Rack Damper Install
 

I've got a bad idle shake that is much worse when the engine is hot (like just getting off the highway) than on a cold start.
It's an 82 300sd so it has the old damper bolt.

It's got good compression, just did a diesel purge, haven't done a valve adjustment or changed the motor mounts yet.

I found a brand new bolt for pretty cheap and I'd like to just swap out the old one as I think it will solve the problem.
However some folks suggest changing the motor mounts and doing a valve adjustment before changing the damper bolt.

Why is this?
I can't imagine anything going wrong except tightening it too much and having a hard problem with cold starts...

The damper bolt is half the price of changing the motor mounts and engine shock, the rubber on the mounts seem to be in good enough shape (not gooey or cracking). I'd rather replace the damper bolt now and do the mounts later on than end up having to do both right away.
I do plan on doing a valve adjustment, but don't see why that would really change the picture.

A rack damper adjustment cannot cure collapsed engine mounts. Therefore, your observation of the engine will be severely compromised and effecting a positive outcome with the bolt will be difficult.

You might get some improvement............might not. But, any results that you get are invalid if the mounts are compressed flat.

So, if you do change the bolt, refrain from commenting on the results as they are basically worthless until the mounts are changed.

Motor mounts and valves that need adjusting can cause the engine to shake significantly. The valve clearances on these engine get tighter over time, which leaves the valves not closing entirely. This leads to reduced compression. If you have some valves sealing properly and others not you get uneven compression which will cause rough idle. Properly adjusted valves can help idle a lot if they are way out of spec. I'd highly suggest at least adjusting the valves if you haven't in the recent past. It's cheap and easy if you've got the wrenches. If you don't you can find another member that does and get a hand. If you want to come visit me we can do it.

You certainly won't do harm by putting in a new rack bolt, but you may not get the results you are looking for if the mounts are bad and the valves are out of spec.

I just made some wrenches last week, probably going to do the valve adjustment on Friday.

This is a real bad shake. Way worse than my wagon which definitely has lower compression and also needs new motor mounts.

If the valves are tight, will the idle shaking be worse when the engine is hot vs cold?

Valve adjustment is critical, motor mounts are a normal wear item. A rocking motor and shaking felt in the car typically points to motor mounts. The simple fact that you are uncertain as to valve adjustment status might indicate that the valves need adjustment.

Best of luck.

When I had the Hot Idle Shake I bought a new Rack Damper Screw but adjusting it did not help much; rebuilding the Injectors and adjusting the Valves took away 85% of the shake.
Replacing all the mounts and taking care of a small Air leak too away most of the rest.
At the end of it the Rack Damper Screw was able to work and remove a little more shake.

I believe Rack Damper Screw is there to remove a little shake on Engines with everthing else in spec.

I also believe an Engine that shakes a lot because the Injectors are shot and the Valves need adjusting shortens the life of the Motor and Transmission mount and may be one of the reasons the Crankshaft Damper shakes itself loose.

Adjusted the valves, the cold idle is a little bit smoother but the hot idle isn't any better.

Going to do the motor mounts & shock and the tranny mount tomorrow. I guess I'll wait on the rack damper.

Also, now there's a knock that I swear wasn't there before. It did have some slight knocking when I first got it and after I did a diesel purge it smoothed it out.

This sounds louder. Is there something I could have done wrong with the valve adjustment? Not tightening the locknuts enough or?

It sounds very much like an injector knock to me, but it seems strange that it's worse after adjusting the valves. I took a video to see what folks think:
300sd 002.MOV - YouTube

You can adjust the Rack Damper at any point you want to; you may be lucky and that is all that is needed.
I was just saying that in my Case I expected a lot from the Damper and found out it is not a cure for larger problems.

If the Valve adjustment helped up the Compression the knock could be from more Fuel burning.

Try loosening one Fuel Injection hard Line nut (and tighten it back up and move down to the next Injector) at a time and see if you can locate which Cylinder that knock is coming from.

Is the old style rack damper even adjustable? Everything I've read says to just replace it if you have the older style.

More fuel burning can cause a knock? New one to me!

I was actually going to try that injector test, but I was super exhausted b/c my new used jack that I just bought (low-pro Napa 4.5 ton!) got stuck and wouldn't lower, while it was holding up the whole front end by the cross member! Finally got it down after trying lots of different methods.

Yes. In theory, at least.

The Damper adjustment is limited to the travel on the spring loaded plunger in the middle of it.

See the DIY section to find out the undesirable effects of turning it in too far.

The newer one has a stronger spring inside.

Also I noticed that my Old Damper was worn some on the Plunger end.

My 82 shakes only during hot idle, rock solid smooth when cold. Replaced the motor mounts and shocks/shock mounts with all OE equipment from MB. Didnt change it.

Valves were adjusted, compression good, etc.

Sometimes it just is something else, be it the rack damper (OP, have you played with the original one yet to see if it made any difference?), air in the lines, etc.

