1982 300 TDT loses prime

I have been posting about my starting problems with my 300 TDT since last fall. I have things pretty well narrowed down now, I think. I have been starting to suspect that the problem has to do with air
entering the fuel system or some other fuel starving problem.
My Benz has been parked and inoperative for a few weeks because of the starting problem. Yesterday a friend of mine towed me up a nearby mountain and then we turned around and removed the rope and then I began to coast back down the mountain. Once I reached 35 mph I turned on the glow plugs for a few seconds and dropped the car into low (automatic transmission). The engine started right up and ran perfectly. I drove around for a half hour or so and then drove back up the mountain, running the motor hard so it would get a little hot. I shut the motor off and started it a few times, and it started fine.
Today it is 22 C (72 F) here. I just went out and tried to start the car. It began to start normally, and then, just when the engine smoothed I released the starter and it died. Then when I tried to start it it acts like it is out of fuel. I can't seem to find anything wrong with the fuel delivery system and no leaks. I have replaced the fuel lines and the fuel filters and the priming pump. Once the engine starts, it runs great, so I think the injection pump must be working right.
What are the possiblities for fuel leaks? Is there a supply pump in the tank on these models?
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I believe you would see some air bubbles in the clear plastic fuel lines to the injection pump if there were an air infiltration leak.
Suggest you consider the engine shut down device and the "ignition" switch that controls the vacuum to the shut down device. The shut down device is a small vacuum powered bellows inside the aft end of the injection pump; there's a brown vacuum line attached to it. It works by pulling the fuel rack (inside the IP) to its OFF position (to cut off the motor's fuel). The bellows wear out and the device becomes (lube) oil logged and sluggish. Typical symptom is a trailing shut down and finally, no shut down. That could be the cause but then its controller, the "ignition" vacuum switch could also be the cause - for some reason not releasing the vacuum OFF signal to the shut down device.
If you simply pull the brown vacuum line off the shut down and plug it the shut down system will be made inert for testing (and you will need to open the hood and press the STOP lever on the throttle linkage to shut down the engine) - so the shut down can be checked at no $$ cost.
You've covered all the other usual suspects.
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When is the last time you adjusted the valves?
What is the compression presure?
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Yesterday. None of them were more than .05 mm out

I have not done a compression check. What range would I need to be in for a motor to be usable without a remanufacture?
Some one else suggested that it may be a problem with the vacuum diaphram on the shut off. I doubt that, but I will pull the vacuum line on that in the morning and give it a try. I doubt it though. My system holds vacuum for weeks if I don't use the car. I take very good care of this vehicle and make every effort to renew or repair anything that fails right away,
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As far as diesel engine compression... I believe you need at least 300PSI or so to get it started.
I believe the usual range is anywhere from 360 to 400 PSI and each cylinder within 10% of each other or lower
The PSI is tested at the third compression... three needle jumps.
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Thanks. I will see later today if my neighbor who has a lot of tools has a compression tester with the proper fittings for me to be able to check it.
I just was outside working on the car and found that there was a large air bubble in the return line from the bypass valve from the pump back to the big spin on fuel filter housing. I checked my Mercedes 617 engine manual and there is a test for pressure and some valves that are supposed to maintain proper vacuum in the system so the fuel flows around properly. I know that is a poor description, but I haven't worked with that system before so I don't really understand it very well. Are air bubbles in that return line a possible sign of pump failure or failure in the valves in that system I mentioned above?
Paul
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By the way, this engine runs great once it gets going, with good fuel economy and no excessive smoke. Also it runs smoothly and has good power. It is easy to pass at 80 mph and it goes right up mountain passes on cruise control with no problem.
And yesterday when I went to start it it began to start right up and then must have sucked some air into the pump and now is very hard to start.
Do you know any tricks for getting air out of the injection lines? I have opened the fittings on the fuel lines and used the priming pump to remove most of the air from that part of the system.
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Air bubbles in the fuel supply lines confirms an air infiltration leak - that's the problem here. Check all the fuel line connections - clamps, banjos, filters, hand pump etc. - each one from the incoming fuel line to the injection pump - one of them is loose and is allowing air to enter the fuel supply.
Hand prime the system, then crank the motor with the starter and hold the accelerator on the floor to get full fuel to the injectors to expel air from the high pressure fuel lines and injectors.
Information: Specified compression (checked via the glow plug hole) is 22 - 24 BAR (319 - 348 psi); the lowest compression is about 15 BAR (218 psi) and the maximum allowable variation between cylinders is 3 BAR (44 psi).
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218! No way... can't see diesel engine start up that low of PSI...
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The Turbo manual doesn't imply it would start if all were 218 psi; it says 218 psi is the minimum for a cylinder. So with a 44 psi maximum variation the other four would be higher.
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