abqhudson

I realize this is a long shot, but, I have a problem that I need help with. 2009 CLK 550 convertible, 35,000 miles: I get a subtle buzz - sometimes much more pronounced than others - when taking off (C mode) under light/moderate load. Only between about 1500 - 1900 RPM right before the car shifts 2-3 or 3-4. I can feel it also when RPMs reducing after the gear change. The buzz/roughness is only apparent when it is very hot outside and when the A/C is on. The buzz is perceptible thru the steering wheel, gas pedal and floorboard. Made an appointment with the dealer and when I went, I could only feel a faintly perceptible buzz = not enough to demo to anyone.

The motor/transmission mounts have been replaced - no effect on my problem.

CPO car and I'm afraid that I may be experiemcing the prelude to a more serious problem.

Any ideas/suggestions would be greatly appreciated. Any hints as to how to isolate the problem?

Thanks for any help.

Rudeney

There are two things I notice on mine. One is an odd buzzy/brappy exhaust note that seems to occur at certain RPMs right before the transmission downshifts. Sort of like what you'd get in a manual transmission if you were in too high a gear. Also, I do feel some vibration from the A/C compressor through the steering wheel and pedals. I'm not sure why, but I think it has to do with the refrigerant hose routing through the dash specific to the cabriolet.

Fatz

Here is my 2 cents worth.... I have spent an inordinate amount of time with my exhaust over the past 2 years. What you describe is usually what you get (probably times 5-10) when you delete exhaust components and/or make it true dual. 1800-2000 RPM is the sweet spot for drone (as is 2400-2700) in exhaust modified high power cars. The transmission shift point depends on how you are accelerating, but will normally happen around 1600 to 2100 rpm. (Or for me around 5900 RPM sometimes...)

Since it doesn't seem that you have done any of these crazy things to your car, it could be that the secondary cat (resonators actually) are perhaps clogged. They could become clogged if the primary cats are going bad and shedding their guts.

If this is the case, the A/C would be just increasing the load on the engine and that extra load increases the torque which, in a modified exhaust, would lower the RPM start point of the drone and increase the amplitude of the drone. BTW drone is vibration of the exhaust system or components that cause an audible hum and can cause things in the cabin to vibrate as well.

My solution for all problems like this is to take the car out, select "S" mode for speed, and try to put your gas pedal on the top of your transmission.....

On your way to the work or the dealer, you can just run the car around 1900 RPM and listen for any vibration/hum. (mph/gear = 38/4 52/5 65/6 72/7 28/3). My differential is not OE so the mph may be a little different. If it is an exhaust related vibration, it will get worse when you climb a hill (due to the engine loading) at 1900 RPM.

Let us know what you find as a solution.

Rudeney

Fatz, I don't think, at leas on mine, that this is anything "wrong". i think it;s just normal. Mine has done it since I bought the car (at 27K miles) and I've driven many MBZ V8's that seem to have a similar "un-sweet spot" for exhaust notes. I do believe it is caused by a harmonic in the resonators. These are on the car to tone-down the exhaust sounds without creating too much back pressure. As you mentioned, people tend to expose this even more when they do custom exhaust work trying to get a better sound. I myself have often thought about doing some exhaust work, but then I hear a guy in a Mustang drive by and think, "Do I really want my car to sound like that?" No, I prefer what I have. The CLK550 at full-bore throttle sounds more "exotic" than these Mustangs and chragers and such.

Yidney

I can't hear your buzz, but your problem sounds exactly like the one I had on my identical car. It had a buzz only when the air was on and only when it passed through a certain rpm range - like 1600 or so. Turn off the air and it went away. Never happened in cold weather. Mine sort of felt like a dental drill being held against the car frame. Mine was under warranty and they replaced half the ac system with no change. They consulted with MB and it was a power steering line. They replaced it and all has been good for three years. Mention this to your dealer. If need be, I can probably dig out the repair slip and tell you which power steering line.

abqhudson

Thanks to all - very much.

Fatz

Rodney, I'm determined to convince you to open that exhaust....

kb24forlife24

^+1

Rudeney

Yeah, yeah, yeah. You want my CLK to sound like a truck, huh? There are times when I do wish I had a bit more aggressive sound, but then there are times when I am glad it does not. A co-worker just bought a new 20015 Mustang GT. He let me drive it to lunch the last week and it was horrible - if I had to put up to that on my daily drive, I'd buy earplugs!

I recall one CLK owner using a controllable resonator flap, programmed into the Homelink button - maybe that's what I need.

mrbobby

I agree Rudeney. The burbble you get from the CLK v-8"s is something unique to the car. I can remember a number of times, my wife pulling away from the house (her car in the hospital or something) and thinking how sweet that burbble sounded. At WOT the sound was totally different but again easily recognizable as the CLK. My wife would tell me about the dog going to the garage door when I was a number of blocks away.

Fatz

Don't want to steal this thread from abqhudson, but I am about 2 weeks from some sound clips for you Rodney. Will be looking for your honest feedback - I don't think you will think it sounds like a "truck".... I think it sounds like a classy badass.... and it is..

BTW abq, what have you found out about your buzz?

abqhudson

I'm working with the MB dealer, we will see what happens. I'm thinking that it is probably something to do with the Power Steering hose mentioned above. Hard to convince the dealer to try something so odd, especially since it hasn't been too hot = more buzz, I think when AC working harder - and, the buzz has been so subtle when I have been at the dealer. I did find out that the AC hose and the PS hose are in extremely close proximity for a bit and maybe it's a routing problem - something needs a tweak??? We will see what the dealer wants to do (CPO Warranty). Will forward any news to this thread. Thanks again.

Yidney

If your dealer is skeptical just based on my description and sending you the part number, I could scan you the whole thing so they can see what all was replaced in the AC system before the steering line, and the MB tech's comments about the harmonic buzzing. Then it would not just be "some guy on the internet told me..." Or I guess with my VIN they can maybe look at it themselves, although I'm not sure all the comments make it into the system.

Yidney

But I will add that on days when mine did it, it was pretty predictable and consistent.