happened. I wasn't surprised. Online searches for instances of Magnaflow support needed in similar situations always end with frustrated comments about Magnaflow not getting back to the person working the repair.

So.... the metric size AMG used for the exhaust is bigger than 2-1/4". Not much bigger, but bigger enough that the two sizes don't fit into the same tight compression sleeves. I got onto ebay again and found item 170947677044 -- a 5' length of 2-1/4" 14 ga 409 stainless exhaust tubing from "wholesaleautopartsexpress2". Five feet for $35 delivered! And it only took two days, too. Stainless 409 is an ugly stainless that doesn't polish up well and it can form a light surface rust, but it's tough and easy to weld, using the same stick or wire as 300-series alloys. I chop-sawed and cleaned up a handfull of 1-3/4" long sections, and then chop-sawed carefully 1-3/4" off the ends of the two-small 49864 and too-large 49863. This was the most I could cut from the 49864 without getting into the bend.

I took those four 1-3/4" segments to half a dozen good nearby indy shops. None of them had a tubing expander. Finally I relented and went to a local chain muffler shop. They DID have a hydraulic expander, and they were more than willing to stretch out the four little pieces ( two spares! ) gratis. It took maybe two minutes. While I was there I asked them if they might be able to weld the little adapters to the ends of my stainless pipes. They were... but they didn't have any stainless wire in their little flux core MIG. Never did, they said. They said their welds would be stainless because the base metal was stainless. Which is why I had avoided the chain shops in the first place....

Why didn't I use segments of the old exhaust? After 125,000 miles of spirited driving, the old exhaust was very hard and very brittle. The grain structure, I'm sure, technically speaking, is completely fubar. One segment I dropped after cutting it out of the car actually fractured. There's a reason some states outlaw welding on used exhaust pipe.

The muffler shop had accurately stretched out both ends of the new little 409 segments to precisely match a piece of the old metric pipe, but the middles were slightly smaller diameter, their expander being designed more for making joints than for expanding whole sections. I worried about this for half a day before I realized I'd been an idiot. While those silly little manual screw-driven auto parts store expanders are entirely useless to take .060 wall stainless pipe up a telescoping size, the working parts of the expander sans the screw and nut, slid into the short sections of tubing and then popped into a 60-ton hydraulic press, work far, far more effectively than did the muffler shop's expander. I'd wasted my time visiting those shops looking for an expander. I could have done it in my garage the whole time. I popped them onto the bar-and-wedge assemblies from the little manual expanders, stacked that in the press, and five minutes later had four perfectly straight short lengths of 409 tubing with precisely the same diameter as the resonator sleeves ( two plus two spares... ).

I have a Millermatic 210 with a spoolgun with both 75/25 and argon tanks, but I've never welded stainless with stainless. A little bit of research indicates that the BEST way to MIG weld stainless is with solid stainless wire and costly Tri-mix gas, but I wanted to avoid another bottle and another flowmeter -- there's no room on the cart. Which leaves two options: either flux-cored gasless wire or flux-cored gas-shielded stainless wire. The former is EASY to find, and cheap. USA Weld -- MIG/TIG/Stick Welders & Plasma Cutters | USA Weld -- has a nice cost-effective flux cored .035 308 small spool -- part number 308LFC-0-035-1 -- for about $39. I hadn't even known that flux-cored gas-shielded stainless wire existed. But it does. Hobart makes an "elevated service temperature" flux cored wire for the 75/25 mix that's specfically for 400-series stainless alloy exhaust tubing, but it's not cheap, running about $18 per pound on big spools. No one had any locally except in big commercial spools, and since the whole point of this exercise was to save money, I wasn't going to drop $400 for the welding wire. But I'm told it would have worked REALLY well. While I was looking into this, I determined that the common flux-cored gasless wire would produce an even cleaner weld with the 75/25 mix running. It does.

So today I got up early to 72 degree weather and welded the little 409 segments onto the ends of the Magnaflow down pipe assemblies. Perfect. The HTP Weld USA flux cored wire looks like a hot glue gun bead with the gas running. Only one drop-through, easy to fill.

I replaced the destroyed clamp on the resonator with an Accuseal 2-1/4" nominal... and then replaced the other one as well. I had spares. Why not? And now the nuts point down!

I lifted the pieces into place and for the first time had both Magnaflow pipe assembles in the car from collectors to resonator. Tightening everything in place revealed a few more issues. First, the left pre-cat bung was rotated aft, so I couldn't install that EGO without relieving the firewall just a bit. Second, both forward cats had heat shield flanges that interfered with the forward frame rails. I removed about 1/8" of flange outside of the weld, on each outboard side heat shield flange, which fixed that. Third, the way the pipes were bent they wanted to route a little more inboard than the old pipes, so the transmission exhaust brackets were in the wrong place. I dropped them and put them in a vise and introduced three shallow bends in each spindly support, so instead of jutting out straight from their mounting they bend aft and then forward and then sideways again. This "shortened" them on each side about 1/4" without cutting and without heat... and it's reversable, too.
I know why Magnaflow rotated that left pre-cat EGO bung aft: it allows the EGO sensor to be tightened easily from below instead of with difficulty from the fenderwell. But it was rotated just a bit too far.

The final problem was that the mounting loops on the Magnaflow pipes were too small to allow the multi-stud assemblies for the transmission brackets to be installed. You can reverse them, and put the multi-stud side under the transmission bracket, but this leaves you with trying to tighten the nuts inside the tight exhaust mounting loops... so I just installed 1/4" stainless bolts and nuts and used two wrenches to tighten them. Easy.

Final tally is a bit more than $2400 better than dealer repair pricing, and ten hours of labor. Maybe I could have found a good indy to do it for less, but I wouldn't have learned anything along the way.
 
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#5 ·
What's the advantage gained going from the stock resonator/crossover to that tiny little can? Are you still using the stock muffler assembly in the rear bumper, and is the single pipe from your central thingie on aft still that huge size that's stock?

I've read that the best results in terms of getting good engine sound out of the vehicle involve retaining the stock resonator, but replacing the stock muffler with something like an SLP Camaro SS unit.... many of the oldest posts I found started with the Magnaflow downpipes... another reason I used them.
 
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