Theres a fuel return valve on tandem pump which should keep pressure at 14-15psi so obviously its not working or pressure is leaking somewhere else because when return line is pressed together engine starts to run normally almost instantly.
I think you should put a new one in. Not sure if it's the one I posted before, I just searched for the engine and pressure regulator. Fuel return valve as you call it is what I read talking about in the first place and searching have me the results I found before.
This is a decent little video on a 2004 Jetta, different car but a PD. Test should be similar. I lean towards the return valve either way, however, it could be two fold.
I took apart the Bosch tandem pump I replaced. I thought I would post a bunch of pics here to let people see what's on the inside. On the bosch tandem pump the check valve is accessible under the set screw shown here with an allen wrench in it. Under the set screw the check valve is a...
So the rebuild kits I've seen should have two springs for the inlet and return valves. Those springs wear and fail causing the valve to let to much fuel through. I haven't been able to find a valve, so I assume that you simply need to rebuild it, or replace the pump. Rebuild is much cheaper. I think the pressure test could be effected by the springs being worn and not holding pressure giving a false reading of the lift pump.
Also I ordered a URO lift pump from fcp euro I think it was, for around $100...
There's a fuel pressure regulator you need to check. First check the fuel pressure to the rail at the tandem pump.
You should always start a new thread and link the thread you find with information related to yours. However, I'm not sure the above thread will help you much.
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