GerritD

I finally found the problem but how can it be solved ?? :

apparently the metal fueline between the external master fuel pump
and the filter contains a valve just behind the fuel pump.

So normally with sufficient pressure this valve will open and fuel will flow
through the filter towards the engine.

DOES THIS MEAN THAT I HAVE TO FILL THE TANK COMPLETELY IN ORDER TO BUILD SUFFICIENT PRESSURE BELOW SO THAT THE VALVE BEHIND THE FUEL PUMP OPENS ???

Leon Speed

That's what I meant by check valve in my previous post. The valve just blocks fuel running back through the fuel pump, and insures there is fuel pressure at the injectors. Not sure how much pressure is needed to open it but I imagine not much. Did you maybe put the valve back in reverse?

GerritD

Aryan,

I did not touch it when replacing my fuel lines....strange

I now have for about 4 gallons (15l) in my tank. Fuel gauge reads minimum.

I did not find any threads regarding this issue....what if you are running out of gas....could this reproduce the same thing...

I know that some cars need a full tank to startup again after running empty.

Bill Ball

No, you should not need a full tank. The check valve may be stuck. But still do not overlook that kinked fuel line. It may be closed completely - hard to tell due to the braided sheath covering it. Get the proper line. it may not be allowing sufficient flow regardless of the valve.

Also, even if the in-tank pump has a broken hose (most do) or is not working, fuel will flow by gravity. The car will run but may not have good high-speed performance. George Suennen's 91 had a bad in-tank pump. The car ran well until he tried to go over 120 MPH. Anyway, although the in-tank pump should be examined, it's probably not the main issue with starting the car.

Leon Speed

How old is the fuel pump? It's likely still the fuel hose, it does look kinked. Is you want to measure the fuel output should be appr. 1150 cc /30 seconds.

GerritD

I replaced the kinky fuel hose by an original one and still have the same problem. Both pumps are working fine but no fuel coming out of the external fuel pump.

How much pressure is needed to open this check valve..

I'll try to get some more fuel in the tank, you never know.
Logically the check valve needs some retaining force, otherwise fuel would not stay in the hoses towards the injectors.
So the force by the fuel in the tank and pumps needs to be a little bit more than the fuel in the fuel lines in order to open this valve...

I can find little information on the internet regarding this valve.....

However I checked the original manuals and apparently I have a fuel pump of the 1981 model with a separate check valve. Perhaps indeed, this valve got stuck.

Bill Ball

I've never seen one of these check valves not open. Even pumping air should open it. Their usual failure mode is to not close. Loosen it a bit and see if fuel drips or squirts out. At this point you don't know where the block is. When you removed the hose fuel should have run freely out of the tank. Did it?

GerritD

Bill,

it is nothing more than a pin with a spring into a plunger. Blowing air through it doesn't work, so it is really tight. Removing the hose, fuel flows freely out of my tank...
Perhaps due to the fact that my fuelpump was disconnected from the hoses and tank for a couple of months due to restoration of the car, could perhaps have locked the valve since fuel pump went dry.

Here is a picture :

Bill Ball

Like I said, loosen it just a bit and see if fuel flows/drips/squirts from behind it, in which case somehow it got jammed shut. But first I would loosen the line on the downstream side to see if fuel is or is not going through the valve. From what I've read you haven't established whether fuel is really pumping or not.

GerritD

checked, loosen the banjo bolt and check valve and fuel is flowing out of the fuel pump. ..damn something that little can cause such big problems

luckily no major costs ...worst case I will need to replace the check valve if it doesn't come loose.

Hopefully this thread now can help other 928 enthusiasts to solve similar problems

ArthurPE

it's probably a ball or pintle spring loaded check valve
they seldom fail
check it by using a pick to push it open (from the pump side)
the pump may not be making sufficient pressure to open it...
as others ahve said the suction restriction (kinked/constricted) hose may be starving/cavitating the pump
it has very little suction head and needs the static head from the tank to work properly

SeanR

That fuel pump looks really old, they are a known failure points, it just might be time to replace it.

And as said already, get the proper hose for that car and toss that mess in the trash.

GerritD

Sean,

perhaps you are right, I can't tell since the car has switched from several owners and no history know (but engine has run only 100000 miles).
I'll just give it try after loosening the check valve and if pressure is not sufficient, I'll definitely buy a new pump.

Any idea how much a new fuel pump costs ?

Bill Ball

The correct pump for your car is about US$300...maybe a bit more.

Leon Speed

These cars require new parts and maintenance. You can do the very minimum and hopefully have a trouble free ride for many years and kilometers or you can do a lot and hopefully have a trouble free ride for many years and kilometers Bottom line is, these cars do require a budget. Personally I set aside about 2000-3000 EUR a year. Sometimes it is more, sometimes it is less.