Violent shake at idle

[Lonesome]

So another question about the 79 300 SD I'm getting... that thing has a SERIOUS shake at idle but when you give it just a little gas it smooths out. Anyone ever have this problem? I'm thinking motor mount but just wanted to get some thoughts.

[dannym]

Motor mounts
Rack Damper
Valves need adjusting
Injectors cleaned/tested
Timing
Fuel supply/filters
Air intake/filter

Take your pick

Danny

[JimFreeh]

There was an upgraded rack damper bolt to correct this problem. The one I have is gold anodized in color.

Jim

[toomany MBZ]

Originally Posted by JimFreeh

There was an upgraded rack damper bolt to correct this problem. The one I have is gold anodized in color.

Jim

The described problem does sound like an air leak vs. RD bolt, if constant. I cured an always flucuating idle, not violent shaking, by replacing the RD bolt. I have experienced said shaking in the CD and SD occasionally, and chalk that up to ULSD. Won't hurt to install new motor mounts.

[Lonesome]

I'll try the motor mounts first. I really dont think its the valves. When I give it just a little throttle it is smooth as silk. Excuse my ignorance but what and where is the RD bolt, I want to check into it.

[SD Blue]

Originally Posted by Lonesome

I'll try the motor mounts first. I really dont think its the valves. When I give it just a little throttle it is smooth as silk. Excuse my ignorance but what and where is the RD bolt, I want to check into it.

Actually, valve adjustment is what I would check first. I'm not sure why you don't think it's valves unless you've actually checked. The problem is they tend to be too tight.

The rack dampner (RD) bolt is in the middle of the aft side of the IP. It's a bolt with a locking nut. (a weak rack dampner shows up as a rough idle while warm)

(BTW, just a little caution......refrain from using "gas" terminology such as "give it a little gas" or "throttle" as some will give you a little guff. We tend to enjoy "diesel" terms such as "fuel" and "accelerator".)

[toomany MBZ]

RD bolts were on turbos only. I don't think a '79 has one.

[carusle]

Mine acts the same as you describe when a glow plug is burned out. Quick and cheap to check and easir and cheaper to fix. Gook luck.

[veg_hed_300d]

at least on my 300 D turbo i adjusted the rack damper bolt and now it runs so smooth now...also adjusted the valves...

the link at the bottom of one of my old posts should help out...that is, if your 79 has a RD

Where exactly is the rack damper located...

[Lonesome]

Thanks for the input, mine is a turbo.

[Ether]

Originally Posted by toomany MBZ

RD bolts were on turbos only. I don't think a '79 has one.

All W116 300SD's are turbos.

[mespe]

could be a few things out of spec, or close to being out of spec,,,

[VegeMan]

I have a similar problem, so probably the fixes are the same as above. My '81 300D non-turbo has a rough acceleration. It idles smooth but the first part of acceleration (from 500 to 1000 RPM -just a guess b/c no tach) is rough. After about 1000 RPM it smooths right out. I thought it was my drive shaft because I know that needs balancing, but it is noticeable also while idling in park or neutral. I know a few of the valves are bit tight as I checked them the other day but did not adjust them (my mechanic is much quicker and equipped for that job). So, I'm thinking motor mounts or valves. Anything else worth checking?

[toomany MBZ]

Originally Posted by Ether

All W116 300SD's are turbos.

Learn something new everyday.

[scotty449]

Just went through the same issues chasing some heavy shaking at idle.

Replaced the injectors, adjusted the valve clearance, retimed the IP, checked for chain stretch, purged with cleaner, replaced the fuel filters, blew out the fuel line to unclog the strainer, checked the compression (325psi+/-5psi), nothing worked. Drove me nuts.

Then I replaced the injection pump. Voila. Smooth as silk now.