Lowered my idle to 800 RPM, then later stalled when exiting the freeway
 

I just installed a "new" engine into my 300SD, and it is performing pretty well with the exception of this idle issue. The engine came with the idle set at 900 RPM. I wanted to work my way down to 750, and used the adjustment screw on the side of the IP to bring it down to 800 and see how it would do. It idled well in my driveway and on the side streets leading up to the freeway. After about 5 minutes of freeway driving, I exited and the engine stalled. It also stalled on the side streets off of the highway, and I had to shift to neutral every time I approached a stop, and keep the RPM up to keep from stalling.

There were a couple of things I observed under the hood with the engine running at 800 RPM. If I rev the engine and then let go of the linkage, the engine will shake and barely stay alive once the linkage returns to idle position. If I press on the stop button just enough to lower the RPM to around 600 RPM, the engine idles fine. The fuel filters aren't new, but they aren't that old either. Any ideas?

It sounds like you have a slightly sticky accelerator linkage, just enough so that it wasn't really at the idle stop when you set it to 800, but is able to rebound on release to too low of a fuel setting. I would take the linkage apart, inspect all the joints, re-lube, and then re set the idle.

I agree with the man in Carson City...

...but I'd also check out the Bowden linkage from throttle to transmission - make it all clean - take it to bits have a ball.

It sounds like a rack damper adjustment is needed. When the idle speed is adjusted lower, the rack damper is now acting as the idle setting (the way I understand it). If you adjust the rack damper bolt with the idle speed lowered, you will achieve the smoothest idle. With the idle speed raised, the rack damper is not even in play (again, as I understand it). I have always lowered the idle speed first before adjusting the rack damper, and then raise the idle speed adjustment up so it is the same as the speed I adjusted the rack damper at (no higher). That way the idle is at the best quality and speed. This is all the way I understand it, and has been my experience.

Thanks for the feedback. The rack dampener bolt was the first thing I tried, but adjusting it didn't change anything. It appeared to be all the way in and I backed it out about three turns with no results. I'll go ahead and clean up all of the linkage. I've seen ATF prescribed as the best lubricant for the linkage, but I've always used bearing grease since it seems to stay in there longer. Any thoughts on that?

I had stalling long ago with my 300SD and it was the rack damper bolt. I ended up replacing it entirely and fine-tuning it. 0 issues ever after that.

I'll try the rack dampener bolt that I saved from the old engine. It's still not clear to me how the bolt locks into place if say you're 3 turns out from the IP. It seems like everything only tightens up when both the bolt and locking nut are flush against the IP.

If the bolt is yellow cad plated, it is the new style. The old style was zinc plated (silver). Many people do the same thing, and just adjust the bolt without lowering the idle speed first. You should be able to feel the mechanism acting on the spring inside the bolt if it is working properly when adjusting it. If the spring is worn out, a new bolt is needed.
#000-074-26-74
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I installed the damper bolt from my old engine which was also the newer style and started with it all the way in and then gradually brought it out while gunning the throttle and then letting it drop to see how bad the engine would shake once the linkage came back to idle position. I found a position about 3/4 of a turn out to where the engine wouldn't stumble too bad. Right now I'm at around 850 RPM, a little higher than I'd like, but I can live with it. I did a diesel purge and everything feels okay.

One thing to note is that I disconnected all of the horizontal linkage from the final vertical link that presses down on the IP and I still get the stumble verging on a stall after gunning it. This makes me think that the issue is somewhere in the IP or injectors. Could the lift pump also be the culprit? It's like there's just not enough pressure in the fuel system to maintain the idle for that 1 second after the throttle has been wide open.

Some observations:

- not all OM617s want to idle smoothly at the same speed, regardless of what the factory spec may have been when they were new.

- finding the ideal idle setting is often a dance between adjustment of the idle adjustment screw and the rack damper bolt.

- on some cars, the process can take a LOT of time to get just right.

- I was taught by a guy with 40 years Mercedes diesel experience that the idle and rack damper bolt should be adjusted when the motor is at operating temperature, preferably right after at least 20 minutes of highway driving (in other words, with the IP thoroughly heat soaked).

- just to check: there should be a flange or 'ledge' around the base of the idle adjustment screw, that acts as a stop for the vertical linkage in its upwards travel. This has been known to break off, and the result can be behaviour as you describe (or worse).

Okay, so I've ruled out linkage and adjustment screw. The tip of the screw seems to be fine since it has plenty of resistance on it, but will still depress when you push on the stop lever. Speaking of which, when I press on the stop lever half way the engine still idles without any problem, it doesn't start to stumble and almost die like when I rev it and then let off. I'm guessing that this leaves me with bad injectors, IP problems or a bad damper bolt. Both damper bolts that I've tried are the newer style and the tips on both seem to have a lot of resistance, but I don't have a new one to compare them to. I think that I'll install the injectors from the old engine since that engine idled well at 750.

Does the engine stop with the stop lever?

You can adjust it as stated in chap 30-300

This makes me think this is still a linkage adjustment problem rather than anything else

You might want to re-read the Rack Damper Pin adjustment procedure.
I thought you were supposed to set the idle with the Damper Pin backed out and then turn in the Pin only enough to remove any shaking.

I know when My own Engine had the hot idle shakes I bought a new Rack Damper Pin but it was rather ineffective.
I found that what removed most of the shake was a Valve Adjustment and rebuilding what must have been the original Injectors (this was at about 198K on the Engine).

The original Bosch Injector Nozzles have 2 tiny passages drilled in the Pintel/tip of the Injector that on mine and most Injectors I had pulled from the Junk Yard were entirely plugged off with solid Carbon.
The idle Fuel is supposed to shoot through the little passages down at the Ball Pin in the Precombustion Chamber.

A small Air leak or not enough Fuel Supply Pressure or suction on the main line could also cause the Stalling.

If none of the above stuff help if you go to the Super Turbo Diesel site ForcedInduction had some pics of the MW Fuel Injection Pump with the Back of the Governor exposed. In there is another idle adjustment screw.

However the idle speed is being established is it accurate? I might double check it with something else before assuming the 9oo rpms indicated are accurate.

Is the car trying to self launch a little at the 900 indicated or does the idle at 900 indicated sound fast to you?

Yes, The engine stops with the stop lever. I don't have chap 30/300 in my service manual, but after reading through chap 07.1.4 (410/1), it occured to me that something may be happening at the regulating shaft along the firewall.

This is the same amp that read 750 rpm for the last engine in the car and that engine idled noticeably lower. It's pretty smooth at 900, I just don't like the frequency of vibration when it's that high. The car isn't trying to launch itself, but it does feel like I'm holding the brake down firmer than usual.