Ive adjusted the rack damper before and never got the shake to go away completely. It isnt bag but it is noticeable and I want it perfect. No telling if it is the solution for many of these shaking situations though...

If you have the Stock Bosch Injectors Nozzles they are likely worn out and or plugged with Carbon.

The very tip of the Injector Nozzle #12 in the pic; the Pintel (sometimes called the Needle) is drilled with two tiny passages #14 and #15.

When those are plugged it keeps the Idle Fuel from shooting through those passages.
All the Injectors on my Engine and most of the used Injectors that I have pulled from the Junk Yard have had those passages entirely Plugged with Carbon.
When I had that Hot Idle Shake the Valve Adjustment and rebuilding the Injectors (with Monark Nozzles that do not have those little passages) took care of 85% of my shaking.

However, it is better to do all the free or cheap stuff like the Motor and Tranmission mounts before moving on to the Injectors.

Nope I have rebuilt, popped and balanced injectors from Sean Watts, with Monark nozzles. No change from before and after injector pull.

Does the knock in the video sound like the injector nozzles?

Not much left to check.

Air Leak?

Low Fuel Supply Pressure?

Or:
Long Fuel Pressure Relief Valve/Overflow Valve Thread
Fuel pressure relief valve adjustment - PeachParts Mercedes ShopForum

Just an update, I replaced the motor mounts, one of the engine shocks, tranny mount, and adjusted the current rack damper bolt.
All with no luck :(

Going to try to find a new rack damper bolt at the pick n pull tomorrow.

I see a tiny amount of fuel around a couple of the injectors. Fuel lines are new, maybe the hard lines are leaking a tiny bit?

I want to try this as I'm also only getting about 22mpg highway. How do I find the relief valve? I've been poring over the threads and there's lots of useful information but little about where it's located or what it looks like.

It's inside the lift pump somewhere?

The RD bolt (pin) may be just as worn out as your current one.

They are available new, I think, check the buy parts button at the top of the page.

Rack damper bolt/pin #000 074 26 74 and
the seal ring # 002 997 85 45.

If you're referring to the item in the pic, that won't cause low mileage.

Make sure your odo is good, check for fuel leaks.

If it's the newer (gold) style, then it shouldn't be. I don't want to spend $50 on a new rack damper bolt if that's not the problem.
Also, there was oil on the old one when I removed it, I'm guessing that's normal?

In the thread that diesel911 posted above, some folks reported an increase in fuel efficiency.
That's less my concern than the hot idle shaking. Looks like the valve came out of the top left part of the lift pump?

Yeah, a gold one should be okay, there'll be oil on it. Pull it and see how much pressure the pin takes before moving, should be pretty stiff.

Actually, my lift pump doesn't look like that. I pulled the banjo fitting and bolt off the backside of the IP and the bolt looks just like that one in the photo. The spring seemed to be in good shape but I tried to stretch it a little bit just in case.

Pressure on my current one is pretty good, about to head to picknpull, keepin my fingers crossed

So no gold rack damper bolts at pick n pull :(

There was a steel one that had lots of room left to be screwed into it's current IP and the spring is definitely stiffer than the current one. (although I pushed and not pulled, should I try the latter?)
Didn't make any difference.

I also tried a compression test today just to make sure. Glad I did because I was pleasantly surprised:
440 415 440 405 420 (front to back)
It's only got 153k miles on it but does have some blowby so I was expecting a little lower.

The cylinder with 405 compression (#5?) had a little bit of oil on the glow plug when I pulled it. Is that anything to be concerned about?

There's still a small amount of fuel accumulating around the injectors that concerns me.

I'm going to try to track down a gold RD bolt I can borrow from someone, but this really seems like a fuel issue to me. IP timing? Fuel pressure?

Adjust the valves and check the compression again, if you hadn't before the first time.

Already did. They were very tight. See post #7

Broke down and paid $67 for the new rack damper. Lessened the shake a little bit but not much.

I also replace all my injector return lines as they were starting to leak. I'm getting a buildup of vacuum in the fuel tank, I guess I have a clogged vent line. Doesn't affect the shaking if the fuel cap is off, though.
But, I think the pressure was causing the injector return lines to fail prematurely. When I started taking them off they were gushing fuel like I've never seen. When I pulled the fuel tank cap off the gushing stopped.

If I pump the primer pump I can hear air coming from somewhere around the IP but can't tell where.

This is driving me nuts

Oh, and all the fuel lines are good, no leaks. I also made a fuel tank in a jar to eliminate everything below the lift pump.

dang it, I read through this cliff-hanger and found that the last pages are all torn out...any conclusion? I might be stuck in the same novella myself, so I'd like to know how it ends.

Upped the idle a little bit and that's about all.
The shake went down a little but I ended up selling the car like that, I don't think the new owner even noticed really.

Now I've got a 1981 300SD and am having similar issues.

Haven't checked the relief valve spring in the lift pump yet, but have done everything else in this thread (and a lot more, actually)

The interesting thing is that it seems to be getting worse as the weather gets warmer