Firemane

Hello! I've been a lurker here for a while, but I need some input that lurking isn't revealing, so...

I'm trying to find some information for my mother's car, a 2006 C230 Sport that she purchased new in June '06 on my recommendation. Her car currently has ~5800 miles on it. The issue is that there is an intermittent erratic thumping/knocking noise coming from the rear of the car (sounds like someone in the trunk punching the seatback - and I've already read all the 'let the midget out' jokes, thanks) when the following conditions are all present:

• The car is at operating temperature
• The car is at a full and complete stop
• The car has been at rest for >15 seconds and is in gear
• The fuel gauge reads less than full

I've used the forum search facility, and the most recent thread on the topic appears to be from 2005 - with no conclusive cause/solution ever related. Vague references were made regarding fuel transfer pumps, sloshing fuel, and a known issue with the Bose audio system (which she does not have), but nothing definitive. I can however say definitively that the sound is not that of fuel sloshing in the tank.

I'm hoping someone here has found a solution for this, as it is becoming so prominent in the vehicle that she is on the verge of trading the car in. She's had it to two different dealers (MB Escondido and Hoehn MB, both in SoCal) nine times between them; they kept replacing the evap purge solenoid and bracket (which is under the hood, not at the source point of the noise) and sound insulation, and are at the point now where neither dealer will accept the car for further investigation of the problem. One dealer, after trying to find/fix the problem, later came back and said "it's normal" - which it isn't. Neither my '02 C240 nor the several loaner '06/'07 C230 loaners she or I have had have made this noise.

She's also contacted MB corporate customer service, but so far their only answer has been "take it to the dealer when it makes the noise". Which we've done. Which one of their techs did claim to have heard - for all the good it's done. We're at a point now where the only apparent thing left is to contact a lemon law attorney to negotiate a buy-back, or trade the car in and take a bloodbath on the loan.

So please, if anyone has a solution for this knocking noise that either I can perform at home or have the dealer perform, my mother and I would be extremely grateful for the sharing of said solution. Thanks!


Brian
2002 C240

Shake

This is common problem. I get it too at stoplights and it comes from the rear. I don't think that it cannot be fixed. I wouldn't waste time on it. Some other people on the forums have this problem too

Firemane

Hrm... Well is it at least known what is generating the sound? Common or not, someone's gotta know what causes the knocking - and why it happens on some cars and not others within the same model and year.

Shake

Well, Sanf on the forum took his car in and they took it apart to see where the noise is coming from, but the dealer couldn't locate it. I don't think anyone knows where it's coming from, but if someone does, do tell please.

drexappeal

I wish I could say I had a revelation of an answer, but I don't. From what you indicated, this sound happens when the car is at rest and not while rolling. At that point, I have no clue.

If this knocking/thumping sound happened while you were driving, that's a different story. In that case, I would say it's the thrust arm bushings which are known to go bad on all Mercedes chassis. Considering the car has so few miles on it and is a 2006, I don't know why the thrust arms would've gone bad so soon.

Firemane

Yeah, it would be a lot easier if it was an in-motion noise; those kinds of noises make sense. But who ever heard of a "thump" when a car is stopped/at rest?? Weird.

I'm going to get the car up on some stands this weekend and see if I can track down what's causing it (since both local dealers have abandoned us on this). Maybe I'll get lucky and find a solenoid or something that could explain this.


Speaking of thrust arm bushings, can you point me to a drawing or picture showing the location and normal condition of them so I can inspect them while I'm under there? I'm kind of on a "failed bushing" kick right now, as I'm having the devil's own time with bushings on my '06 GTO (pics are broken, but trust me - they're bad), and have been hung out to dry by that brand's local dealers as well. It would be nice to see if bushing nightmares on mom's Benz - and mine for that matter - are in the cards as well.

drexappeal

You know what, I just thought about it and if the car is at rest and you are hearing a "thumping" sound, have you ever checked the exhaust to make sure that one of the hangers isn't loose? It's possible that the thumping is from the exhaust pipe vibrating and pushing up against somewhere at the rear of the chassis.

Ooh, unfortunately I don't have a pic of the thrust arm bushings. I can see if I can find the part number in my old work orders done by the dealership.

czachari

Seriously, my car did it from day one. The one redeeming thing is that it does it infrequently. Now, I ignore it, it's no biggie. The dealer gave me that b.s. about the transfer pump moving fuel from 1 tank to another, I don't know if it's true. The thing is that if something bad happens to the car especially in the rear, I have the paperwork to prove that there was something going on back there. I figure that if nothing has gone wrong in 4 years, it's probably nothing serious. From my investigative skills, I think it's exhaust related. It only happens when I drive the car hard, in hot weather. After I stop, it thumps 2-3 times. Must be some metal contracting or expanding. It almost NEVER happens in the winter and that's a clue. It also does not happen in everyday "sedate" driving, again reinforcing my hypothesis. The dealer inspected the exhaust in front of me and saw nothing wrong. I hope this puts your mind at ease.

Firemane

Well, I'm gonna put the car up on stands this weekend and see. Man, if it really is just a loose exhaust hanger, I'm gonna have to go knock some skulls down at the dealership...

