So I've had 2 problems since I bought my 2008 C300 4Matic.

1. Air leak sound when brakes not engaged. You can hear it over the engine idiling. Goes away soon as you engage the brakes.

2. Rough Idle and excessively rough idle with accessory load (power steering, acc).

I was thinking issue #1 was probably a brake booster leak. It wasn't till today that I realized #2 is probably the same issue. If there is a leak in the diaphragm then the engine would have a Vaccum leak.
I'm resonably mechanical, but I've never replaced a brake booster. Anyone have any tips for doing it on the C300? Is it a pain in the *** job? I trust myself for than my local dealership so if it's not to bad I'll tackle it myself.

Cheers guys.

 

Got your PM. Remove the vacuum line from the booster & plug it. See what that does to your idle.

Replacing the booster is easy if it's faulty.

Sometimes noise is just a ruptured boot where the pedal rod goes through the firewall. We had this on another car recently. Let me see if I can find the thread.

 

I had the exact same problem, luckily I was still under warranty and they replaced the whole booster for me, at least it says so on the receipt (and the booster did look new). I noticed it happened right after a hard short stop.

 

This started as a hissing noise on another thread.

https://mbworld.org/forums/c-class-w...ghlight=brakes

Edit - and here is the other thread. Please excuse Ivanrad - he is non technical.

https://mbworld.org/forums/c-class-w...ghlight=brakes

So from a noise perspective make sure it's not a bad boot. If it's a vacuum leak go further. If you are mechanical the booster swap is easy.

 

Awesome. Thanks guys I will report back once I look at it a little further.

I was a little reluctant to work on the Benz with the lack of service manuals but a car is a car. The Germans can't build them THAT much different.

 

An Ate booster is an Ate booster

 

good game. my master cylinder is toast after 21000KM 1.9 Years. What is the retail price on that part? Luckly im still under warranty.

 

I'm out of warranty as of 4 months ago.. I swear HQ has a switch that sends a sattelite signal down to your car and triggers all of you're problems the second warranty runs out.

 

my idle is still very nice. maybe the vacumm leak isnt that strong yet?

 

I removed the vaccumm line over the weekend. Hissing noise dissapeared but there was no improvement to the idle.

I have 36,000km. Pretty shocked I'm having to diagnose these types of things on a Mercedes at this stage.

Next step for idle is plugs. After that. I have no idea.

 

Is your air cleaner sealing over the MAF properly? Pull it off & reseat it or the MAF hunts to get fuel trim correct.

 

i guess we all have to pay to play.

maybe get someone to run some star dianostic or try seeing if HQ will do warranty work out of good will.

 

From previous cars, the way it feels my first thought was MAF. I took it out over the weekend and cleaned it with some electrical contact cleaner. When I was reassembling I did focus on making sure the filter/cover was seated correctly over the MAF. I remember thinking what an odd setup it was.

I will double check though just to be sure. I'm not sure I mentioned it, but the rough idle/vibration is significantly worse in D on the brakes then it is in N or P. I'm assuming the engine is idiling the same either way but with the tranny in drive, it is just vibrating through the drivetrain more.

I've heard of that a few times, I've never had much luck myself getting the "Good will" treatment just out of warranty. Depends a lot on your dealer I'm sure.

P.S Thanks Glyn M Ruck for checking back and following up.

 

Its funny when I replied about the brake booster I didn't even notice the part about the rough idle, well just like the brake booster issue, my car has the exact same rough idle vibration issue!

Just like you described, it's horrible, when the engine is idling (warm at 600-700rpms) and I turn the steering wheel I get these strong pulses of vibration from the engine, seems like its from the load on the engine from the power steering. Exact some thing at a stop light in drive with my brake on I feel this vibrating strain on the drivetrain that goes away when put in neutral. When I am about to come to a halt at the stop light everything is smooth till I am fully stopped and I get a strong pulse of vibration from the engine for about 2-3 seconds and then from there on I just feel the smaller strain vibration. I took in in when it was under warranty for this twice and they didn't do anything for me. They said they checked the codes and saw no evidence of the "idle dipping" and they found nothing with power steering vibration, but now I'm out of warranty and it's there!!

here's my thread about it, Glyn had some good ideas which I mentioned it to my mechanic when having my trans fluid changed but he never really pursued it, soon I'd like to take it back to him and have him focus on it:

https://mbworld.org/forums/c-class-w...ping-halt.html -there's also some people that responded here with the same issue, one said they had their dealer "go over it with a fine tooth comb" and they found nothing.

It's nice to see I'm not the only one, I would love it if we could find a solution. This has been the only imperfection on how my car drives.

I see we have the exact same cars (08 c300 4matic), what kind of gas do you use? and has this vibration been there since you owned the car or did it develop?

edit: haha just realized your one of the ones who responded to that thread..

 

It definitely sounds like we have the same issue. I'm not sure if thats a good thing or not considering your dealership couldn't work it out..

I've always used 91. I've only had the car for 6,000km though. Bought it used. I've tried to think back to if it existed when I bought it. I really don't think it did, but at the same time with a new car you're a little more wrapped up. You don't start to notice things like that till a month or two in.

Did your dealership just not have a fix or did they not even acknowledge it?

 

When you unplugged the vacuum line from the booster did you remember to plug it?

 

I did.. Problem existed before I removed the Vac line also. I was only removing it too see if the booster leak was related to the engine idle/vibration.

The rough idle doesn't seek or dip like it usually would with a Vac leak either. Not ruling it out, but if it is a vac leak it's definitely not the hose connecting to the booster.

 

i use regular. but it shouldnt effect the brakes. 87 has a 91 RON rating which meets the gas cap requirement.

 

The advisor just said the tech drove it, checked the codes and found nothing. To be honest I don't know if that means they couldn't fix it or just couldn't find it. Then again the car really only does it once it's warmed up because thats when the RPMs drop to their lowest and I have found the roughness and vibrations occur at around 500-600. Perhaps they didn't test drive the car long enough to let it warm up.

Regarding Glyn's idea of a vacuum leak on the air cleaner, I noticed my car (08) and I believe 09s do not have a metal clip in the back of the air cleaner that slides over it and locks it down (but the plastic stop is built into the cleaner for it), I did notice that the 2010 and 2011 cars have this clip. All it does is lock down the air cleaner. I know if it came this way from the factory in 08 and 09 it should be okay, but ot may be worth the try of getting the clip somehow.

 

hmmm 500 seems a bit low......

 

I'm mistaken when I say 500, more like 550-650, it sits a tad above the 500 mark.

 

If the ignition system, coils & plugs are up to snuff & the fuel pressure is reliably > 3.8 bars I'm starting to think you may have a throttle body problem. The actuators chatter on some throttle bodies & the only fix is replacement. To make matters worse this condition throws no fault codes.

Good luck!

 

Let's hope not.. $900 throttle body on a "maybe" doesn't sound too fun to me. That will definitely be my last resort.

Haven't had a lot of time to go much further, and I'm waiting on my plugs in the mail. Yes, I had to order Spark Plugs on the internet as noone stocked them.

 

Glyn M Ruck.

What is your opinion on a sticky valve? I personaly would hope not at this mileage, but perusing around I seem to see a lot of talk about valve seating/sticking and idle issues for Mercedes engines.

As always though, you only see the negative on forums. Is it actually a common occurence on the M272 in your experience?

 

No - I've never EVER heard of a valve hanging up on a M272 engine. Only 2003 & early 2004 M271 engines in poor fuel areas like US & SA. Aus has some of the best fuel around & matches Europe after your latest refinery upgrades.

Easy check - Compression or leak-down test but I would not waste my time.