1985 300D Engine Rocks at Idle - PeachParts Mercedes-Benz Forum

79Mercy · #1 07-12-2011, 02:51 PM

My new 1985 300D has a case of the engine shakes. When I first got it she would idle at about 850 RPM in park, and at that speed the motor rocks back and forth pretty badly. I lowered the idle to about 750 RPM and its much smoother, but it still rocks.

What is causing the rocking back and forth at 700-900. Its MUCH worse at 900. The engine is smooth at 950 RPM and up...

I just replaced the rack damper pin with a new Bosch one, little to no improvement. Has new Mercedes motor mounts, i just put them in a week ago.

I'll post a video shortly.

Thank you!

79Mercy · #2 07-12-2011, 03:18 PM

Here is the video, the camera is completely still, its the motor that's shaking around.

http://www.youtube.com/watch?v=FmMAd8spIZo

leathermang · #3 07-12-2011, 03:25 PM

Do any of the engine mounts you put on require the ' self made' tool to check to see that they are mounted in the correct position ?

79Mercy · #4 07-12-2011, 03:43 PM

Not that im aware of. Everything went together just fine. The new motor mounts actually made ZERO improvement, old ones weren't to bad.

The injectors don't sound that bad, but maybe that's whats causing the rocking... I'll have to test them. If those are okay then i don't know what could cause it to rock so badly.

leathermang · #5 07-12-2011, 10:37 PM

Some of these engines have very specific and quirky instructions on the motor mount installation.. including that tool for locating them correctly..
some say things like ' tighten this one....and shake engine with hand ..then tighten that one...

Diesel911 · #6 07-12-2011, 11:34 PM

Sound like you may have an Air Leak.

What I had to do to get rid of the my when Engine hot vibration/shaking:

Raise the Idle speed within the normal range (this helped some; it was around 700rpm). I raised it to about 750 rpms.

I bought a new Rack Damper Bolt but it did not help much. I have a turbocharged 300D Tubo and it has a Rack Damper Bolt.

Rebuilt my Injectors with new Nozzles (This took away most of the shaking)

Doing a Valve Adjustment (took away some more shaking). If you can adjust the Valves yourself do that first except for the new Valve Cover gasket is free.

Changed Motor and Transmission mounts (took away some more vibration)

Found that I had a small air leak that showed up when the Engine was hot due to hard fuel inlet lines that did not seal even with the clamps tight.

Finally was able to use the Rack Damper Bolt to tune out a little more shake. (that bolt was made to reduce the shake on a new engine; not to compensate for a bunch of worn and out of spec parts

Diesel911 · #7 07-12-2011, 11:41 PM

If you have the Stock Bosch Injectors the small drilled passages (#14 and #15) in the Injectors can become completely plugged with Carbon.

The passages are there for low Fuel quantities; like at idle.

Annual Diesel Purges may keep the passages clean but I believe if the are plugged solid a Purge may not help because the chemical will go around the plugged area.

79Mercy · #8 07-13-2011, 01:07 AM

Good point's Diesel911. I will be doing a valve adjustment on this motor soon. I will also rebuild my injectors soon. AFAIK they have 242K on them.

I will use a spare set of injectors with new Bozio nozzles that a fellow forum member is allowing me to use while i rebuilt mine.

I just find it strange that the motor only rocks at certain idle speeds....

carnut · #9 07-13-2011, 09:20 PM

Can you isolate a faulty injector by cracking open the hard line at the injector, engine running? Much like removing a plug wire from a gasser. As each line is cracked, there should be a noticable affect on the idle quality. The one that has the least effect is the culprit. If all react the same the problem is elsewhere.

leathermang · #10 07-13-2011, 09:22 PM

Quote:

Originally Posted by 79Mercy

...I just find it strange that the motor only rocks at certain idle speeds....

It only ' grooves ' to a certain beat.....

79Mercy · #11 07-13-2011, 10:00 PM

Quote:

Originally Posted by carnut

Can you isolate a faulty injector by cracking open the hard line at the injector, engine running? Much like removing a plug wire from a gasser. As each line is cracked, there should be a noticable affect on the idle quality. The one that has the least effect is the culprit. If all react the same the problem is elsewhere.

I did that today, they all seem to make it rock even worse that it already does.

I contacted my previous owner, turns out the motor hasnt had a valve adjustment in 25-30,000 miles....

thats probably why she rocks....

charmalu · #12 07-13-2011, 11:08 PM

When I started reading the first post, I thought "Valve adjustment".

Do you remember this thread I posted a few months back? that engine almost jumped out of the engine bay it was rocking back and fourth so much. Adjusted the valves, and it was soooo smooth you couldn`t hardly tell it was running.

http://www.peachparts.com/shopforum/showthread.php?t=295588

After doing that car, valve adjustments made a believer out of me.

Charlie

79Mercy · #13 07-13-2011, 11:17 PM

i dont have bent wrenchs, im gonna pull the VC hopefully in the morning and do a valve adjustment with straight wrenches, i have done it before, i guess i can do it again without bent ones haha.

colincoon · #14 07-14-2011, 12:13 AM

Transmission mount! It makes a huge difference, although my engine rocks a bit too.

79Mercy · #15 07-14-2011, 12:16 AM

Tranny mount looks fresh to me.