300TD Hot Idle Shaking - apparently solved - PeachParts Mercedes-Benz Forum

surfblau · #1 08-08-2002, 02:44 PM

300TD - om617 Hot Idle Shaking - apparently solved

So I have been using this board for several months to fix up my 1984 300TD, and things have been really working out. One elusive problem that I have been working on since day one is a moderate to severe shaking at hot idle only.

Symptoms:
Shakes (sways in the engine compartment, harmonically, side to side, wide osscilation) at hot idle in park or in neutral at 600 rpm.
Completely smooth in any gear at 900 rpm.
Runs smooth at cold idle at any rpm.
Always starts easily.
Never smokes.
Always idles smoothly (rpm wise), never hiccups, stalls, misses, etc.

In short, everything was perfect except the sometimes violent shaking that at times seemed to be lifting the wheels off the ground.

Based on what I could read on the board and what I could find out from other people around town, here is what I did in order.
- Changed motor mounts (one was 50% pancaked- no real improovement, but everyone says this is the first thing to check)
- Changed transmission mount (no real improovement, but learned the importance of liquid wrench)
- Changed the front shocks (I did this unrelated to the hot idle, but figured that there was a snowballs chance of helping - it didn't)
- Adjusted valves (no real improovement, but got a little more power and mileage)
- Diesel purged - twice (idled smoother, but still shook)
- Injection pump governer screw (rack damper screw) replacement (the new gold one) (no change, but could make the problem worse if I wanted to). Current adjustment is all the way in which seems as though it should not allow startup, but it starts easily
- Biodiesel - 2 and 10%, seemed to help one time, the other time had no effect
- Injection pump delivery valve seals - made problem worse, which was interesting, probably because of crud in the valves getting liberated into the injectors
- Injector testing - all had the correct opening pressure and spray pattern, testing was done by Gus at Pacific Injection who gave me some ideas on this post

At this point I was getting fustrated and was pondering
- compression test
- IP timing check
- IP replacement (in fact, I got a used one on ebay as a contingency ($50), but Gus (who has somewhat of a legendary reputation in injection pumps- and drives a 300CD) said it would be very unusual for that IP to ever need maintainence, and that a rebuild and bench test would cost a lot and probably not work in the end because when installed back in the car, his experience was that the car would continue to shake. There are just a lot of variables in the system)

Before I went that route, I figured that I would change the engine shocks. The techs at the dealership didn't think that they would matter and thought that they only affect shut down, but weren't too sure. The board here doesn't really talk too much about them. No other sources I could find seem to say much about it. Furthermore, when I was changing the motor mounts, I unbolted the shocks and they appeared to have at least some resistance, which is all the dealer techs said that they needed.

As a side note, Gus said that the 5 cylinder engines are notorious for having problems with resonance and shaking because of their design. This got me thinking that the shocks function was at least partially to dampen the ossilations of the 5 cylinder engine.

So I changed both engine shocks, but not the shock mounts or external rubber pieces.

And the shaking is gone, though there is still some vibration, but it seems fairly normal.

Both old shocks had only resistance to finger pressure, and had a grinding sound on compression or extension.

My guess is that at cold idle, the shocks still provided some resistance, but after 20 minutes of driving, the heat thrown off by the engine reduced their shock absorbing ability and the engine ran with it. Wide ossications were the result. ( it could still be a compression problem, or the IP could be crudded up, maybe someday when I have a garage I will get around to tinkering with it more.)

In summary, ENGINE SHOCKS MATTER.

I hope this helps someone like me in the future....

Holson Adi · #2 08-08-2002, 03:25 PM

Wow!
Thanks! I think I'm gonna replace my engine shocks too!

McRoth · #3 08-08-2002, 03:37 PM

Are there engine shocks on an 82 240D? I've never seen anything like that, just the engine mounts, but I'd be interested if they are there!

Thanks!

#4 08-08-2002, 04:11 PM

Wow! I would have NEVER guessed they were that important. I thought they were only for when you shut the engine off and the ensuing vibration thereafter. This post is going to help a lot of folks. Hey, if we were college budies, I would have moved you up to #1 on the speed dial for this one!!

