Polo35m

Hey guys I've got a new phenomena with my 08 S550. Occasionally and randomly when braking it feels as if the car is going to stall and then it downshifts and it feels exactly like you've been rear-ended. It's actually quite startling. The car is extremely well maintained I recently replaced the spark plugs and they air filters and it seemed like the problem went away but now it is back. The air filters were extremely dirty and I thought that that was the problem. But again the problem is back I should mention that I do live in Florida and it has been brutally hot here with temperatures in the mid-to-high 90s on the road. Any advice would be greatly appreciated. No engine codes whatsoever.

Sal_Monella

Wow, that sounds like what my car was doing. Does it "flare up" at gear changes?
Mine did, and then when it would shift up it felt like a Smart Car was rear ending me. Turned out to be a clutch pack in the transmission lost a retainer ring and was expanding way too much (the upshift flare) and then when that pack would collapse it would shift real hard. Metal shavings all through the trans and TQ Converter. My car had 83k miles on it.
$7500 later and it runs like it should.
My 2004 F250 diesel has over 200k miles and has never had any problems other than injectors..

Polo35m

Oh Geez!!!!I will wholesale this car before I spend 7500 it's 10 years old. I don't think it's transmission related because it shifts perfectly in both sport mode and comfort mode. I've been searching like crazy and the throttle body keeps coming up which I'm not ruling out but mine I'm like I said is clean as a whistle. My filters were extremely dirty and when I first changed them the problem went away but now after a few months is back. I did further research and folks are saying you should use OEM Mercedes filters rather than aftermarket because the air flow is significantly better. So I guess it's worth a $50 investment to at least try.

The Thomas J

Would motor or tranny mounts cause something like that?

Sal_Monella

Great point! I had to change my trans mount and drive shaft coupler because of the damage that the trans did on it's way out.
Do some research on engine mounts Polo35m. Many people have had to replace them.

Polo35m

Brand new engine mounts and brand new tranny mount last year car drives absolutely Palm Beach perfect 99% of the time except when it does this stumble...idle down shift thing.

Sal_Monella

Is there a trans code that would indicate the Torque Converter isn't unlocking?
I know that on GM vehicles touching the brake pedal you can see the RPMs move up slightly as the converter unlocks, then the trans starts to downshift.