It was requested to repost this thread,
OH MY, brothers and sisters of the Benz, I am writing today with experience to be shared. I am so fortunate enough to drive a 1978 Mercedes Benz 300CD. This vehicle is a joy to drive, as most of you know the 123 was a serious bid for one of the best-engineered cars of all the Mercedes models. Although it is a coupe with no turbo, it scoots along, as it should.
I have recently done about all tune up procedures that can be done (adjust valves, check timing chain, adjust start of fuel delivery, changed all filters, changed injectors, replaced return lines, checked glow plugs.) Then I was really determined to make this car idle smoothly; I have heard the non turbos can be tuned to a real smooth idle. This was a 32,180.00 luxury coupe in its day.
So I went by the manual and adjusted the locking idle screw on the engine side of the pump, and replaced the cable for the idle adjustment knob on the dash, new motor mounts, new engine stabilizer, new transmission mount, new tires. ran diesel purge through the pump to insure cleanliness. This produced good results but not luxury smooth. The timing chain is approx 4 degrees stretched. I thought about putting a woodruff key in to counter the stretch but I have a new chain and feel this will insure the proper specs, as I don’t have a dial gauge.
Now as many of you in the forum are aware the Turbos were not introduced until 1982-1985? Anyway, what was also added was an idle adjustment screw (among other things) on the back of the injection pump. This gave all the turbo drivers the ability to make the engine idle smooth, due to a spring-loaded adjustment screw. The screw holds some sort of metering shaft stable so while idling, the injectors are getting a smooth consistent flow of fuel. Unfortunately, any one who owns a 123 from 1978-1981 does not have this handy feature, UNTIL NOW!
I went to my local shop that only works on M.B. ‘s. I spoke with the owner who never seizes to surprise me with the plethora of knowledge that he possesses. He suggested that I swap the plate off of the back of my injection pump with a newer one that has the idle adjustment screw installed. He said that they all have the same internals so by adding the screw, it will stabilize the metering shaft. So I looked on Ebay and found an 83 turbo injector pump, won the bid for $50.00 +$ 25.00 shipping. Received the pump, which included the updated injector pump plate! This plate has the vacuum shut off switch bolted in the top with four 10mm bolts. The remaining four flat head screws were removed and I was holding the improved plate. Installing was a little trickier but patience and several breaks enabled me to accomplish this trivial procedure. I replaced the supplied idle screw with the updated (gold ) version to insure trouble free operation. I went through the idle tuning procedure once more and amazingly the car is MUCH SMOOTHER!
I called to thank him and he stated that he thought that the dealership he once worked for had this replacement plate with gasket. I called the local (only) Benz dealer and asked them for a search of the plate, they new nothing about it or had any part number for it other than the complete injector assembly. The plate has no numbers on it. Anyway I have one, and it works, maybe a pump rebuilder could have a supply, or someone here could find out more about where to get them. $75.00 was cheap for my problems.
SO, any of you pre-82 123 diesel drivers that are just accepting the fact that when they come to a stop, they need to A) throw it in to neutral, B) set the idle too high, C) power brake it, or D) deal with the(((shakes))) in D. THIS COULD BE YOUR DAY. I am no mechanic, I don’t make a living working on vehicles, I may know a fair amount about the cars I own only because I care, I love to tinker, and I get a charge out of the accomplishments. Please read the above carefully, this plate is not the get all, be all of your (((shake))) problems. I have performed many maintenance procedures to this engine to get the results that I am experiencing. Each and all procedures help make the vehicle run better. ALL OF THIS IS JUST .02 from me to you, I hope it helps, and to all you Turbo 123’s go play with your ALDA adjustment? CD in SC
The timing chain is next so stay tuned, I wonder if I can balance a champagne glass on my valve cover?….mmmmmm

Be sure it is Romainian crystal filled with Dom Perignon!
Good job and thanks for the info.

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1982 240D 313,000 (4 speed)
1984 300CD 172,483
1985 German Shepherd Dog -Lacey- R.I.P.11/04/05

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