OM617 Shimmy - PeachParts Mercedes-Benz Forum

BHutch · #1 01-21-2010, 05:32 PM

The car in question is an 85 300SD with about 99k. I've been trying to trouble shoot a slight shimmy or lope that happens at lower RPM's but I wonder if I'm chasing a white whale in that the 617 is not supposed to be the smoothest of engines.

The Problem:
At Idle it has an extremely subtle lope(air cleaner doesn't move but sound is prrrr-prrrr-prrrr instead of prrrrrrrrrrrrrrr) that becomes more pronounced as soon as any throttle is given(even the smallest amount). It has a period of about 1 cycle per second at 800 RPM's. It smooths out slightly up till 1300 or 1400 RPMs then has a pronounced lope of about 2 cycles per second. The travel from side to side is between 1/4 and 1/2". Above 1800 it seems to be smooth.

What I've done so far:
-Adjusted valves
-Popped and balanced injectors
-adjusted IP timing
-motor mounts are about a year old but I have reason to think Indy did not use MB mounts
-Motor shocks are a few months old
-Fuel hoses seem to be tight and filters are relatively fresh

I guess a compression test is in order but I use less than 1/2 qt. of oil per 3000 mi. and most of that is leaks. No abnormal smoking. My last oil analysis din't show high levels of fuel in the oil (or anything else for that matter). MPG seems to be average. Starts very well in the cold.

First, am I wasting my time tring to iron this out?
Second, if not, what are the possible culprits that I've missed?

snookwhaler · #2 01-21-2010, 06:01 PM

Assuming you have ruled out and replaced the fuel filters. I had a similar problem. It turned out to be an air leak on the lines going from the primary filter to the lift pump. Fixing that leak fixed the problem. This was after I updated to the new rack pin and cleaned the tank screen.

You could pull the rack pin and see if the tip is worn down. Mine was worn down quite a bit, but was still working. I replaced it. You can adjust quite a bit of the shake out by adjusting that pin. It is a PITA and it is really hot around that oil filter. You need to adjust it only when the engine is "warm" (up to operating temps). If you peer down between the IP and the oil filter housing you can take a peek at the rack pin and see if you have any adjustment left between the lock nut and the pin. If so, try adjusting it first. The silver pin is the old one. The gold pin is the new one FYI.

Engine mounts can do it too, if they are worn.

Just some other things to check.

Diesel911 · #3 01-21-2010, 06:22 PM

You might that a look at the Overflow Valve threads and see if any of the symptoms match your problem.
Here is two of them:
http://www.peachparts.com/shopforum/showthread.php?t=244990

www.peachparts.com/shopforum/showthread.php?t=234609

BHutch · #4 01-21-2010, 06:38 PM

I forgot to mention that the rack dampender pin is less than a year old. I don't have a really high degree of confidence that I have it adjusted correctly, though. I was under the impression that it only affected the IP at idle RPMs. Could it have an influence at higher RPMs?

The IP overflow valve is intriguing but I don't think I have any fuel starvation symptoms.

I though about it some more and if the period of the wobble just above idle is really 2 cycles per second and not 1 as I previously said (I didn't measure with a stopwatch) wouldn't that about line up with a piston stroke at say 700 or so RPMs? Is it possible that I could have lower compression in one cylinder with not other syptoms? Only one way to find out, I guess.

toomany MBZ · #5 01-22-2010, 10:28 AM

^ Maybe the RD bolt needs some fine tuning.

BHutch · #6 01-22-2010, 02:19 PM

I tweaked the RD and it seemed to help a little. I still have a strange wobble that happens about 1300 RPM. I can't really imagine what would cause a shimmy at only a certain RPM. You can actually hear/feel it inside the car. Not a lot of travel from side to side but very noticable nonetheless.

winmutt · #7 01-22-2010, 02:55 PM

Rack dampener is just a bandaid for the underlying issues, unbalance delivery from the IP. That will cost you about $2-300 to get adjusted $900 for a rebuild.

toomany MBZ · #8 01-22-2010, 03:04 PM

A worn out rack damper bolt is just that, a worn out part. It could very well be a harbinger of things to come.
I did not have any issues above idle speed tho.
If you can, check the timing chain stretch.

snookwhaler · #9 01-22-2010, 03:42 PM

Quote:

Originally Posted by BHutch

I tweaked the RD and it seemed to help a little. I still have a strange wobble that happens about 1300 RPM. I can't really imagine what would cause a shimmy at only a certain RPM. You can actually hear/feel it inside the car. Not a lot of travel from side to side but very noticable nonetheless.

It sounds like you have 2 different things going on there. One at idle and one while moving. Are you sure "the wobble" is not driveline related? That almost sounds like you are talking about a driveshaft related problem.

BHutch · #10 01-23-2010, 09:22 AM

The "wobble" happens in park and can also be felt/heard slightly while in drive, moving. I'm curious to know more about the IP adjustment. I assume that requires removing the pump and taking to a bosch shop. Could a valve-related compression issue cause a vibration at certain RPMs?

BHutch · #11 01-25-2010, 05:21 PM

So, I listened around with a mech. stethiscope and noticed a ticking noise coming from the #1 cylinder. The ticking was loudest right next to the injector. All of the other cylinders had a very slight ticking but #1 had a TACK sound that seemed to match the phase of the wobble. I loosened the fuel line and the sound went away (but of course the wobble got even worse). Tightened it back up and the sound returned. I realize that to know more I have to move the #1 injector to another hole to see if the sound follows it but I was just curious if this information matched with anyone's experience. Also, does anyone know where to find a 27mm deep socket that will fit over an injector?

dadette123 · #12 01-25-2010, 05:37 PM

Harbor Freight for the socket?

winmutt · #13 01-25-2010, 05:53 PM

Lowes Kobalt 12 pointed. I have a variety of 27mm that WONT go all the way.

toomany MBZ · #14 01-25-2010, 08:21 PM

Craftsman 1 1/16 deep socket.

theo3000 · #15 01-25-2010, 09:49 PM

Quote:

Originally Posted by BHutch

First, am I wasting my time tring to iron this out?

Try the simple stuff, but it's easy to get carried away and start messing with things that have nothing to do with the problem.

I replaced the RD pin, motor and tranny mounts, engine shocks, injector nozzles, and fuel lines and was never able to completely eliminate the shake. I finally decided that the shake was the result of low(er) compression on one cylinder, so I've learned to live with it.

Good Luck

BTW - Autozone has a "Great Neck" 1-1/16 deep socket that works fine on the injectors. 6 bucks