"Thunk" sound in 300TD after motor mount & flex disk replacement - PeachParts Mercedes-Benz Forum

aaronkobi · #1 03-20-2008, 03:16 PM

I recently had my motor mounts and front flex disk replaced in my 1984 300TD. About a week after having them replaced, I started to notice a "klunk" or "thunk" sound when slowing down. Sounds like it's coming from the engine compartment. When accelerating, I would hear the "thunk" sound again as the car started. I also REALLY notice the sound when turning the car off. As I turn the key off, as the engine gives its last shudder before shutting down, I hear a THUNK, a little louder than the one I hear while moving.

Thoughts? Reminds me of a loose muffler bracket sound, but, doesn't explain that it only happens when slowing down and starting up.

Chad300tdt · #2 03-20-2008, 03:26 PM

Did you have the engine shocks replaced too? Is your air cleaner on properly?

Sounds like you need a valve adjustment and/or idle adjustment too.

aaronkobi · #3 03-20-2008, 03:31 PM

Nope, I supplied the engine shocks but my mechanic said the old ones were fine. Valve/idle adjustment was just done about a month ago. Time to take it back to them :-(

What I'll do is have somebody turn the car off and on while I have the hood up. That way I'll be able to track down the general area from which the sound is coming.

Chad300tdt · #4 03-20-2008, 03:37 PM

Good luck Aaron. Hopefully it's something simple like worn out rubber bushings on the air cleaner bracket. I also put a piece of coolant hose between the head and the air cleaner to keep them from smacking together.

sd300td · #5 03-20-2008, 03:40 PM

Quote:

Originally Posted by aaronkobi

I recently had my motor mounts and front flex disk replaced in my 1984 300TD. About a week after having them replaced, I started to notice a "klunk" or "thunk" sound when slowing down. Sounds like it's coming from the engine compartment. When accelerating, I would hear the "thunk" sound again as the car started. I also REALLY notice the sound when turning the car off. As I turn the key off, as the engine gives its last shudder before shutting down, I hear a THUNK, a little louder than the one I hear while moving.

Thoughts? Reminds me of a loose muffler bracket sound, but, doesn't explain that it only happens when slowing down and starting up.

Mine does the same, but only when starting or shutting off the engine. The thunk comes from the passenger side, under the floor board. Almost sounds like it comes from within the passenger side front door.

Is your thunk a one-time thing, or does it go thunk thunk thunk as the engine stops moving?

Folks here thought it was the air cleaner mount, gelled fuel, or GP's.

aaronkobi · #6 03-20-2008, 03:48 PM

Mine feels like it's coming from the drivers side, possibly under the floorboard or just in front of the firewall. When slowing down, I can actually feel the thunk in my feet on the floor. When I shut the car off, most of the time it's a single THUNK, but sometimes there is one louder thunk and then a 2nd minor "after-thunk".

When engine mounts are replaced, the engine has to be jacked up about 3-4 inches, correct? I'm thinking that when they did this, something came loose or broke. Possibly an exhaust bracket? At first I thought it might be something in the suspension, but since it's worse when the engine shuts off and shakes, I've decided it's something actually attached to the engine and not suspension or structural.

Guess I'll just need to get in there and start trying to see what's loose. If that fails, back to the shop it goes. I could live with it, but, don't like the thought of something loose that could possibly be damaged from shaking around over an extended period of time.

Biodiesel300TD · #7 03-20-2008, 03:58 PM

I would take a look at your transmission mount. It is easy to get to, it's at the rear of the transmission.

aaronkobi · #8 03-20-2008, 04:40 PM

Just spoke with the mechanic that replaced the motor mounts.

The mechanic's thought that replaced the mounts is that the "klunk" is due to aftermarket motor mounts (I supplied the motor mounts). He said the rubber is softer in them than in the OEM Mercedes mounts and it causes the brackets within the mounts to make contact when the engine shakes (such as when shutting down or coming to a stop). His recommendation was to switch them out with OEM mercedes mounts and remove the engine shocks (not sure why that is, but he said the engine shocks are pretty much pointless and usually add to the problem).

Chad300tdt · #9 03-20-2008, 04:42 PM

It sounds like some double talk to me. I would think more about going back to someone who is advising the permanent removal of engine suspension components.

Does he work on diesels regularly?

aaronkobi · #10 03-20-2008, 04:58 PM

Yeah, these guys are pretty reputable... Don Pool Mercedes. Specialize in older benz models. But yes, I thought the same thing... sounds a little fishy. Would OEM motor mounts really make THAT much of a difference? Should have just left the old ones in... didn't have this problem before they were replaced. Grrrr!

Biodiesel300TD · #11 03-20-2008, 07:39 PM

I would check your tansmission mounts. But you could also jack the front of the car up and put jack stand under it. Then start the car and climb under. Then have someone shut the car down for you and listen and see if you can identify were the sound is comming from. If you can here it in the floor boards I'm guessing it's not the motor mounts. When the new mounts are put in the enigne is generally lifted up a fair amount, the mechanic could have damaged the trans. mount. Also check the engine shock mounts, make sure the top of them aren't torn out. If the nut wasn't removed from the shock the shock the mount will tear apart when the enigne is lifted. Ask me how I know.

I have heard bad thing about cheap motor mounts. What brand did you put in?

aaronkobi · #12 03-21-2008, 09:31 AM

Andrew, I'm with you on the transmission mount thing. I just can't really imagine engine mounts klunking like that regardless of how cheap they are, but, who knows...

The brand I bought are Febi-Bilstein/Meyle.

Dee8go · #13 03-21-2008, 09:38 AM

Quote:

Originally Posted by aaronkobi

Yeah, these guys are pretty reputable... Don Pool Mercedes. Specialize in older benz models. But yes, I thought the same thing... sounds a little fishy. Would OEM motor mounts really make THAT much of a difference? Should have just left the old ones in... didn't have this problem before they were replaced. Grrrr!

Don Pool? You must be in N. VA. I'm leary of people who start telling me stuff the manufacturer put into my car, especially when the manufacturer is Mercedes Benz. I don't think they usually put a lot of parts into a car that are unneccessary.

aaronkobi · #14 03-21-2008, 10:53 AM

Quote:

Originally Posted by Dee8go

Don Pool? You must be in N. VA.

Yep, in NOVA indeed. I've been working with them since I bought the car in January and they've been great for the most part. What I really like about them is that they'll take the time to really explain what they've done (when I pick the car up, they take me outside and show me what was repaired). They obviously ENJOY what they do and are always happy to answer random questions. Plus they're between my house and work, which is a huge plus. Basically, they've been great to deal with, so I don't want to give a bad impression.

They said they'd replace my engine mounts with OEM Mercedes mounts for the price of the mounts only (with their shop markup added). I may take them up on it. If I do, I'll ask them to leave the shocks in place. I agree, I think they're there for good reason, and while they probably don't do MUCH, they do help dampen the vibration a bit (I've actually watched them in action while the engine is running).