Mercedes-Benz W123 Diesel Owners Repair Guide

This guide was deigned to help owners of W123 Diesels. It will help those who own Mercedes-Benz 240D, 300CD, 300D, 300SD, 300TD or any vehicle with a OM615, OM616 and OM617 Engine. Lack of maintenance is the most common reason for a catastrophic failure on old and high mileage vehicles.


Info About the W123:

The Mercedes-Benz W123 is a range of cars produced by German manufacturer Mercedes-Benz from November 1975 to January 1986. The W123 models surpassed their predecessor, selling 2.7 million units before the production ended in the autumn 1985 for saloon/sedan and January 1986 for coupé and estate/station wagon.

Like its predecessors, W123 gained the reputation of being over-engineered, well-built, durable, and reliable. Many taxi companies chose W123 for this reason, and they were common sight in Germany. Reaching 500,000 or 1,000,000 km with minor mechanical issues. Once the W123 reached the end of service life, they were often shipped to Africa and third world countries where they were highly esteemed for ability to travel on rough roads and further without frequent maintenance.


Model Designation Info:

Model designation corresponded to the engine displacement, chassis type (coupé or estate/station wagon), fuel type (for diesel engines only), and availability of fuel injection system (for petrol engines only):

  • C for Coupé
  • T for Tourismus und Transport (estate/station wagon)

No designation was given for the long-wheelbase: it was simply called "Lang" (long) in the brochures and order forms.

  • D for Diesel
  • E for Einspritzung (fuel injection)

When the diesel engine is turbocharged, TURBODIESEL is affixed to the right side of the boot lid.

As for the chassis codes, W123 is the saloon/sedan, S123 the estate/station wagon, C123 the coupé, V123 the long-wheelbase saloon/sedan a.k.a. Lang, and F123 refers to the bare long-wheelbase chassis as used for ambulances and other conversions.


Engine Info:

The world's first turbocharged diesel engine for passenger vehicle, OM617 A was finally introduced in W123 models in September 1979, exclusive to 300 TD TURBODIESEL for European market. The same engine is offered in the North America and Japan in 1981 as 300D TURBODIESEL, 300CD TURBODIESEL, and 300TD TURBODIESEL. 

The OM616 was a 2.4 L (2,404 cc) engine with power output of 48 kW (64 hp). In August 1978 the pre-combustion chamber was updated for more swirl and more efficient combustion. The new camshaft pushes the valves deeper so the air and exhaust gases have less resistance. The engine capacity was lowered to 2.4 L (2,399 cc) to satisfy engine displacement tax laws in Europe, but power output rose to 54 kW (72 hp). A similar technique power update around these time was also done on the OM615 and OM617

  • OM616.912 53 kW (71 hp)
    • 1976-1978 W123 (240D)
  • OM616.916 48 kW (64 hp)
    • 1973-1976 W115 (240D)
    • 1976-1985 W123 (240D)
  • OM616.936 53 kW (71 hp)


This was introduced with the W123 series. Power originally was 80 PS (59 kW; 79 hp) @ 4000 rpm, torque 172 N⋅m (127 lb⋅ft) @ 2400 rpm. From September 1979 - 88 PS (65 kW; 87 hp) @ 4400 rpm, torque 172 N⋅m (127 lb⋅ft) @ 2400 rpm.



The .952 is identical to the .951 apart from minor changes to fit the North American 123 chassis. It was also fitted to the European market W123 wagon, the 300TD. This was the only turbocharged OM617 fitted to a European W123. Power originally was 121 PS (89 kW; 119 hp) @ 4350 rpm, torque 230 N⋅m (170 lb⋅ft) @ 2400 rpm. From October 1982 - 125 PS (92 kW; 123 hp) @ 4350 rpm, torque 250 N⋅m (184 lb⋅ft) @ 2400 rpm. Vehicles sold to the North American market had EGR equipment fitted.



Maintenance and Inspections:

  1. Inspect or replace the air filter
  2. Oil Change
  3. Inspect or replace Belts
  4. Diesel Purge to clean out the fuel system. After the purge change the Fuel Filters. There are 3 Filters. Primary(Large Spin On) Secondary(Inline Filter) Tank Screen(Strainer Type) The tank screen can be cleaned but if its too dirty it must be replaced. Inspect for any fuel leaks while the car is running. The rubber fuel injector return lines are common to leak.
  5. Valve Adjustment!!! It is very important to do a valve adjustment. Its recommended to be done every 12-15K miles or 1 year. If you dont know when it was done last just do it anyways. Here is a link to my guide on how to do a valve adjustment:
  6. Hard Starts or white smoke on start up is a sign the Glow Plugs or injectors need to be replaced. I recommend to use a reamer to clear the carbon from the chambers, this helps improve glow plug life. 
  7. Injectors! I recommend having your Injectors Rebuilt. You can also buy new ones.
  8. Flush the radiator with a process called Citrus Flush. Refill with MB Coolant or Zerex G-O5. Green Coolant is Harmful to the cooling system!
  9. Check or Adjust IP Timing
  10. Inspect Timing Chain Stretch
  11. Check The Lug Bolts
  12. Inspect or replace the Motor Mounts and Engine Shocks
  13. Vacuum Leaks can cause the car to not shut off, the door locks will not work, transmission shifting issues and more. Check all your vacuum lines are connected and the rubber connectors are not torn. 
  14. Clean or Delete the EGR System
  15. Check Or Replace Rear Diff Fluid, Transmission Fluid, Power Steering and Brake Fluid
  16. Check brake pads and inspect Control arm bushings and steering components for torn rubber boots. Grease if applicable. 
  17. Inspect the brushes on the Voltage Regulator it is recommended to inspect them every 50K. Replace when the brushes are near 5mm or Every 100k Miles.



Parts and Tools for Maintenance:


Mercedes-Benz W123 Diesel Engine Tune Up Kit w/ Valve Adjustment Wrenches



Valve Adjustment Wrench Kit

EGR Delete Kit

Diesel Purge Kit

Lug Bolt Replacement

Glow Plug Reamer

Magnetic Engine Oil Drain Plug

Fuel Tank Screen Removal Kit

Magnetic Rear Diff Plug





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