924Board.org

[gml]

Been smelling too much fuel and my brain can’t keep up, I think I need to ask some people who know what they’re doing.

My cold control pressure is 4.6 bar, and my system pressure is the same at 4.6 bar.

I ran the fuel pump for 30 seconds and got around 700 mL (a bit shy of the 750 mL benchmark). Testing fuel coming out of the return line

I took apart the WUR and ran it with only the top half of the WUR, so no strip applying pressure to the diaphragm in the regulator. Same thing; 4.6 bar with no pressure, no significant change if I push on the diaphragm with my finger.

My guesses at the moment:
1. probably a blockage in a fuel line somewhere. How do I find it? I found this quote in the “wurfix” PDF:

[72rallye]

If loosening the return line of the WUR at the FD fixed the control pressure, then there is a blockage in the return line of the FD, or a blockage internally in the FD between the WUR and FD return line ports. The WUR is ok.
When you opened the FD return line to measure the flow volume, did the control pressure drop? If so, then the blockage is in the FD return line. If not, it is inside the FD.
The other line connected to the WUR return port that you loosened should be the return line of the frequency valve.
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81 931, 86 951, 86 928 5spd, 75 911S coupe

[gml]

Thanks 72rallye. Something clicked and I now have a basic understanding of these fuel system pressures. I will try loosening the FD return line and see what happens with control pressure, I did not think to look at the pressure gauge while fuel was dumping out of that line.

If the problem is at the FD, what is the fix? Complete disassembly and rebuild of the FD itself? Would you just run some pipe cleaners brushes through the FD to clear out blockages? It sounds like this guy was able to blow on the FD somehow to clear out a blockage (step 3)

If the problem is not in the FD but the return line, I think I will disconnect the return line at the fuel tank and put it into a gas can, then try to run compressed air into the return line.
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George

1982 924 NA 5-speed

[72rallye]

If there is a blockage inside the FD it doesn't have to be a fault, such as accumulated dirt.
At least on the 931, if the system pressure falls below whatever the fuel pressure regulator is set to it will block the WUR return line. This is meant to maintain pressure inside the FD after the engine has been shut off to facilitate a hot start. The symptoms would be the same as what you observed.
The NA FD is different and from the manual I cannot figure out how it works, but there has to be a similar mechanism, since the manual also specifies a minimum pressure after shut off.

Thomas

EDIT: I should have first read the link that you provided. The "push valve" apparently is what maintains pressure after shut off.
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81 931, 86 951, 86 928 5spd, 75 911S coupe

[gml]

thanks Thomas. I just tested control pressure with the return line after the FD disconnected. Dropped to 4.2 bar (4.6 when it was connected).

I’m not sure what to do with that info. I’m still running only the top half of the WUR so I’m expecting control pressure to be 0.5 bar.

My guess is that there’s a blockage in the FD and the rest of the return line…
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George

1982 924 NA 5-speed

[gml]

Interesting point about the minimum system pressure, although at 4.6 bar my system pressure is within the test spec for my NA, so I don't expect that would be it.

Going to disconnect the return line at the tank and blow compressed air through the hard return line after the FD.

I am also trying to figure out how to clear out the FD... Based on my understanding from this post I will need to...
1. Remove the pressure regulator at the FD (14 mm hex head, mine for the NA has just a single copper washer and just a spring underneath the bolt)
2. Plug the hole from the pressure regulator with my finger (edit... or why not just remove the spring and reinstall the bolt?)
3. Blow compressed air into the WUR return line

Hope that sounds reasonable...
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George

1982 924 NA 5-speed

[gml]

I took off the bolt on this regulator (13); looks like I am missing number 16 and 15. I did not take out any more pieces during my quick inspection (looked like 18 was in place, not sure about 19).

Keeping in mind that my system pressure is within spec at 4.6 bar, is this any cause for concern? I don't see the legend for these parts in the workshop manual where I grabbed the image.


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George

1982 924 NA 5-speed

[72rallye]

My manual seems to be more complete:
13 Plug, torque 1.3-1.5mkg, 9-11 ft lb
14 Flat seal A 10x13.5
15 Shim 0.1mm thick quantity as needed
16 Shim 0.5mm thick
17 Spring
18 Piston
19 O-ring
I don't see a problem with leaving out the shims. You add or remove them to get to the desired pressure.
It is possible that the system pressure was high because of a return blockage and someone tried to lower the pressure by removing shims, which in this case doesn't work.
After cleaning out the return line, the system pressure may be too low, in which case you can add shims back in.
What was the control pressure when you opened the return line of the WUR. You only said it was lower, but not by how much.

I have never seen a disassembled NA FD or a drawing thereof with the "push valve". Therefore, I cannot comment on how to clean it.
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81 931, 86 951, 86 928 5spd, 75 911S coupe

[gml]

Thank you for the labelled diagram 72rallye, and your explanation makes sense. I will try to get some shims because once I sort out this high control pressure I'm guessing system pressure will drop, and I'm already on the low side of the spec.

To answer your question: yesterday I re-tested loosening the WUR return line at FD and this time there was no drop in control pressure (stayed steady at 4.6 bar) even as fuel was spraying out of the loosened fitting. Tried this test twice, no drop either time. The first time I tested this, I only quickly cracked open that fitting and noticed the gauge bounce down, but that could have been my error after all.

But I'm still not sure what to make of this; if my first test was a fluke, then why did I see a drop when I undid the return line after the FD? Maybe yesterday's test was a fluke and the only way I'd notice the pressure drop is with the WUR return line fully removed?

Anyways, I already got a WUR rebuild kit so I took it apart. It was very clean overall except for some surface rust on the inner circle under the diaphragm where the pinhole fuel passages are. The inlet filter barely lets any throttle body cleaner through it... but it was clean, aside from a bit of rust around the perimeter of the filter. I read in a PDF that if there's any rust on the machined surfaces of the WUR, it's trash. Is that true? It doesn't seem great for the fuel flowing through the WUR to receive consistent exposure to a rusty surface.

Between my test results being non-repeatable and inconsistent, I am not sure what to do except try to do everything.

1. Still need to try clearing out the tank return line
2. Will put a new diaphragm and O-ring in my WUR
3. Will order some pressure regulator shims in anticipation of system pressure issues
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George

1982 924 NA 5-speed

[72rallye]

>>> The inlet filter barely lets any throttle body cleaner through it
I think that's your problem. After removing the diaphragm there are two small holes visible. One goes to the inlet port and the other to the outlet.
With a can of carburetor cleaner and the attached plastic tube I could press the tube against either hole and a large amount of cleaner would spray out off either port.
That fixed the control pressure for me.
Thomas
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81 931, 86 951, 86 928 5spd, 75 911S coupe