Air in Fuel Delivery Lines or Bad Lift Pump?

Guys - here's one that has me head scratching. I think I've got some air in the suction side leading into the fuel filters - 5psi on start up and then drops to 0 psi. The engine is a 3208 NA 210 hp (4000 hrs). The 200 gallon tanks are pretty low(60 gallons between the two sides on the sight glass) When coming across the bay, the engine lost power, the tach began to wander and then the engine died. I put the Admiral on the helm and was able to get it started and back into port by pumping the priming pump continuously to pull fuel into the lift pump.

It seems like it is out of fuel despite what I see in the sight glass(yup, I opened the sight glass valve!) I'm old enough to have done some shade tree mechanical but I'm no diesel mechanic. In the gas engines I have some experience with, there would always be a low pressure fuel pump delivering fuel to the carb where it was atomized. Does the diesel lift pump provide low pressure suction of fuel from the tank AND high pressure fuel delivery to the injectors?

Does a low fuel condition make t more difficult to get fuel to the lift pump?

Today I changed all the filters, and tomorrow I'm going to attach a small pump to the tank/primary filter line to confirm fuel flow from the tank and that the pick up isn't clogged. Then I was going to pressurize the line between the primary filter and the secondary on the engine and see if there is a pressure drop anywhere along that line. Other ideas/suggestions?

What would the symptoms of a bad lift pump be?

Thanks for any/all info

I would think your lift pump is gone.Your tanks are low so you might have been running on gravity flow before .Just a thought.Best of luck pin pointing the problem.

Could be a lot of things, as well as a bad lift pump. I've had two pumps fail over the years. In both cases, it continued to pump oil just fine. But put part of the diesel it was pumping in the crankcase lube oil.

I would have suggested changing the fuel filters as a plugged filter or air leak can overwhelm the lift pump and cause the engine to stall. But sounds like you have already covered that. Have you tried running the engine since changing the filters? Are you sure you are not sucking air from one of the tanks, if they're that low on fuel?

Your lift pump is most likely a diaphram pump just like on a carbureted engine and provides relatively low pressure oil to the injector pump. The injector pump is where the fuel pressure gets stepped up and goes through the ceiling.

It's usually the simplest thing that's the cause of the problem!!
LB

lift pump sucks fuel to it [maybe gravity assisted] and sends it under low pressure to the injection pump that distributes it to the injectors under high pressure, filters and water seperators in the system and a return line for excess fuel.

Thanks Guys - I pressurized the system and found an air leak in one of the tank selector valves. Whew! Thanks for the suggestions - you guys have saved lots of sailor's bacon over the years - appreciated!

Up and running again,good for you mate.Funny most solutions are so simple once you find the problem.Never have thought a selector valve would give trouble .