And thanks for looking for the thrust arm bushing info!

Firemane

Yeah, see when ours first started doing it, it was very infrequent - like a couple times a week. These days it does it several times a day. And we are talking my mom here; going around a corner at 15 MPH is her idea of a "spirited drive"

I find it interesting that yours seems temperature-related though. See, here in San Diego we really don't have seasons, so we wouldn't have noticed that... Actually, that's not entirely true. She has noticed that it does it a lot more on warm-hot days (mid-80s & up), but it never really gets cold enough for it to go away completely. Only a full gas tank makes it stop completely.

Thanks for your post though - it definitely gives me more information to go on (and more ammo to tell my mom to just turn the radio up )!

-B

mleskovar

My understanding is that the gasoline 'tank' is actually two tanks and a pump does activate to balance the amount/load between both of them. The difference between a full tank and a half a tank is about sixty pounds. To see if it's weight related put 60 lbs (or even a person?) in the back seat when the talk goes down to 1/2, the noise starts, and see if the noise goes away. At least then you can concentrate on the cause. Also, I'm guessing there's a balance tube between the two tanks to equalize any pressure buildup....maybe it's pinched off and you're hearing bubbles/gas escaping through the equalizer pump? I'm no expert and I've never had to go into my tank setup so a lot of this is speculation (it's the internet isn't it )

czachari

I am looking forward to you looking under your car. I actually hope you find something. I think if it was something loose it would make that noise all the time and even more while moving. The fact that it ONLY does it while stationary kind of shoots down the possibility of having something loose down there.

dnegri

I have the sam exact problem in my '06 and the dealer stated that since the gas tank is horseshoe shaped that there is a pump inside the tank feeding gas to the opposite side which is why you here this noise. He mentioned that it was normal. I just learned to live with it.

Shake

I just ignore it now. I used to be annoyed about it, but it's not that big of a deal with music.

Firemane

Alrighty then! I'm surprised to hear that this actually might be normal, but I guess any car can have some quirks. Still gonna lift the car this weekend; if nothing else, I like to be familiar with the undercarriage and other mechanical aspects of my cars, so I'm sure it'll be edifying regardless of if I find anything loose. Thanks for all the responses everyone!


-B

markmam1

I think I hear what you guys are talking about it happens mostly when you are about to stop or your on a stop. Its not really that bad you just get use to because I did. I brought mine to the dealership a few million times and they tightened everything on the rear end and still hear it checked my bearings and bushings and still there so I guess it will never go away

dnegri

If you are experiencing the same thumping noise, which it sounds like you are, then it has nothing to do with bearings, bushings etc. Listen closely to the noise and the statement I made in my previous post about what the SA said actually does make sense. It sounds like something in the fuel delivery system. The noise is definitely a pumping/thump type of sound. Which is why the first thing that came to my head that the fuel pump was going bad. But unfortunately we have a secondary pump as well and its normal although I still don't agree.

dnegri

Btw.....my dealer first tried to tell me that it was the purge solenoid as well until I made him sit in the car with me in drive but stationary and point out where the noise was from.........the back not the front. He determined that it was the fuel transfer pump.

Firemane

Well, I didn't get to lift either one of my C's today, having wrenched the hell out of my back moving servers at work last night. Gonna try next Sunday if I can survive "round two" of the data center move from hell...

Devereaux

Came across this thread from Google so I joined. My son gifted me his 2005 C240 4 matic with 151k miles. There is a thumping nose from the rear only when a) putting it in reverse and slowly backing up, b) coming to a stop at stoplight, c) slowly accellerating. Doesn't seem to do it until the car is warmed up some. Doesn't do it when shifting (tranny seems smooth) or going over bumps or when turning. Mechanic can't find anything wrong. Is there anything new in this thread? Right now seems like "drive it until something else happens" is my option.

vaxhu

hello to everyone. I had the same problem and at the mercedes benz service they repalced with a new torque converter, new propeller shaft pump, new oil shaft and other changes and the problem still exists.it happens always after 4-5 secs to a complete stop. Any ideas or solutions? Please

Devereaux

I drove it for a long time and the problem persisted. Especially when it was hot. When the weather was cool it was less. A couple of times it kicked so hard I thought I was going into the car in front of me. Bottom line was I never did find a solution, I just got rid of my old Mercedes. Sorry I can't help but I did drive it for a long time without anything really bad happening.

vaxhu

I have x204 glk 220cdi 4matic. The the car has 3 types of driving. E for economy, S sport and M manual. if i drive it on E or S the thud is heard almost everytime i come to a stop after 4-5 seconds. But if i use it on M i never get that thud/kick/sound.The strange thing is that when i stop while driving on M i dont hear any thud but when i changed on E (i was still stopped) , guess what after 2-3 seconds the kick is heard.
i have tried it for many times when i drive on M and i stop and start again there is no kick/thud, when i stop on the next stop and im still on M i wait for 10 seconds no kick is heard, just after i change on E 2-3 seconds later i hear this f***ing noise.

Any suggestions or help ?

thank you