Fuel Pump Opening Mystery

[Dennis Vogel]

Can someone educate me on what this hole on the bottom of the early fuel pump is for? I can't believe it would just have an exterior opening for no reason.

I did a rebuild and don't seem to have any extra parts from the kit. I went back and reviewed my photos from before tearing down the engine, and don't seem to have any angles that reveal what might have gone there. Though there were pieces missing from the engine, so that's not a real reliable indicator anyway. Parts catalog and workshop manual haven't helped me either.

Thank you.

[Jay Darlington]

i'm not sure but It could be to equalize the pressure at the diaphragm. its pumping on one side and needs equalization on the other side. one would think its a drain if the diaphragm were to leak.
Jay D.

[David Jones]

The 356 was not the only engine that used that fuel pump. VW did and I am pretty sure if memory serves me right that Opels of the day had that pump and no doubt many other vehicles manufactured in that period in Europe. This means there were applications where that hole may have been a necessity even if it were not on a Porsche or early VW. They all have it so accept it and go on.

[Craig Richter]

I believe Jay's got it. Take a loose pump and push on the actuator while covering that hole with your finger and you'll feel the little pressure pulses. I sure hope it's not a drain to get that flammable stuff all over my hot engine!

[Dan Epperly]

Hmmm, then why don’t later pumps have it?

[Mike Wilson]

Like Jay, I thought about pressure on the other side of age diaphram or, msybe a drip hole for any oil that might leak past the O ring?

Mike

[Craig Richter]

Marine applications go to great lengths to plum fuel pump vents so the bilge won't fill up with fuel if a diaphragm leaks. That could cause a huge BOOM! Porsche's method, or lack of a method, does seem pretty safe by comparison.

Good question Dan. I'll go down to the garage after dinner and poke around, see if I can find an answer.

[David Jones]

I think that considering the very large opening to the crankcase via the actuating rod opening and lever assembly that no pressure pulse from the diaphragm would be felt under the diaphragm. Rather what you may feel would be crankcase pressure pulses from piston movement which would be twice the frequency of the pump diaphragm which in steady operating state would more or less vibrate rather than stroke.

[Craig Richter]

Dan, The brand new late fuel pump I just looked at has two holes on the backside, but they seem to be covered by the red diaphragm fabric. Dunno how they work as vents that way...

[Bruce Smith]

Most mechanical pumps will have such a vent to prevent back pressure from interfering with pump pressure.

[Dennis Vogel]

It could be to equalize the pressure at the diaphragm. its pumping on one side and needs equalization on the other side. one would think its a drain if the diaphragm were to leak.

Sounds like this is the consensus Jay. Thanks everyone. I was worried I had lost a part or missed a step. Not used to empty holes...

[Al Zim]

With the hole you describe when the diaphragm leaks the fuel will flow to the ground instead of into your oil. Which should keep the engine from destroying itself. If the car is in the garage with your gas water heater then make sure that you have good fire insurance on your home.

[Eric Lenius]

Generically know as a telltale drain. In addition to limiting flow into the crankcase, it lets you know that there is a problem. Fairly common on diaphragm actuated valves, etc.

[Dick Weiss]

Referring to the so-called vent or drain hole(s), the A/B pump has 1-hole and the later B/C/912 pump has 2-holes--just under
the underside flange.