1992 124 Mercedes 300D 2.5 Turbo shakes at idle after about 20 minutes of driving
 

Hello,

I own a beautiful 1992 300D 2.5 Turbo which has treated me very well. Lately, however, I have come across a problem. The car shakes at idle after about 20 minutes of driving. I took it to my mechanic who suspected it was the OVP Relay and we replaced it but the problem still persists. We also suspected it may be the ABS relay but it was not.

The wierd thing is as then when the car shakes the RPM does not fluctuate at all, it still stays at the same spot. And, it is intermittent! It will sometimes stop shaking and then start again.

Would it be the wiring harness? Or could it be an engine mount? I am really worried about this car. Please advise:)

Cas.

Not the ABS relay. That is about the same as saying my coffee isn't hot because my toilet seat is up. Wiring harness is unlikely as well - you have mechanical fuel injection (and no spark plugs).

Motor mounts are plausible, but that would just be something that allows the vibration something else causes to be more pronounced.

If you have leaks at the individual fuel lines coming from the delivery valves on the fuel injection pump you could be getting differing fuel pressure to the cylinders. On a five cylinder motor that does not go over well.

Here is a thread worth reading; although, not for the exact same engine. http://www.peachparts.com/shopforum/diesel-discussion/344531-new-engine-mount-now-engine-rocking-bad-way.html

I had the same problem with mine...same model year and engine. I changed out the motor mounts and it fixed the problem.

And another thing, when the car goes into reverse the shaking is not there, but when it is in drive, there is shaking and when it is in neutral, there is no shaking at all, or in park. When it takes off, there is no loss in power at all, the transmission shifts smoothly and I just finished a long 1000KM drive in which the car returned me almost 1000KMs on one tank. Climbing mountains was no problem for the 2.5 Turbo, so I am really confused, as soon as the car stops at a light it starts shuddering without any fluctuation in the RPMs at all....

On my 87 with the OM603, I ran into the same problem and it was my motor mounts. The vibration and shaking were very different when I put it in reverse or neutral versus drive. I am guessing it was because of how the motor mounts had settled and the way it would torque the drivetrain in drive vs reverse. The shaking would also get worse after the engine warmed up (I am assuming because whatever hydraulic fluid was left in the mounts would thin out). Can you do a visual inspection and see if they are collapsed? I think yours would still have the hydraulic motor mounts like mine does.

The fact it does not happen in reverse essentially confirms that it needs new motor mounts.

When the car exhibits the shaking does the engine speed drop to 600RPM?
Do the RPM's drop at all?

Thanks Everyone for your advice. My mechanic changed the motor mounts which were very collapsed, and now the car runs smooth and there is no more shaking at idle. The Benz was also treated to a diesel purge and new fuel filters and is now quieter and faster. We had also bypassed the OVP relay as well just to be on the safe side. Reason being was when we touched the relay the idle would decrease significantly.

Hello TimFreeh
The idle did not drop at all. It was the engine mounts all along.

I guess I'm confused...... doesn't the last sentence of post #8 directly contradict the first sentence of post #9?

The OVP feeds power to the ELR circuit that provides engine speed idle control, when the ELR isn't operating the engine idle speed will fall to approximately 600RPM and it will noticeably shake. You may have needed motor mounts but I'm guessing that by-passing the OVP to enable the ELR to operate independently of the normal OVP relay was a bigger portion of the fix.

I would be careful bypassing the OVP relay. Not only will it blow the fuse to protect some of the control units, it also will clip off high voltage spikes that are not long enough to blow the fuse but are long enough to damage sensitive electronic parts.

Just a thought that is worth about $.02

Paul

Thanks for your help!
 

Thanks Paul, yes, we decided to go back to the OVP relay so that sensitive electrical problems will not be damaged. The car does have some vibration noise, but no shaking.