I read in another post that some cats are cracking due to a collapsed motor mount.

I thought it was weird that mine cracked at the seam on the top of the first cat on the right side. I had it welded by a local shop after I took it out and then reinstalled it myself. It is working fine so far.

The question is that should I replace the motor mounts?

How do I know if they are bad?

How hard would it be to do in my garage?

I don't want to have to buy or fix the cat again as it was a huge PITA and they are very expensive to buy and do yourself if it cannot be welded.

Thanks
Paul
2000 ML320 (out of all warranty)

 

So, this is yet another common problem...at around 60K, my CATs and motor mounts both had to be replaced...

I was of the opinion that the motor mounts could have contributed to the CATs failure, but the SA kept saying, the two were unrelated...

In my case the motor mount problem kicked in first...it started with a mild vibration of the truck at idle time on cold starts, but then gradually the vibration became persistent and I took it in to the dealership, and they thought the motor mounts needed to be replaced...and true indeed, the vibration disapperared after the motor mounts were replaced...

I was the dealership watching the mechanic work on the motor mounts and I thought it was not that simple, the belt has to be taken off the pulleys, the fan unbolted, and then of course the motor mounts have to be loosened.

Finally, with a lift, the whole engine was taken out and new motor mounts put in place...and the whole process reversed.

All in all it was not that bad on the pocket, $293 out of the door. I heard, the motor mounts themselves cost about $34/piece.

Anyways, I drove back out of the dealership only to come back next week with my truck sounding almost like a steam engine....the inspected it again and said it was the CATs. I strongly felt it was the motor mounts that caused the CATs to fail, but my SA insisted they were un related issues...

The CATs were done for 0$ out of pocket since they fell under the 80K emissions warranty.

One week after the CATs are replaced, I am back at the dealer with the complain of a significant drop in mileage - down to 12 mpg in city...

They took a look at it again, and said, it was the MAF sensor, and that the broken CAT was compensating for the faulty MAF for a while..

Now I felt all three problems were related...the motor mounts caused the CATs to fail, and the CATs caused the MAF to fail...but my SA insisted that all three were unrelated...I find it hard to believe though...

Anyways, I got away with cleaning the MAF and that was the end of that major ordeal at 60K. I have since put 20K more miles on the truck and haven't had any major problems yet..

If they are doing the CATs for free, I suggest get the motor mounts done with them...its not that expensive a fix after all...

 

Doing without the proper tool and a hoist can be a real pain in the A$$ it takes me about 2hr to do both sides. the fan belt and a/c compressor needs to come off. then the motor needs to be raised so you can remove the old mounts and put in the new ones...i am seeing mounts fail as early as 30k....not a job you want to do on your garage floor

 

Not worth my time on my back, he he...

I have no vibrations or anything wrong as far as I know. Should I wait or do them as preventative?

Thanks,
Paul

PS
It is nice to have a quiet truck again!

 

Especially considering that this vehicle was built with off roading in mind...would you suggest any alternatives to MB motor mounts? Something that lasts longer and can withstand some off roading?

I am an occassional off roader, so do subject the truck to some occassional bumping and jumping....I wouldn't want to replace my motor mounts that often...

 

I wouldn't jump on changing the motor mounts...you can always get them inspected at the MB dealership for a small cost.

CATs can fail due to bad motor mounts, but that's not the only reason why CATs would fail...unless you are sure that your motor mounts are bad, don't jump on getting them replaced...