mbe4000 fuel problem

You have a filter on the return line?

 

I've never seen a filter in the return line. (maybe that's not what you are saying.)
Does it look factory?
Is it a spin on filter because some of those have check valves/anti-drain back valves built into them. If it wasn't plumed incorrectly it would not make it back to the tank (or make it to the engine if it was setup to filter after being pulled from the tank.)
I can't see why you would need a filter in the return line myself but that's just me. Maybe they are common place

 

I know this thread is 9 y/o but anyone who comes across it will benefit from the information. I spent a week chasing low fuel pressure and fuel aeration on the return side and have studied and now understand this fuel system.
The MBE 4000 has a check valve on the bottom of the fuel filter housing that will CLOSE when the return line is removed for service or replacement. This eliminates dumping a quart of fuel on the ground.
The original poster said that the engine will run only when that line is disconnected. By doing this you are closing the valve and effectively taking the return circuit out of the system, causing pressure to build up in the return part of the system. It will then bypass to the suction side of the fuel pump by way of a built in bypass valve. This engine recirculates a lot of fuel in a short period of time so by removing (closing) the return line it will increase fuel pressure in the loop. My guess is that he had an air leak on the suction side somewhere from the tank to the pump or the fuel pump is internally bypassing and causing low pressure. The system while running should be about 60 psi at idle and 80 psi while at WOT.
I've attached a diagram of the fuel system. There are two metered orifices in the system to allow for a controlled bleed of fuel to be returned to the tanks. Despite the two small (.080) orifices, with the backpressure on the return side about 60 psi, there is a lot of fuel returning to the tank.

 

This 2007 Sterling will not run.but will start with ether.One check valve was stuck open and had carbon in it and we replaced the filters at the engine and the water filter separator. We are not sure whether somebody may have installed a check valve at the wrong

 

Did put a new unit pump at the front of engine. I still pull lines and get no fuel flowing