1977 300D 123 series - Mercedes-Benz Forum

 
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post #1 of 5 (permalink) Old 12-18-2012, 10:35 PM Thread Starter
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1977 300D 123 series

Earlier this year I bought a run down mercedes 300D in the hope of doing a few repairs and it being a minimally problematic... After spending much more money than I expected there is a persistent problem in that mostly the car runs fine but the car loses power and stalls occasionally on hills and has an extremely slow pick up when accelerating, road side assist has come out a few times saying its the pumping air through the fuel line and said its the fuel pump. Each time this has happened I've been able to drive it post after it stopping for a little with nil intervention. The mechanic I took it to charged me another $700 to not fix it (replaced fuel lines, cleaned out the fuel tank etc) and am back to where I started with the problem persisting saying that it is indeed the fuel pump/injector pump. I am wondering why its an intermittent problem and if any one knows a Diesel mechanic in the Brisbane area?? Or if I should just give up! Ive spent so much money already! I'm told its gonna cost $2000! omeone else said its just a matter of replacing the pintles? I don't know what to do!
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post #2 of 5 (permalink) Old 12-19-2012, 12:44 AM
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Did your mechanic change the fuel screen in the tank? Have you changed the fuel filters (prefilter and main filter)?

The problems sounds to me a clogged fuel system, not a faulty pump.

For the W123, there is a seperate forum:
W123 E,CE,D,CD,TD,TE Class - Benzworld.org - Mercedes-Benz Discussion Forum
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post #3 of 5 (permalink) Old 12-19-2012, 04:30 PM
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you could have a bad prime pump handle , seen this problem before , to test screen in tank , remove hose and let it drain in a bucket to see the flow . if you have the old style prime pump handle that screw in after use , you can actually feel its action while pumping fast .if tank is empty , the handle will move freely , if the spin on filter is plugged , the handle will be hard on the down stroke , if the prefilter or screen in tank is plugged , the handle will feel springy on the up stroke , if you release it , it will pop back down .
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post #4 of 5 (permalink) Old 12-20-2012, 03:30 AM Thread Starter
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The screen was actually extremely clogged which we thought fixed the problem but unfortunately the problem persists! I feel like I've spent too much money to keep going and on the flip side have spent too much money to abandon the project.
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post #5 of 5 (permalink) Old 12-20-2012, 04:54 AM
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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.
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