Mattman928

I suspect a plugged internal fuel pump (my car has sat for 3 years) so I want to pull the internal pump and check it for debris. I got the car running over the weekend, and it ran great, but yesterday when I started the car and let it idle is ran for 10 min and then the engine died. When I tried to restart it there was no pressure on the fuel rail and I didn't hear the pump running.

Now, I wish to drain the tank (I have about 10 gallons in there). I rased the car, removed the pump cover, disconnected the hose from the "Inlet" to the external pump and tried to drain into a 5 gallon container, but no luck nothing came out.

Not sure of the internal pump is a positive displacement pump thus not allowing the gas to drain, but when I read on the forums indicated it would be able to flow freely.

Help! Any thoughts?

G.P.

On my 87 the fuel drained slowly after disconnecting the soft hose from the external pump.
I was told that the internal pumps are prone to failure, so I pulled it out and replaced it with the internal filter only. You will probably need to change the soft hose also (I did). Part numbers are in pet, and Roger will probably stock them.

Mattman928

Can the S4 and GT models run without the internal pump?

G.P.

Yes, I have eliminated the internal pump and replaced it with the in tank screen (filter). This filter has a slightly larger diameter external hose connection, therefore you also need a new hose between the fuel tank and the pump. (That's what I found on my 87)

Kiln_Red

The external pump is the one that you should hear running. Pull the fuel pump relay and bridge terminal 30 to terminal 87 with a jumper wire. I'd be sure that the external pump runs first before I did anything else.

The fuel level sending unit is likely to be frozen so the gauge may not be trustworthy.

dr bob

Those living/driving in hot climates might consider replacing the internal pump rather than eliminating it. It's there to prevent vapor lock issues with hot fuel at the suction of the main pump. The factory had good reason install that little in-tank pump.

Roger mentioned recently that the in-tank pump will be NLA from Porsche at some time soon. Might be time to start looking for a spare now, while they are still available. Even if you don't need one today, it's a cinch that you will at some point. For me that point is usually some time soon after they go out of production.

G.P.

[QUOTE=dr bob;12366586]Those living/driving in hot climates might consider replacing the internal pump rather than eliminating it. It's there to prevent vapor lock issues with hot fuel at the suction of the main pump. The factory had good reason install that little in-tank pump.

If the tank is venting properly how can this happen? I had no problems during last summer in Sydney with 30+ degrees celsius, but I have only had my 928 for about 18 months, so I am not so experienced.

Mattman928

So I guess the intank pumps aren't god for 25 years! I pulled this out of the tank today, wasn't expecting to see this

Ill try bench test it, but I don't think it's worth trying to salvage

dr bob

[QUOTE=G.P.;12368489]Problems were common in other CIS cars starting when CIS started really. Fuel is pumped through the hot engine bay as it circulates through the metering unit, warm-up-regulator, and back to the tank. The heat added on each pass though eventually left the liquid so hot it would flash to vapor in the hose between the strainer and the pump inlet.

On later cars, the fuel grabs heat as it passes through the rails on its way to the pressure regulator, then returns hot to the tank. The in-tank pump solves this by adding a small amount of pressure, with very low pressure drop into the pump itself. The in-tank pump is at the very bottom of the tank, vs. the main pump suction stages that are several inches higher. This keeps liquid in the in-tank pump, while the extra pressure from that pump is enough to avoid boiling in the main pump suction and the connecting hose. This is a pretty delicate balance of fuel at the bubble point at the top of the hot liquid in the tank, to less than a third of one PSI at the bottom. Less fuel and/or hotter fuel reduces that pressure while dropping the boiling pressure. Changes in flow and pressure drop through suction piping have the biggest impact on pump operation with barely-not-boiling fuel.

James Bailey

The fuel pump also adds heat as they are rather inefficient so each pass as it gets pressurized and sent back up to the engine it adds up. Enough so that the factory started sending the fuel into the cooler mounted on the firewall which borrows cold from the A/C ....presumption being if it is hot you will be running the A/C and chilling the fuel.
When sitting idling the vast majority of the fuel sent up to the engine is simply returning to the tank since the pump flows more than enough to run the engine wide open at 170 MPH...

Dave928S

What's wrong with it?

Here's a thread if it's the common hose disintegration ... https://rennlist.com/forums/928-foru...with-pics.html

If it is that, and a piece of rubber has jammed/blocked it, you may be able to free it up by reversing polarity, and then immersing in some kerosene to allow it to pump/flush clean. It might also just be worn out and need replacement, or substitution with a strainer if that's the way you want to go.

Herman K

The piece of rubber hose between the internal pump and the connecting flange is the Achilles heel of the 928 fuel system. The internal pump body is suspended inside the tank by being connected to his piece of hose every bump in the road makes it flex and depending on the quality of the hose and fuel the life cycle of the rubber hose may vary.

I for one after have been stranded (10 years ago) by a bad pump practice internal fuel pump hose replacement every (2) years as part of my preventive maintenance procedure and I always carry a spare internal and external spare fuel pump (and tools to replace them) when driving beyond my free AAA towing range (have not had any pump issues since).

jpitman2

FWIW I ran my 83 CIS car without an in tank pump for a year in Riyadh (>1000 ft ASL) at temps to 120F without any issues. The car had lived there for 9 years before I got it. Its never missed a beat in Oz at up to 100F in the last 10 years. Replaced the feed hose and the in tank filter a few years ago .
jp 83 Euro S AT 56k

G.P.

Thanks for explaining the vapour issues Dr Bob.
Looks like there are different opinions about the need for the in tank pump, but as stated by a couple of posters above, they designed it that way.
I will be keeping my old in tank pump and if I start having problems during hot weather I will follow Dave's procedure for changing the hose. Will check the brushes also.
Thanks to Matman928 for asking the initial question. Hope you are getting it sorted.

Cheers,
Gary.