A friend of mine just got a 1986 300SDL. It drives great as fast as you care to go when accelerating normally. If you really put the pedal to the metal, it shakes violently!! When you let off, all is well again, what is it? thanks

Is the shake RPM related? Or load related? If so when does it start and stop? With the car in park does it vibrate if you rev the engine to 4,500 or so? Describe the problem more please.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

it is load related, not sure of start and stop, only it does it the minute you accelerate fast, and will do it until you let off. revving at a stop to 4500 is smooth.
forgot to mention, their are leaks on top of the injection pump, around several of the six outlets. could it be? is that a hard fix?

Sounds like its a driveline problem. Either: flex discs (CHANGE THEM IF OLD) engine mounts....or tranny mount. Likely flex discs or center bearing/carrier if it happens under load though. Have it checked out asap!

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Sounds like a driveshaft issue. The more load is put on that shaft, the more it will shake. Had a center mount give me that problem before. The flex discs can cause it too.

As for the leaks, the SDL probably needs the delivery valve seals replaced.

Yeah its drive shaft related. I would stop driving it until you can get under their and inspect the flex discs, and center bearing.

The engine mounts and trans mount get tired causing everything to sag, leading to drive shaft problems. Replace the mounts, and troubleshoot the drive shaft itself.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200