OM603.971 'rocking' or pulsating at idle

[glenlloyd]

My W140 has had this problem for a while now...it's not bad but gets worse as the weather gets warm. I'd like to get it taken care of. Also a friend with a twin 300SD says he has the same problem.

The engine will pulsate or rock at idle ever so slightly, just enough to make the car rock just a tad bit. It almost seems like it's matched to the pulsation of the lift pump. The condition seemed to be reduced (maybe) when I replaced the pump valve o-rings and seal washers but it's still there a little bit.

In the summer it's worse when the AC is on and engine heat is higher IIRC.

Off-idle there's no problem of the sort but I'd like to get it sorted out.

Any suggestions as to what the cause is? I searched through the forum but found nothing similar that I could draw from.

Forgot to mention the lift pump has been overhauled and IP has good pressure.

Thanks
Steve A

[NJ300sdl]

Might be the electric idle control. Unplug it at the pump and see if it helps. Mechanical base idle has an external adjustment.

[glenlloyd]

Originally Posted by NJ300sdl

Might be the electric idle control. Unplug it at the pump and see if it helps. Mechanical base idle has an external adjustment.

Thanks, I will look into that and see what I find.

Steve

[sixto]

Could be tired engine mounts. Is there at least 1cm gap?

Sixto
87 300D