mlandis

Hi, folks, when I start my 2007 SL550 in the morning, it makes a single loud THUNK a couple of seconds after the engine catches. It sounds like it comes from behind me, not in front of me, so I don't think it's coming from the engine.

It was too sporadic for the dealer to hear it until today when I was driving on vacation (of course) and when I took it in to another place, they said it was a normal sound from an older transmission. (My car has 275,000 miles on it. I'm aiming for a million.) I find that hard to believe, as it sounds like someone whanged the underside of the car with a rubber mallet. It's startlingly loud.

Has anyone had a similar experience?

Thanks,

Michael

Rudeney

Wow, that's some serious miles on the R230! How has it been in terms of maintenance?

As for the sound, I don't have much to offer, but maybe it's exhaust related? At that mileage, your cats are likely dead.

Heatwave

Have you ruled out flex discs?

mlandis

Interesting! I'd read under search that someone had a leaky fuel injector in an older E class that caused gas to build up and explode after it was pushed out of the piston. This is 2 references to exhaust possibilities. I'll have to look into this.

I feel like I've been lucky. She's had the usual SL problems - replaced the ABC/steering pump a couple of times, had 3 hydraulic lines blow over that time, replaced both accumulators, a sensor here and there, countless tires ;-). But the engine and transmission are still purring nicely. No body rattles. Then again, this is kinda a love affair I've got, and what would be annoying in a stranger is endearing in the ones you love, so I may be forgetting some things.

​​​​​​​​​​​​​​Right now she's getting into uncharted territory. None of the techs have worked on an SL with this many miles. ;-) This noise has stumped them, too.

What worries me is that I've had two serious hydraulic issues seconds after this thunk, which has me wondering if one of the valves in the valve blocks is sticking. Thankfully the impacted parts were srill under warranty. But nobody is willing to open up those boxes on an off chance, and I can't really blame them.

mlandis

I'm pretty sure they're fine. The noise is only at startup, no other time, and doesn't seem tied to the actual engine startup. It was also quite inconsistent to begin with, only appearing every few days or once a week, but whenever it did happen, it was the same loudness. It's only recently that it's happening consistently. So it hasn't been a gradual increase in severity, or a sudden start of the issue, which is how I'd expect the flex discs to go.

Heatwave

Lemme ask one more question, Mate: I'm assuming you go to your car, open the door, get seated, then start the engine ... and then you hear the clunk.

Have you tried just getting in the car and just sitting there ... not starting the engine ... to determine whether something in the suspension might be making the thunk sound?

ItalianJoe1

I'd suspect something in the rear ABC suspension, it does it before you shift out of Park correct?

next2pool

I wonder about that as well since as soon as you start the engine the ABC pump starts pumping and pressurizing the system. If you are still in park, no parts of the driveline will see any rotation or torque.

mlandis

I haven't done a specific test, but I know I've done different things before starting - plugging my iPod in, settling in, and it's consistently 2 seconds after start.

mlandis

Right, before I shift gears.
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Heatwave

Have you tried turning off ABC at the console sometime while driving the car. Leave it off when you shut down the engine. Then see if there's any change the next morning when you start up and shift into gear. I'm now with Italian Joe ... it's something in the ABC, including the struts.

ItalianJoe1

You can't "turn off" ABC, the pump is hydraulic and runs all the time when the engine runs.

I'd bet you have something in the rear valve block or pulsation damper that's failing.

On my CL, the suspension won't adjust until you move the car, I assume the SL is the same, so I don't think it releases the valves right away. But the system does pressurize as the motor starts, which sounds plausible in your situation.

I'd say have it on a lift when you start it and have someone try to localize the noise underneath.

next2pool

My 2003 will level as soon as I start it. It only sags a bit after sitting for weeks and not enough to do anything about it, but like i said, as soon as it starts it comes back up to ride height. I'm thinking the symptoms definitely point to the ABC system.

ItalianJoe1

Interesting, both of my '03 CLs and my friends '03 S600, if sagged down, will only pop back up to normal height if I put them in gear and move a couple inches. I guess the SL is different, I know a lot of the ABC system changes from the 215/220 to the 230 chassis.

kbob999

My SL does the same, it levels after moving a few inches, so I'd say the behavior is the same as the CL.

Lee Wireless

I believe Italian Joe is correct. The rear ABC components are located inside the left wheel-well and suspended with a rubber mount. My car was making a similar (although not as loud) until I changed the Air Reservoir. It only takes a few minutes to remove the wheel-well liner. You can then have someone listen and feel for the clunk. Good luck.

ItalianJoe1

The accumulator? The round black ball?

That would be my guess, it's got air trapped somewhere and as soon as it pressurizes the system, it pops back into there.

As above, gotta gain access and start it with someone checking back there, if it's a loud, repeatable noise, should be simple enough to track down.

Hakosuka

My 2008 CL 65 has started to do this. Did you ever figure out what was going on?

Bill Meador

my 95 c220 sometimes wont engage when i turn the ignition on then 20 seconds later there is a huge thump behind the passenger firewall, after which it starts normally , seems like an electrical relay perhaps the starter?
but i have no clue , never seen anything like it