Your diesel tank can be invested with microbes. This causes the screen to clog up. Even cleaning the tank might not be sufficient. You need a microbe killer.
This is certainly not a problem with the injection pump (IP). Repairing an IP is certainly very expensive, but they almost never fail. On the side of the IP is the lift pump:
Yours might look a bit different, but it is basically the same shape. This pump is driven of the camshaft of the IP and it pumps fuel from the tank through the filter and to the fuel chamber or gallery of the IP. This fuel chamber supplies the high pressure cylinders with fuel and from the high pressure cylinders the fuel is pumped at high pressure into the engine.
The route the fuel takes is:
Tank > tank screen > pre filter > lift pump > main filter > fuel chamber.
Then there is a return flow. The fuel chamber has a relief valve and there is a fuel line attached to it. When the fuel chamber is under a certain low pressure, the valve opens and the fuel is pumped out again. The return fuel line connects at the top of the main filter with the return flow from the injectors and the return flow from the main fuel filter (at the top of the main filter housing there is a tiny hole, so that any air in the main filter is pumped out to the return flow).
The common fail points in the fuel system are:
- clogged tank screen, mostly as a result of a microbe investation.
- clogged hard fuel lines from the tank to the engine bay. Rare. Usually pressurised air will free them.
- clogged pre filter, can be observed. Replace if needed.
- clogged main filter, replace if suspected. It needs to be replaced as normal maintenance anyway
- leaking hand pump. The old screw-type hand pumps can leak. Replace with the newer push-button style pump. Buy a Bosch one.
- relief valve opening too soon. Screw out banjo fitting, extend spring of the valve.
- valves of the lift pump failing. Rare, but it can happen. The valves are in the fuel line fittings. Screw open and replace if necessary.
- fuel tank vent not working. Rare, but it can happen, depending on the tank vent. Try driving without the tank cap. If it improves, the tank vent isn't working. The tank vent is at the left rear wheel.
Some tests you can do:
- disconnect the fuel line from lift pump to main filter. Put a container under the fuel line and pump by using the hand pump. Is fuel pumped out? That means the fuel supply isn't completely clogged. Should be easy to pump, any resistance means that there is some clogging. If the hand pump is leaking, replace the hand pump:
Here you can see a lift pump, detached from an IP:
Here is the old screw-type hand pump, detached from the lift pump:
Here is a new push-type hand pump:
- After the first test, with all lines connected and when you pump with the hand pump, you must see fuel flowing through the return line from the IP. You must also hear the relief valve opening. If not, the main filter is clogged.
There are numerous threads about the fuel supply in the W123 subforum, with numerous tests and tips.