Fuel Flow but No Pressure

#1

I’m having a start problem and bench testing the fuel pump as there’s no fuel to the fuel rail.

Testing the new pump (using 12v battery reading 11.75v) has flow but can’t produce 10psi. Unlike some posts here the pump looks to be in good condition.

Any ideas? Just a bad pump, not enough voltage…?

1989 325i

Thank you

#2

Sounds like a bad pump or blockage

#3

Check and see if it has a exterior fuel pump

#4

Actually it should and the fuel filter cannister .

#5

Thanks, but to my understanding only the earlier E30’s had two fuel pumps. Mine is an '89, so just the one in-tank fuel pump. There is a fuel filter canister under the floor below the drivers seat.

Thought it might be a bad pump so ordered a second once, same thing, less then 10psi. Used the same method to check my Toyota fuel pump and immediately went over 60lbs, so it’s not the battery or check system.

??

#6

Certainly the voltage is a bit low. With the car running you’d prob see 13.8V or so, depending on the condition of the batt and charging system. But it does not seem reasonable to me that the pump’s output pressure would be so low just because the V is a little low.

Tell us more about your bench test. Maybe it’s a bad test. If a lot of fuel is flowing during your test, the pump would not be able to maintain pressure. You will either get a bunch of flow out of it, or a bunch of pressure, but not both at the same time.

#7

When I test for flow I just run a hose from the pump into a bucket. When I do a pressure test I don’t have a way of minimizing flow, so I just run it straight to the pressure gauge. I bleed the system via the bleed button on the pressure gauge so there’s no air in the system.

With this method I got good flow and 65psi with the Toyota pump, and I get what looks like good flow and 8psi with the two BMW pumps.

Thanks