On my 300sd, I have been having problems with hard start and plugging the primary filter. I decided to change the main filter and did so without filling the filter. Yes I knew better. I couldn't get the injectors primed. The pressure from the lift pump was 5psi cranking. I had a new paint pot with a pressure regulator, filled it with diesel, Set the pressure at 20psi using air pressure and cranking with the lines all loose, got diesel instantly at the injectors. Started right up. Fuel pressure was 20psi at idle and around 30psi at high idle.
Before this took place, I had disassembled the lift pump and cleaned it up.
And had 5psi at cranking RPM.

What should the fuel pressure be at cranking RPM?

Thanks, Tom

Here's the FSM chapter (for a W123) for the OM617 turbo lift pump test

http://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/617/07_1-145.pdf

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

At the lift pump around thirty pounds. A seriously restricted tank filter can keep it low as well remember. Dead heading the lift pump is fine and will do no damage.

I would suspect at even just twenty pounds at the output of the lift pump there is a problem. I think the best quick test is to disconnect the tank line at the tank and look at the flow. It should be a decent gravity flow.

Also make sure the tank vent is functional or the vacuum accumulation in the tank will fight the lift pump until it is unable to provide any pressure at all sometimes. The pressures quoted in the factory manuals are rock bottom I would think.

When you factor in the fuel filters resistance to flow etc. A good relief valve wants at least 14 pounds pressure to open. So on average a pump at one bar will not do it. One bar being about 14 pounds pressure. Without the relief valve open you eliminate the cooling effect of fuel being returned to the fuel tank helping to cool the injection pump. As well as putting the cigar hose function out of the circuit.