Yah - I know " when it doesn't pump fuel"... but I'm desparately trying to save the cost of a new pump.

73 914 1.7

The fuel pump was totally siezed from sitting with gas in it for a couple decades. I was working fine before the coma. We pulled the pump and removed the housing. As you would imagine, the spinner thingy and the roller bearings were all gummed up. Little bit of car cleaner and elbow grease cleaned it up just fine. Reassembled, bench tested ok - ie motor spun and there was a little splash from fuel down in the body of the pump.

We tested outside with a small container of gas and only the supply and discharge lines hooked up. It wouldn't pump. In fact there were bubbles coming out of the supply line. We kinda scratched our heads and decided to put it in the car anyway in case it had something to do with priming. Still no joy. BTW we completely rebuilt the lower fuel lines, cleaned the tank, replaced the strainer sock ... Just to be sure we pulled off the supply hose. It didn't gush immediately but as we lowered it we got a good rush of gas. Makes sense - the rear end is jacked up and there is only about a gallon of gas in the tank.

Oh yes - one other oddity. When it was in the car we broke the fuel return fuel line where it joins the plastic hose going back under the car. When we did this we got bubbles in the fuel tank.

So some questions:
Does the system need gravity feed for priming the pump? If it does then lowering the car may get it going.

There is a pressure valve between the D and R ports. In it's "at rest" state it blocks D and opens R. If you push it outwards (toward the crimp cap) D opens up and R closes. Is this the way it is supposed to work? (keep the lines pressurized when the pump pressure goes down?)

Can you point me to some place that would explain how that type of pump actually works? I can't for the life of me figure out how spinning roller bearing can create fuel pressure?

Is there anything else we can try before biting the bullet and buying a new pump?

Thanks
Steve

Last time I tried to rebuild a Bosch pump, same result--who knows? Anyway, don't spend the multibucks for a 90day wonder from Bosch...Autozone, Advance, etc sell replacements with lifetime warranty, so long as you purchase and "install" a new fuel sock in the tank. (the 914 filter being prior to the pump, just put the sock on the shelf with the receipt). Work great in my Fiats, and since they use the same pump (at least the later years), you should be fine for about $80.

good luck!

John Hisey
Two Fiats, 75 914, and an SRL311 awaiting reanimation

Thanks John. Not the answer I was looking for but it is good to know that it is not just me.

After your post I went looking for alternatives. There has been a couple of threads on the Mallory pump so I did some research on it and found a warning about mounting location to avoid loss of prime. I got excited and took the beast off the rear jack stands hoping that it would gravity feed and reprime. No such luck - still nadda.

So I'll just have to get a replacement to get on with the project - then go back and see if I can figure out the pump. I did find a short statement about the theory of operation at:


http://www.volkscafe.com/sp/8691V/8691Vtool.html

QUOTE

The fuel pump is a roller cell design. It is driven by a permanent magnet electric motor and is located near the fuel tank. Steel rollers are held in “cut-outs” on the rotor. Centrifugal force seals the rollers against the walls of the pressure chamber as the rotor spins. Fuel is trapped between the rollers and is forced out the delivery port. The pump is designed to be both cooled and lubricated by the fuel flowing through it. The pump delivers several times the amount of fuel needed to operate the engine at any time. Excess fuel is returned to the fuel tank via the fuel pressure regulator.

Sheer speculation here but I think that normally open pop off valve is to ensure that there is fuel running through the pump for cooling before opening the fuel supply (D) to the engine. The fuel enters the motor part of the pump through a pin hole fed by the the roller bearings and then up through the axle. Get's deflected through a journal to the pop off valve closing it and allowing the fuel to be delivered to the engine. My problem is probably one of two possibilities:

1. The pop off valve is too stiff due to crud in the valve. Feels pretty smooth but I have nothing to compare it to.

2. The clearance of the spinning disk is probably critical. Too much and fuel would blow by. If the O rings aren't seated right - pinched in the assembly - you probably won't get any pressure.

I guess there is a third possibility - that the brushes are worn and won't carry enough current to put the motor under load but I don't think that is the problem. It kicks pretty well when you put 12 volts to it.

Anyhow - nothing that money won't fix - probably true of anything in the 914

Here's a photo of the pump taken apart if you haven't seen it.

yuck - sorry about the size guys. Have to get out the photo editor before adding more pictures.