My lift pump broke the main spring and wasn't working at all for a while but the car still ran (not perfectly) on diesel but not on VO. I'm almost sure you could run it on diesel with a gravity feed of a foot or 2, probably could also with warm VO. Anyway it would be easy to set up a temporary setup to try it. It's unlikely the air is leaking into the lift pump though - everything is put together very tight before the piston and after that it's positive pressure. Could be leaking where the hose clamps onto the lift pump input.

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1983 300SD with bad undercarriage rust, with old greasecar 2-tank conversion. About 200K miles, just an adolescent but with premature bone disease.

On a single tank system, why did you defeat the stock lift pump and replace it with an elec lift pump? You said you removed the spring of the stock lift pump??? Will that allow fuel to still flow through it, being that it's a positive displacement piston pump?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I did it just for fun to see how well the electric pump delivered fuel and voila, it did. In my experience I have never had any air, leaking, burst hose, etc.., issues running a pressurized system with those pumps. That would be close to 100K miles with our cars alone. Many other people run the same configuration with the same results. It's soooo much easier than chasing air leaks.

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'70 F100 shortbed
'82 Diesel Westy
'83 Euro 300TD
Curtlo Viper
Yeti ARC
Surly long haul trucker

I'll have to try the pressure system at some point, but for now I have to stay with a vacuum system in the Mercedes. I haven't done anymore testing. I wonder if a leaky injector halfs is a source of air intrusion? One of my injectors still has a very minor leak after I put new nozzles in.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

That's on the positive pressure side of the system, so it should only leak fuel out.

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1983 300SD with bad undercarriage rust, with old greasecar 2-tank conversion. About 200K miles, just an adolescent but with premature bone disease.