Originally Posted by barry12345

I have no arguments with anyone posting other information. They all could be right.

Since you did not hear the relief valve open to discharge. There remains a problem in the fuel supply system. That will probably totally fail you at some point. Is it the problem now? No way to tell. At the same time if left alone will ultimately totally fail I suspect at some point.

Cheap and not hard to address getting the fuel supply back to normal. I call it a fuel system tune up. Even Mercedes literature recommends a periodic checking of the system.

Still some systems have gone about forty years with only fuel filters being changed. Still operational but many are compromised in many ways. A decent percentage of roadside breakdown issues will eventually occur with these fuel supply systems not being periodically checked.

Your system is already partially diagnosed. You cannot reach relief valve opening pressure because the lift pump valves are leaking too much. Or the relief valve is open already. Simple tests can indicate what it is.

I may as well describe the tests. With the return line from the injection pump removed. The primer pump should build pressure before any fuel is discharged. If the pumping did not build resistance before the fuel was discharged the relief valve is faulty. To verify this close the relief valve output. If you can now make pressure when pumping it is the relief valve for certain.

If not the lift pump valves probably need renewal. The check valves in the lift pump are letting the pressure escape back through them. Actually they will disable any attempt to build enough pressure if leaking enough.

The other alternative test to use. Is to put a gauge on the output of the lift pump. Cranking the engine should produce at least 25 pounds of pressure. When the cranking is stopped the pressure should not drop off to nothing in an instant. Dropping slowly is acceptable but with really good valves it should hold the pressure.

Another indicator is how quickly does the engine start after siting for a day. If almost instantly at 65F degrees or above. Chances are strong that the relief valve is totally closing and the lift pump valves are in good condition. The engine did not have to crank to get enough fuel available to start as it did not drain fuel back when sitting. If it where not for your description of how the primer pump was acting. I too would have suspected fuel issues.


Multiple engine rotation startups can have many causes. One often ignored is there is just not enough fuel sprayed by the injectors. Created by lack of fuel present in the base of the injection pump so the lift pump has to resupply and build some pressure before enough fuel can be sprayed to start on. On a tight system as soon as the engine starts to rotate fuel pressure in the base of the injection pump is almost instantly increased. Fully filling the injection pump elements.