Hello, I have been trying to locate the injection pump check valve on a 1979 300D but no luck. Can anyone tell me where to look ?
Thank you ✌

AKAIK, it should be located on the lift pump.
If you have a horizontal fuel feed line it is sandwiched between the plunger style primer pump and lift pump on the DS of the injection pump.
If you have a vertical fuel feed line, it is under the nipple for the fuel inlet on the lift pump.
It prevents drain-back of the fuel when the engine is not running

There is also a pressure release valve (fuel overflow valve) on the engine side of the injection pump.
It is located in the banjo bolt for the clear hard fuel return line, and is serviceable unit comprised of a spring and ball bearing.
It helps maintain fuel pressure in the low pressure fuel circuit of the injection pump.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

It (the suction valve) primarily prevents the fuel pump from pumping fuel back to the tank. Which is exactly what happens if the suction valve fails to function properly.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

They symptoms of valve in the Fuel Supply lift pump not seating well are if the vehicle sits over night it takes a lot of cranking to get it started.

If that happens let it sit unused over night again and before you start pumping a lot on the hand primer. If it starts OK that is an indication your Fuel supply Lift Pump needs to be rebuilt.
Depending on which pump you have kits were between $17-$37. Bosch kits are more then the generic kits.

However, a small air leak on the suction side could also cause the same symptom.

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84 300D, 82 Volvo 244Gl Diesel

I have not read of anyone adding a check valve on their Mercedes but just for reference there is inline fuel check valves that you can add on if that is what you want.

Some of them can be taken a part as in the pic. Some are metal but cannot be taken apart and there is also plastic/nylon ones.

I bought some plastic ones and some Metal ones on ebay but have never used them on anything. But, anyway eBay is one source and likely Amazon.

I have never seen any at Auto Part Stores nor even industrial hardware stores but it is likely NAPA would sell them or Mc Masters Karr or Granger would have them.

Where I worked we added one a similar type of check valve on a Caterpillar Truck Engine. We knew that the fuel was migrating back to the fuel tank and even pulling fuel out of the fuel injection pump housing but never figured out exactly why it was happening.
Installing the check valve in the fuel inlet line cured the symptoms cheaply and the Truck Driver was happy to get in the shortest time possible.

I have also read of inline check valves being used on Equipment where the Fuel Tank is a long ways away from the Engine.

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84 300D, 82 Volvo 244Gl Diesel