electrical fuel pump losing prime or not sucking

[alpinems]

Hi guys, my car has been acting funny lately. After a period of time (5-10- minutes or even greater) I would just lose power and I would have to limp home. Ignition system is all new, timing's ok so we looked at the fuel. I pulled the fuel line off the carbs and measured the flow rate into a milk jug when cold for 30 secs. Then the next time it stumbled, made it home and tried the test. Aha! just barely spurting any fuel, maybe 1/10th the normal rate. Fuel pump going bad.

I put in the new Pierburg pump (and new filter while I was at it) but that didn't solve the problem. The pump runs fine but eventually (if ever) loses the prime and can't suck any fuel from the tank. I have a 3/4 - full tank of gas. The fuel lines from the tank to the filter and pump are new and not kinked so not pulling in air.

My next suspect is the fuel tank. Is there a screen or something in there that can be clogged? Where do I look next?

[DanOKC]

There is a small screen filter on the bottom of the fuel sender, it can come off & end up floating around bottom of your tank, it just snaps in place.

[c.d.iesel]

.....and WHERE IS THE PUMP MOUNTED ?

Above , or, below the gas tank ???

and

was this 'pump' install to cure the same problem your having now ?

also remove the gas filter and blow out it contents

into a clear jar - in the reverse flow direction to see what you have accumulated

[alpinems]

.....and WHERE IS THE PUMP MOUNTED ?

Above , or, below the gas tank ???

and

was this 'pump' install to cure the same problem your having now ?

also remove the gas filter and blow out it contents

into a clear jar - in the reverse flow direction to see what you have accumulated

below the gas tank

I pulled the fuel sender unit and the screen was laying in the bottom of the tank. I couldn't see any rust, but that doesn't mean anything.

And the "pump install" was a replacement of the pump already mounted.

That's a good idea to backflush the filter. I'll try that and see if it's clogged with junk.

[peteinjp]

Check for an air leak between the tank and the pump. If there is a slight dry/ cracked hose the pump will not prime and just suck air. Are all the lines tight and hoses in good shape?

[Beaner7102]

What about the tank breather hole. As you pump fuel out of the tank, air has to fill its place. If your breather hole/line is blocked the tank is in vacuum and the pump runs out of steam. Try removing the filler cap when it does it again, if it runs good again then you just found the problem. HTH Beaner7102

[alpinems]

I'd thought I'd update this with the resolution of the issue.

It turns out that the new fuel line I was running to the pump from the fuel tank was too soft and was collapsing under suction. My fuel line runs from the tank into the passenger compartment and then turns in a nice curve and goes to the pump which is mounted under the car. Even though I nicely radiused the curve so it wouldn't kink, under load from the pump it would eventually kink at the apex of the curve. That would prevent much of the fuel from moving forward and starving the motor. The new fuel line was standard 5/16 fuel hose from the local auto store.

Looking at the original BMW fuel line it was made of much harder construction, less likely to kink. I replaced the new line with fuel injection hose which was much stiffer, not as much as the OEM stuff, but good enough. The car has run great ever since. Hope that helps some folks down the road.