Fellow W123 owners, I recently purchased a 1983 300D that surges at idle. My current idle is set at about 900 RPM. However, I would rather have it set at 700 RPM, but the surge is more prevalent when I lower it to that speed. I have had the car die at that speed. The surge is intermittent, and usually is more prominent in the summer, after the engine has gotten hot, driven for more than 10 minutes. I must admit, the surge is much better than it used to be. I believe it is due to following maintenance issues I have recently addressed, based on reading various postings in this forum.

In the last few weeks/months I have:

1. Replaced motor mounts
2. Purchased the "gold" rack damper bolt and adjusted all the way in. This resulted in the smoothest idle.
3. Adjusted the valves
4. Bled the injector lines to be sure that there was no air in lines
5. Run one tank of Red Line fuel additive through fuel tank
6. Ran 2 cans of Liquid Poli diesel purge directly through the fuel pump (not just added to tank)

I must admit that the surge is better but not gone. Any other suggestions?

Note the following:
It runs very strong and has good acceleration
It shifts smoothly
It is just about to turn over 300K miles
It smokes some, but not excessive. Not dark black.

I have no other issues with its performance, except at idle.

Thanks for any insight. Willing to keep trying based on new recommendations, haven't given up yet!!

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02 E320 Black (198K)
85 300D Blue (147K)
83 300D Blue (299K)
Plus 2 300D Parts Cars
05 ML350 Silver (165K)
02 ML320 White (207K)

No. To adjust the rack damper screw you back it out and adjust your idle speed with the idle speed screw. Then you slowly turn in the rack damper screw until it removes as much of that surge/shaking as it can without raising the idle speed.
The rack damper screw was meant to remove some shaking from an Engine that is with in spec.

On mine the emission sticker has 800 as being the highest idle speed. But, it would not be unusually on a worn out engine for it to be like 825-850 rpms.

If you still have issues after that you need to look else where for the problem. Unfortunately there is no singular issue to point to.

What appears to be a banjo bolt on the Engine side of the Fuel Injection Pump Fuel Line is actually a pressure relief valve.
If you have the type that can be taken apart inside is a spring and a ball bearing that is used as the valve. The Mercedes manual has that you can carefully stretch the spring out till it has a free length of 27mm and put it back together.
If you have the type that you cannot take apart you have to get another valve.

Next is air.
If you still have the old style hand primer pump which has a usually white hard plastic knob on it (some are brown or black hard plastic) it is a notorious source of allowing air to be sucked into the system when they are old.
When you pump with the hand primer and it leaks fuel that is a indication the pump is worn out and could be the cause of a large air leak.

Look in the Plastic filter and see if you can see bubbles moving through it. If you can replace the Hose and otherwise troubleshoot where air is getting into the system.

Try idling the Engine without the Fuel Fill Cap on. If that helps your Fuel Tank Vent may be restricted.

A lot of the owners people bought cars from did not know the valves needed periodic adjustment or did not have the skills to do it themselves and would not pay to have it done. In use the valve clearances get tighter and tighter and has an effect.

Un-even compression. Do a compression check. If the Car has sat a long time and not been run accept for occasional starts you could have some sticking piston rings. There is various stuff you can use inside of the cylinder to free up the piston rings. Also not adjusting the valves has an effect on the compression.

Have your injectors checked. If you remove your Injectors and look at the bottom of the Injector Nozzle (you can clean it gently with a brass brush and Carburetor Cleaner) if you see that the metal has a burnt out indentation or that the hole is ragged that is a good visual indication your nozzle is no good and needs to be replaced.
The one in the attached pic is really bad and there is supposed to be a tiny hole where the red line goes and that hole is entirely plugged.

Beyond that the Injectors like your cylinders have a specific pressure range they are supposed to open at and the stock nozzles have that tiny hole I mentioned that gets entirely plugged with carbon and that tiny hole is where your idle fuel is supposed to come out of.

Well there is some stuff.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

A less obvious one is that crankcase pressure can push on the inside of your Vacuum Shutoff Valve that is on the Fuel Injection Pump. While the Engine is idling remove the Oil Fill Cap and if that has an effect clean out your Engine Breather.

