Fuel pump question

So I finally had some time to get the car up on ramps and take a look underneath the back end. The previous owner had removed the tank and cleaned it out. I've found some pictures online of the fuel system but still don't understand it a whole lot yet and I don't want to start putting things back yet. Do the 2 pumps work in unison all the time or does one work while the other doesn't depending on the fuel in the tank? I see a large rubber fuel hose that goes from the external pump up and around the passenger rear wheel well and there seems to be a filter attached in the wheel well. I'm assuming this is a stock setup? Then at the top of the tank there's a fuel line that runs straight under the car towards the engine.

new928 wrote: So I finally had some time to get the car up on ramps and take a look underneath the back end. The previous owner had removed the tank and cleaned it out. I've found some pictures online of the fuel system but still don't understand it a whole lot yet and I don't want to start putting things back yet. Do the 2 pumps work in unison all the time or does one work while the other doesn't depending on the fuel in the tank? I see a large rubber fuel hose that goes from the external pump up and around the passenger rear wheel well and there seems to be a filter attached in the wheel well. I'm assuming this is a stock setup? Then at the top of the tank there's a fuel line that runs straight under the car towards the engine.

Nice looking 928!
Ahh, I did this job on mine last year.
I'm pretty sure that Both pumps run at the same time, but I didnt put the inside pump back in. What you describe is what I have on my 79. There's an accumulator in the wheel well too that keeps some fuel line pressure so the car starts quickly. The top line is the fuel return line coming out of the fuel distributor (if I remember right).
Roger (928sRus) sells a fuel line replacement set. You might consider changing the lines now since everything is out and because there's nontrivial pressure in the lines.
When you get to the old lines in the engine compartment, it's really hard to take off the coupling next to the firewall so Roger suggested that I use a dremel and cut the line and leave the coupling In place. Then attach the new line back on the coupling. Hope that makes sense! Have fun! I may some pics in my post. (Joe's 79)

Just a heads up on all this fuel discussion. I have a very experienced Porsche mechanic friend who once worked for Peter Greg on his 917. the guy is an aeronautical engineer and probably the smartest Porsche car guy I know. He nearly burned up his racer and garage by having a dud fire extinguisher close by when working on a fuel cell and pump. Fortunately he had three extinguishers in the garage. Two turned out to be duds. Fortunately for him, his car and his garage number 3 was the charm. PLEASE, no matter how smart you think you are, before engaging in fuel related repairs make sure you have triple redundancy on the extinguishers (and none of those bitty bottles for oven fires). Best insurance you'll ever have is placing them prominently around your garage. My friend said he performed olympic high jump across the car to get to number 3. Very, very lucky man. Be safe out there and drive'm like you stole'm... safely of course!

Great point Balz. When I get to the point of adding fuel to the tank I'll definitely do that.

Ok, so this is starting to make a bit more sense to me. I'm thinking I can add one of these (in the picture) into the tank instead of the pump. That goes from the tank to the external pump and then through the filter (I might have that backwards) , then to the accumulator in the wheel well towards the engine. Then there is a line that comes from the engine back to the top of the tank in the sender unit?

new928 wrote: Great point Balz. When I get to the point of adding fuel to the tank I'll definitely do that.

Ok, so this is starting to make a bit more sense to me. I'm thinking I can add one of these (in the picture) into the tank instead of the pump. That goes from the tank to the external pump and then through the filter (I might have that backwards) , then to the accumulator in the wheel well towards the engine. Then there is a line that comes from the engine back to the top of the tank in the sender unit?

Yes, you got it! Just to be perfectly clear, working from the tank forward, the rubber hoses part of the lines that you described, eventually connects to hard lines going forward to the engine compartment. The rubber hoses pick up there next the the firewall. All of these rubber hoses are of a particular rating for the pressure. You can't use just generic rubber fuel lines! Just get the kit from Roger. It has the right grade hoses already cut and clamps.
If you have any questions, just post and I can send you pictures from my 79.

When you get it all put back together, you can locate the fuel pump relay and the right terminals to jump so you can test that power is going to the pump(s) properly check for leaks and such without having to crank the engine.
BTW, I had to replace my sending unit too because it was frozen. hopefully yours works, but don't be surprised if it doesn't.

Martinss shared this with me when I started and it was incredibly useful:
"I find the pictures in PET invaluable Joe. Mostly I go to the Autoatlanta site and 'Shop hard parts' for the correct model year; yours would be: www.autoatlanta.com/porsche-parts/928-78...orsche_928_parts.php

The full PET Catalog is available in PDF for each model ( www.porsche.com/canada/en/accessoriesand...ginalpartscatalogue/ ) and I have it saved locally on my computer, but while it is more complete, most times the Autoatlanta site is more convenient."

I know you have probably already completed your fuel system work but I'm a newbie to this forum, but tons of 928 info ..the reason Porsche put a small helper fuel pump from the tank to the main pump is that fuel pumps push fuel forward from the outlet, fuel pumps were not designed to suck fuel from the tank, that's why they are mounted at the lowest point of the tank, the helper pump ( now like $700) was able to suck and feed the main pump and ran at the same time as the main pump...what I did when I deleted the helper pump -$70 filter or $700 pump..was get the high flow Bosch fuel pump and ditched the old pump- the one with the internal check valve- very prone to clogging, and putting on an external check valve-$20 so easily replaced instead of replacing a whole $200 pump. The high flow Bosch commonly referred to as the 044, same pressure needed for cis fuel injection but 300 liters per hour, and upgrade your power wire from the circuit board to 10 gauge and upgrade the ground wire ..Works without any problems and very quiet !!