#5 08-08-2002, 04:58 PM

Question. you show you replace the govenor screw? I am thinking this is the rack dampner screw, on the back side of the injector pump.
Good news on the engine mounts. I will be changing mine out soon. Get a little shake on the 78 300D. And I thought it was just old age.

surfblau · #6 08-08-2002, 05:20 PM

For me, governor screw = rack damper screw.

The naming issues on these cars sometimes get pretty bad. I am looking to do my "subframe mounts" on my 126 this weekend. Depending on which post, they are called

front rear subframe mounts (or bushings)
differential mount (or bushing or carrier)
rear (or back) subframe mount
etc.

Lots of nomenclature for two donut sized pieces of rubber and a brick sized rubber/metal frame. Just plain confusing.

Like I said in the post, most people I have talked to think that these shocks are intended to prevent sudden loads to the engine mounts, like during ignition and shutdown. I don't think the guys in d-land had that soley in mind when they designed the engine mounting system.

With regard to the presense of shocks on a 240. I don't know. If you have them, they will be attached through the engine carrier (the wing from the engine that bolts to the motor mount) near the engine mount. You can see them from below the car looking forward from behing motor mount (which should be an easy landmark to find).

later

Palangi · #7 08-08-2002, 09:20 PM

I think I read somewhere that only the 240D's with automatics have engine shocks.

Diesel911 · #8 05-12-2008, 04:30 PM

Quote:

Originally Posted by Palangi

I think I read somewhere that only the 240D's with automatics have engine shocks.

If this is true it brings up a suspicion that I have. I have been thinking that maybe it is something in the automatic transmission that is getting hot that is causing the shaking at idle when the engine is hot.
Is there anyone out there with a stick shift 5 cylinder 300D? Do you also have more shaking at Idle when your engine is good and hot?

Bajaman · #9 05-13-2008, 09:03 AM

Quote:

Originally Posted by Diesel911

If this is true it brings up a suspicion that I have. I have been thinking that maybe it is something in the automatic transmission that is getting hot that is causing the shaking at idle when the engine is hot.
Is there anyone out there with a stick shift 5 cylinder 300D? Do you also have more shaking at Idle when your engine is good and hot?

Yes, I do.

Diesel911 · #10 05-13-2008, 11:31 AM

Quote:

Originally Posted by Bajaman

Yes, I do.

Thanks for answering; another theory down the drain.

rcounts · #11 06-14-2008, 12:38 AM

What if you only have a mild case of the "hot idle shakes" sometimes?

Sometimes after a run on the highway, the first time or two that I stop, it will shake a bit, but after a couple of stops it pretty much quits doing it.

Now what the heck could cause it to come and go like that?

MVK · #12 08-09-2002, 12:25 AM

Excellent run down. You almost sound like a doctor dealing with an unknown disease. But the result was worth the trouble I guess!! Nice work Surfblau.

I have very similar symptoms, will check the shocks on mine too. Thanks

MVK

#13 08-09-2002, 01:44 AM

Engine mounts took a great amount of " torque shake" out of my engine (or rather off the frame) during acceleration and a little at idle also. I am very curious to see if your subframe mounts make a difference. Please keep us posted.

coleman kelly · #14 05-22-2003, 08:54 AM

Great post. I too have a shaking 300, side to side only at low idle, but mine does it when cold as well. Were you exagerating when you said the wheel almost came off the ground?
People have also told me that it's the injectors. Thanks for saving me great time and effort. bowlcole

surfblau · #15 05-22-2003, 10:25 AM

My shaking has pretty much disappeared. Yours could very well be injectors, or worse, differential compression. Or rack damper.

The easiest possible fix would be to drive it very hard (flooring the accellerator) every chance you get. After 10k miles, you should see some improvement.

This is the so called italian tune-up, and it seems to work over a period of time, especially when everything else is eliminated. It all depends on your desire to get covered in grease.

alec

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