Repair Links
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84 300D, 82 Volvo 244Gl Diesel

IMO, if you have an old school IDI diesel with uneven idle, the FIRST thing you should do is check compression. IF compression is the culprit then go for a leakdown.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

There was at one time two different rack damper screws,one with a stiff spring,one with.. a soft spring. The stiff one is hard to move.
If soft one adjusted all the way in and still lopes,stiff one was suggested.

Surging idle could be a governor issue inside the injection pump. If you where to post a video might help. There is a difference between a rough or loping idle and a surging idle.

Rising quickly and falling back. Like someone was randomly pulling on the throttle linkage and letting it go. Or does it just surge higher and stay there?

A surging idle to me is one where the engine idle speed is changing at idle. Depending how much of a difference is present when it is doing this to me seems important. Plus what it actually sounds like. I suspect a rapid change. This is where a video may help. I doubt it is something that can be adjusted out.

To me without the video it sounds like the engine gets more fuel then the fuel backs off. I think it was 911 that mentioned it could be crankcase pressure. I wanted to add that you should remove the vacuum hose from the injection pump shutoff. To eliminate any chance of erratic vacuum action there.

Plus critically check the transmission fluid level. Just in case the level is low and the transmission is loading the engine and unloading it. It was not surging yet a long time ago I had some strange effects when my transmission fluid was just a little low.

It probably is doing the same surging at the elevated idle rpms. With the transmissions increased loading at high idle rpms somewhat dampening the effect.

To attempt to diagnose this on site. A video or sound clip may prove helpful. As to me at least the amount of surge is important. The frequency of it and even the sound. Another alternative perhaps is letting a person at a injection pump repair place just have a quick listen to it.

1. I did 'reset" the damper screw but it did not help. I was able to remove the majority of the shaking without screwing it all the way in. But it did not resolve the rough idle.

2. I removed the pressure relief valve and measured the spring. It was about 18 MM, way shorter than spec. So I stretched it to 27 MM and replaced. Ran the car for a while with no rough idle but after a few minutes it returned. That is the usual condition. Able half the time it idles relatively smooth and the other half it is rough.

3. I don't have the old style primer pump. I have the black one. I do not see any leaks around the base.

4. I looked at the clear plastic filter and saw no bubbles. Maybe one single bubble at the top that never moves.

5. I removed the fuel filler cap but saw no change in the idle smoothness.

6. Like I said previously I just adjusted the valves. Some were tight, some were loose and some needed no adjustment.

7. I have not done a compression check yet.

8. I have not "inspected" the injectors yet. This is what I was planning to do next. From what I've read, you should not reuse the crush washers. Is there a place to buy just the washers?

9. Note that I did loosen the oil filler cap while it was running to see if it would "dance", which would indicate significant blowby. It did not dance!

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02 E320 Black (198K)
85 300D Blue (147K)
83 300D Blue (299K)
Plus 2 300D Parts Cars
05 ML350 Silver (165K)
02 ML320 White (207K)

Isn't the "gold" one the one with the stiff spring. That is the new one I bought a month ago. Should I be looking for something else?

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02 E320 Black (198K)
85 300D Blue (147K)
83 300D Blue (299K)
Plus 2 300D Parts Cars
05 ML350 Silver (165K)
02 ML320 White (207K)

Barry,

I will get you a video clip soon. For now, to answer your question in writing, I believe you would define it as a loping idle. The idle does not "get higher and stay there". It is more like a "rise and fall", but the primary issue is more the shaking/vibration than the RPM. It rocks side to side, so signficantly, that it can feel a side to side movement, sitting in the drivers seat. This is why I initially replaced the motor mounts because it seemed like it was caused by to too much play in the mounts, thereby allowing the whole block to move side to side. But the new mounts did not fix it.

Like I said it happens about half the time, usually after the engine has warmed up some. Like I said, the annoyance is not so much and movement in RPM as it is side to side rocking.

Will get a video clip soon.

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02 E320 Black (198K)
85 300D Blue (147K)
83 300D Blue (299K)
Plus 2 300D Parts Cars
05 ML350 Silver (165K)
02 ML320 White (207K)

To the best of my knowledge the gold one is the last update. If it is not curing the issue you have an other issue or issues.

This is from my notes on what I did.

What I had to do to get rid of the my when Engine hot vibration/shaking:

Raise the Idle speed within the normal range (this helped some; it was around 700rpm). I raised it to about 750 rpms.

