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.

MVK · #8 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

#9 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 · #10 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 · #11 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

lrg · #12 05-22-2003, 12:48 PM

Surfblau,
Great color on the mount shocks. My local tech told me they were for shut down only but obviously he was not entirely correct. I wonder if the shocks help only with violent shaking or if it helps with the more minor stuff too. By the way, my new, gold rack pin dampener screwed in all the way and I could still start up without a problem just like you. On cold days though I found it took longer to smooth out on my morning startup so I backed it off about 1 turn. It still runs smooth and seems to be better at cold startup. For fun you might want to back yours off a turn and see what happens. No harm done if you just end up screwing it back in.

dieseldiehard · #13 05-22-2003, 07:09 PM

surfblau, what an intersting chapter in the life of your 300TD.
(You want to sell me the old parts that were good after all... ?)

Say I just saw Gus yesterday too! He is a rare resource isn't he?

Wanna hear my story? I had the IP for my TD (well, one of them) rebuilt by Gus last year. He did an asolutely beautiful job, complete with a computer print out of the performance, idle to test RPM, in multi-color bar graph form. Plus I got to see him doing his magic on a pump calibration station!

After installing the IP I always thought it was running rough, it had a slight loping when the engine was hot (OK cold) and a high idle speed too.
I wasted a can of Lubro Moly injector cleaner, finally changed injectors, installed another rack dampener pin (warranty by Gus), and a few other things I have intentionally erased from my mind.

The mechanic who installed it had said he couldn't adjust the idle speed down below about 920 RPM, which didn't really matter much until I had him install a new rebuilt transmission (due to a sudden surprise when the existing one locked up in low gear - another story!!) Now, the new tranny came up with a new torque converter which has a higher lock rate and caused the car to creep or maybe I should say, accelerate rather violently when the brake was released in gear. Clearly 900+ RPM was no longer acceptable, back we go to the drawing board.
Just as the mechanic said, adjusting the idle speed control made no change. So I go to see Gus for a quick consultation. In under ten minutes he showed me the workings of the IP and suggested that first the linkage between the IP and the rest of the throttle rack be disconnected to see if the idle speed or roughness was affected.
He also informed that if the rack damper pin was adjusted "in" too far, the idle speed would be excessive.

Voila! First the linkage to the IP was removed, and it was noted to be a little too long, causing the throttle to be pressing towards shut off (this is the same as pressing the manual shut-off button) and the damper was turned out slightly. Clearly a lash-up snafu.
Now the engine idles properly and smoothly and the mechanic gets an "F" grade.
Maybe we have passed each other on the freeway and didn't know it!
Dieseldiehard
'71 220 (gas) 4-speed manual "present project car"
'79 300TD w/ '85 turbodiesel engine
'83 300D
'85 300TD

#14 05-24-2003, 05:01 PM

This make total sense Surfblau. Thanks for laying it out so well, but aren't the mechanics supposed to tell us what's wrong.

dieseldiehard · #15 05-26-2003, 02:53 AM

Welcome to the group, 85turbosd.
Many of us here are shirtsleeve mechanics, weekenders, or DIY'ers that like to work on our own cars or have a mechanic do a specified job. I can't speak for everyone on here, but over the years I have been taken advantage of by shops that wanted to shotgun a particular problem, or worse yet pad the bill by performing some work that wasn't necessary.
If you trust a mechanic (I mean you know him well!) its probably OK to let him tell you what is wrong. To walk into an unknown shop and let them tell you what is wrong is open to ones own expectations. Some problems seem simple at the outset but turn out being rather complex in the end.
I am not faulting mechanics in particular as its often the shop manager or owner that drives greed, or in some cases doing a "better" job is what they set out to do.
Anyhow, one can get a lot of satisfaction figuring out on their own what is wrong with their car and fixing it without going overboard, and usually saving some money in the process.
At least an individual in this situation has no one else to blame when errors are made.
Dieseldiehard
1971 220 (gas) 4-spd manual 102600
1979 300TD w/ ’85 turbo engine 287690
1983 300D 234200
1985 300TD 202,820