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  #1  
Old 10-17-2002, 05:52 PM
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Question Good idle but Rocking and Shaking after Hwy. Run

Hey all,
Recently my car has developed a shaking and gentle rocking motion after long-ish highway runs when stationary, like at a traffic light. Could this be a bad rack damper bolt? In the mornings and around town when the engine isn't being stressed too much, the car idles fine and doesn't shake at traffic lights. Actually, in the morning when it is just started, it sounds and feels great with just a small bit of vibration. Could it be bad motor mounts? I ordered a new rack damper (which I think is called an "Idle Adjust Bolt") from FastLane but have not installed it.

If it's not motor mounts/shocks, then I will try to install the idle adjust bolt (if you all think it is that) myself. If this is the case, what is the procedure for installing this bolt? I know that it's a 14mm and 12mm nut on the back of the IP. What I don't know is when to stop screwing it in. Thanks for the help.

Alex

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  #2  
Old 10-17-2002, 08:28 PM
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Interestingly enough, my car did the exact same thing until I did the Diesel Purge thing 3 days ago. It now idles nearly perfectly at all time. I used 1.5 cans and a clean mayo jar. I'd try it before messing with the rack dampener pin.
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  #3  
Old 10-17-2002, 10:50 PM
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people may feel that this has been belaboured to death, but I am having the same exact problem and would love to hear the responses.
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  #4  
Old 10-17-2002, 11:13 PM
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My 123 does it too sometimes but I've been afraid to mess with the rack damper pin because like yours Alex, my car idles really smoothly except after a hard highway run. I recently did a purge and it doesn't seem to make a difference. Ditto the new engine mounts I installed about 5,000 miles ago. Maybe I'm paranoid but I'd be careful fooling with the IP if it's running great 95% of the time. Unless you guys can convince me otherwise I think it's best to quit while you're ahead.
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  #5  
Old 10-17-2002, 11:21 PM
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Mine runs soooo smooth after a nice highway run. Oil pressure is nice and low at idle, temperature stays regular, has better response (faster acceleration and gas pedal is more responsive), and the idling is smoother. I don't do highway driving all that often so I always open that throttle wide open and shoot a nice cloud out when I get on.
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  #6  
Old 10-18-2002, 07:03 AM
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Recently I did new motor mounts (all three), new engine shock absorbers and their upper mounts (both), and a new rack dampener pin (last in the sequence of events).

The motor mounts helped some - particularly since I found the one on the driver's side had no bolts to hold it to the frame! But what really helped was the new rack dampener pin. In my research reading here I found that the new pins have a stronger spring, and that was observed when I had the old one out next to the new one - harder to push in the new one.

So I just guessed on how far to put the new pin back in. I threaded it in and kept going until I felt the spring tension just start to engage - and went in another turn or so.

Now the idle at all times is smoother - no difference whether the engine is cold or hot. Before the new pin the car's idle after a highway run was like a 60's hot rod, bouncing the car from side to side. Now I can fix the hood ornament on something visually and the car is not moving.

Unfortunately the new motor mounts lifted the engine higher and changed the angle of the driveshaft U-Joint, exposing the fact that it and the center bearing were worn out (after 325K miles). Did the center bearing and support first - and it helped - but now I'm dealing with a U-Joint replacement in a driveshaft where the U-Joint is not designed to be replaced. It is staked in place by punching the metal around it. Great. I could tell the U-Joint was worn because it felt like it had a "center detent" in one direction of motion. U-Joints should be totally smooth as they are moved around.

Car idles very smoothly now. But has a vibration in the driveline at middle speeds (45-55 mph).



Ken300D

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  #7  
Old 10-18-2002, 09:31 AM
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Ken,
Thanks for the great post. I will try changing the rack pin like the way you did it and see what happens.

Alex
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Old 10-18-2002, 10:14 AM
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Just a quick reminder- in case someone already installed a new rack pin, the old one is clear zinc plated (silver), and the new design is gold zinc plated. Installing the new one really helped my turbodiesel!
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  #9  
Old 10-18-2002, 12:46 PM
bigjoe
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basic diesel performance reply

I'm new here and i admit i have no experience on mercedes automotive diesels, but let me give u a nother possibility other than those i have seen. i have some questions for u.... how does the performance seem while running @ high speed? is there any noticable deficit in power? when returning to idle, does engine try to die out completely before settling on rough idle. if there is a power deficit thru the entire range of speeds and engine does try to stall when returned to idle, i would suspect that air may be entering the fuel system, causing an insufficient fuel supply and rough idle. a strong tell tale sign of this would also be extended cranking the first time u start it for the day. most diesel systems are not pressure fed to the injection pump and they rely on a closed system and suction provided by the transfer or lift pump to get fuel thru the primary fuel filter to the transfer system. if there is even the smallest of holes in this part of the system, u will notice it in the ways i have described. once fuel has made it thru the transfer pump, it is lightly pressurized and any leaks further downstream should be evident by leaking fuel. leaking nozle return lines can also let air enter the system, but usually only after shut off ..and then the symptom is the hard startup and messy driveway. again l&g, i am no mercedes expert, this is just basic diesel characteristics. hope this helps.
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  #10  
Old 10-19-2002, 03:08 AM
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Joe,
Thanks for the advice but it starts great, does not leak fuel, has great, smooth power and the idle is very smooth, it just rocks the car a bit.

Alex
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  #11  
Old 10-22-2002, 04:27 PM
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DieselHead,
Let us know when you install the rack damper pin. I'm curious to see if the newer pin helps at all. I was out in my W123 yesterday and it started shaking a bit at idle after some hard driving so I'm beginning to think you're correct in thinking that something else needs to be done. Since our problems seem to be identical and I've replaced the mounts and completely purged and changed all the fuel lines there's not much else left except for the engine shocks (which I am told won't dampen relatively mild rocking, they're meant mostly for shutdown) or the damper pin. Thanks.
LRG
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  #12  
Old 10-22-2002, 05:49 PM
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I'm with you guys. My '84 rattles the fillings out of my teeth. I currently have the rack pin in all the way. Ordered a new one and it just arrived.

Perhaps a dumb question, but do you install with car running or not?

Thanks

Don
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  #13  
Old 10-22-2002, 06:04 PM
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I believe the procedure is to replace the pin with the engine off but screw the new pin only part way (about 1/2 - 2/3 of the way). Start the engine and tighten the pin until it just smooths out. Check to make sure the engine starts properly, if it doesn't the pin may be in too tight. It can take a bit of fooling with to get adjusted properly. Anyone who has done this if you have more to add please do.
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  #14  
Old 10-22-2002, 06:06 PM
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Irg and Diesel Don,
Unfortunetely, I think I will be having my mechanic fool around with this spring loaded pin so I won't be able to speak to the actual procedure. AFter it gets installed, I will take it on the highway and see if anything changes.

Alex

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