I bought a new Rack Damper Bolt but it did not help much. I have a turbocharged 300D Tubo and it has a Rack Damper Bolt.

Rebuilt my Injectors with new Nozzles (This took away most of the shaking)

Doing a Valve Adjustment (took away some more shaking). If you can adjust the Valves yourself do that first except for the new Valve Cover gasket is free.

Changed Motor and Transmission mounts (took away some more vibration)

Found that I had a small air leak that showed up when the Engine was hot due to hard fuel inlet lines that did not seal even with the clamps tight.

Finally was able to use the Rack Damper Bolt to tune out a little more shake. (that bolt was made to reduce the shake on a new engine; not to compensate for a bunch of worn and out of spec parts.

It got the most benefit from adjusting the valves and rebuilding the Injectors next best was changing the Motor and transmission mounts. Like yourself I thought that the rack damper was going to cure the issue but it did not.

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84 300D, 82 Volvo 244Gl Diesel

My latest update:

1. I have tried to upload a video clip but get an error message. Sent a question to the administrator on how to get it to load. No response yet.

2. Plan to address injectors next. Just purchased a pop tester from Mercedes Source and am waiting for it to ship. Guessing there is a good chance I have some "injector issues" since this engine has 300K miles on it. I have only owned it for the last 3K and have no idea what level of injector related maintance was done in the past.

3. When I finish the pop test, I may need to "balance" the injectors. Does anyone have a recommendation on where to buy shims, if I need them?

4. Will do a closer inspection of fuel inlet lines to see if any could be leaking.

5. FYI: Tried removing the shut off valve vacuum hose while running. No change in idle.

6. Performed a visual inspection of the transmission mount while idling. Did not see any movement in the mount even though the engine was moving side to side. Also did not see any cracked rubber or collapsed rubber. So I don't believe there is a tranny mount issue.

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02 E320 Black (198K)
85 300D Blue (147K)
83 300D Blue (299K)
Plus 2 300D Parts Cars
05 ML350 Silver (165K)
02 ML320 White (207K)

The Stock injector Nozzles have a central hole that the idle spray is supposed to go out of.
The pop/opening pressure is adjusted with different thickness of shims.

In post number 12 is a picture of the central hole and a diagram of the passages and the whole injector.
W123 1985 300D - Shaking at idle

To lap the injector flat surfaces you can use a piece of glass as a lapping plate and at least 500 grit compound. Some say 1200 grit but where I worked we used 500-600 grit with no issues. OF course you need to clean off all of the gritty compound before re- assembly.

I got my lapping compound on ebay from a seller that sold lapidary/rock polishing stuff. The finest valve lapping compound you can get in a auto parts store is only as fine as 220 grit which is too course.

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84 300D, 82 Volvo 244Gl Diesel

Vince Walden how to rebuild IDI Injectors.
vincewaldon.com - HOW-TO: Rebuild Diesel IDI Injectors

Leaking Injectors after Lapping them
Injector problems - argghh

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84 300D, 82 Volvo 244Gl Diesel

A general attempt at an overall tune up unfortunately is pretty much mandatory. I never acquired even one example with evidence of maintenance. Some would have been done and perhaps others not. I just never knew.

For example your injectors could be three times past the Mercedes recommendation for periodic 100K checks. Or they could have been done fifty thousand miles ago. It is somewhat painful that we never know in all too many cases.

Establishing a baseline and proceeding from there does make sense. A compression test is first on the list. It lets you know that the basic engine is capable of running well. Instead of trying everything first and finding it is not.

You will have the injectors out for checking soon. It is a good time to do that compression check. The tool loan programs that are in place might have one. These are places where you pay a deposit consistent with the value of the tool. That deposit is returned when you return the tool. I have no ideal of how widespread that type of program is. It does exist in my area.

The harbor freight cheapy diesel compression kit is usable for around 25.00. The quality is not that great though. But far better than nothing. I have one but if I did it again I would try for a decent looking used one on ebay and similar sites. Or one from a free rental program. A lot of engines at this age and milage for this type still check out compression wise. To me it is ultra important to know if you have one. If you do there is something to work with. That is why it is called a baseline.

On mine the Transmission Mount was soft due to a leaky tail shaft seal.

Anyway the whole engine transmission unit sits on the 3 mounts.

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84 300D, 82 Volvo 244Gl Diesel