I've been searching this forum for the last few days and think I have pinned down how to describe what is going on with my newly acquired '83 300SD with 288k mi.

At start-up, she purrs like a kitten. After she warms up, she shudders at idle speeds. I can feel it once the RPM's drop to under 1K, the engine starts to fluctuate up and down.

I've been told and read that it is something to do with the rack dampner bolt, valve adjustments(which was just done 50mi. ago) and possibly the injectors.

She does not smoke and drives VERY well aside from this little hiccup.

This is my first Mercedes and my first post in this forum.

Hello all!

Is the fluctuation in idle speed a rhythmic one, roughly the speed of a ticking clock? Mine does that, but only in the winter. I'm waiting for the cold weather to see if all the work I've done has made any difference. Curious if there's a known cause as well.

If something other than that, you might try to refine your description or even post a video.

-Rog

A number of issues can cause or contribute to idle issues, including: injectors in need of overhaul, air leakage into fuel system, broken motor mount, the rack damper issue, or simply the idle not being set correctly. As these cars age not all of them "want" to idle at the same spec.

The rack damper bolt normally causes a rhythmic rocking motion that you can feel. To check if yours is original, clean off the back area of the injection pump and look down at the bolts there. If they are silver, you have the original and should upgrade (regardless of whether this is your issue).

If by rough idle you mean vibration and/or buzzing through the chassis, that is most likely a collapsed motor mount that is no longer isolating the frame from the engine.

Can you make a video and post to YT? Idle issues can be a bit awkward to troubleshoot via text posts.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Rack Dampner: Try re-adjusting it. Back it off and adjust your idles speed with the proper Idle Speed Screw and Nut to what on the Emission Sticker. On mine it is 750-800 rpms.
The turn the Rack Dampner Screw in until it takes a way some of the shaking. If you turn it in and it starts raising our idle appreciable it is adjusted wrong. It should have no effect are barley effect the Idle.

If you get a new Rack Dampner you will find the Punger is longer then the worn one you have and the Spring is stronger. However, a new one may or may not fix the issue because in my opinion it was only designed to remove excess shaking from an Engine that is with in specs.

Injectors and Valve Adjustment"
Of the 2 thinks I did that took away 80% of the hot idles shaking it was adjusting the Valves and reguilding the Injectors with new Nozzles.

You could re-check the Valve Adjustment but that is up to you.

I have posted pics in a lot of threads on this. The Stock Injector Nozzles have 2 small drilled Passages in them;for the idle and low voumes of fuel to go through. On mine those passages were entilry plugged sold with carbon.
Added to that if the opeing pressure are low and out of balance that just makes it even worse and then of coures if the Nozzles are worn out.
There is some concensus that at about 150k your Injector Nozzles are worn out. But, for sure they should be checked unless you know they were checked or rebuilt.

New Motor and Transmission Mounts took away some more shaking.

After I did all of the above then new Rack Dampner Pin/Screw was able to remove a bit more of that hot idle shake.

Other causes: The Fuel Pressure Relief/Overflow valve is what controls the Fuel Supply pressure inside of the Fuel Injection Pump housing. If the Spring is weak it will not hold the proper pressure and idle speed.

There is at least 2 long threads on the Fuel Pressure Relief/Overflow Valve. Over time the Spring inside gets squashed. If you have a valve that you can take a part you can go inside and get the spring (keep track of the Ball Bearing as that is the valve) and stretch the Spring till it has a free length of 27mm and assemble it.

An Air leak in the Fuel Inlet. In my own case the Fuel Hose was old and hard. The Clamps were tight but when it got hot under the Hood the Plastc Filter under the Hose warped and it would suck Air.
Replacing it with regular 5/16" ID Fuel Hose from a regular Auto Parts store fixed that and that Hose is rated for Diesel Fuel.

If your Hand Primer has a shiny Aluminum Body and a hard Plastic knob on it they are know source of some sreious Air Leaks.

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84 300D, 82 Volvo 244Gl Diesel

https://youtu.be/6gp4enmqtWg

This is my new problem. I'm not sure if the sound is audible. Please let me know if you can hear it.

Is the fluctuation in idle speed a rhythmic one, roughly the speed of a ticking clock?

Yes, sometimes. So far I have found no consistency aside from it always purring like a kitten when its cold. The last time I went to pick'npull I spent the whole time cruising at 70mph on the freeway for about 30-40 minutes and when I took the exit and stopped at the light, it idled just fine. It confused me, i was expecting it to shake and vibrate like it usually does.

Although yesterday it started making a very weird noise in park, drive and reverse(only when I've got my foot on the pedal though in reverse) it's still shaking and vibrating like it was before but now its got this really low toned grumble. I was thinking it may be the transmission mount?

I don't think it has to do with the fuel delivery system because I can accelerate like nobodies business.

And as far as I know, its shifting better since this new noise.

I tried posting a video yesterday of me shifting through the gears while parked but it doesn't seem to have made it.

I haven't checked the RDB quite yet, I'll do that tomorrow.

I also haven't checked the mounts yet. I just had that epiphany to check them today.

Do you think it may have something to do with the timing? And if so, is the timing adjustable on these cars? I had a 22re toyota truck before this and all i had to do was adjust the rotor cap and it would change the timing. This may seem like a silly question but this is my first diesel. I'm use to gassers.

Have you checked out your air filter housing by any chance? I know that sounds weird, but when I installed new (solid) engine mounts I had a terrible noise and vibration. Like, seriously worrisome. Then I took out all the bulky air filter stuff, which is something I was planning anyway since the filter housing mounts were long gone, and replaced it with a more compact custom setup. After that the noise and vibration was gone. I confirmed it by setting the old air filter box back where it was and sure enough it was rattling the whole car.

Even if it's not that, I guess my point is that it could be something simple and not so obvious.

-Rog

Thanks for the leads 911, like I said in a previous post, It has started making a new, very low toned noise along with additional vibration. It seemed to have started out of thin air while I was running errands the other day. It only makes the noise in park and drive and sometimes in reverse. It is more intense in drive than any other gear. It stops making the noise after I reach about 15mph. And if anything it is shifting smoother than before. I am stumped.

I see your point, I'll take my air filter off tomorrow for a moment and see if that helps. I may be thinking too hard about this whole thing.

A good friend of mine who has owned a number of cummins diesels told me not to worry about this new noise and that diesels are weird like that.

What filter set-up did you end up using btw?

I basically followed the lead of mach4 (using the large Superduty housing and filter):

http://www.peachparts.com/shopforum/3127556-post6.html

Although I wasn't able to fit mine in quite the same way due to the coolant pipes being in the way, which is a bummer because his looks neat. I don't generally post about my mods since touching the holy Mercedes engineering seems to be upsetting to most on here, but I have yet to regret anything I've done to make my car more enjoyable to drive and work on.

-Rog

Oh I just remembered. The only thing I have done mechanically to this car is changing the oil with Delo 400.

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"When the going get's weird, the weird turn pro." -Hunter S. Thompson

'83 300SD - 288k - Sarge

Good input above, if the shaking is violent, like happened to me, it was the rack damper bolt/pin, once replaced and "tuned", all's well.

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83 SD

84 CD

I've read the RDB needs to be adjusted after some good freeway driving. Is this true?

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"When the going get's weird, the weird turn pro." -Hunter S. Thompson

'83 300SD - 288k - Sarge