924Board.org

[coup85]

TL;DR: Proper CIS kit plumbed after the FD T/restrictor (valve on WUR side). System = 4.9 bar; with valve open “control” still ~4.8–4.9 bar. Residual OK. With WUR return to a bottle there’s zero flow (pump on), but fuel does reach the WUR inlet. Opened WUR: clean; with diaphragm off I can blow through, assembled it’s shut; earlier the pivot/pin felt loose, not sure if normal. Looks like a WUR stuck closed. Before I ship the WUR out, am I missing any obvious 924-specific gotcha?

Long version:

I finally got a proper CIS pressure kit and spent the weekend testing. Car is a 1979 924 2.0 NA. The fuel distributor’s top port feeds a T: one branch to the WUR, the other to a hot-start solenoid, and their returns join back to tank.

I plumbed the gauge in series after that T/restrictor, with the tester’s shutoff valve on the WUR side. Closing the valve cuts flow to the WUR and the gauge shows system pressure, so plumbing/orientation seems right.

What I’m seeing: with ambient around 25 °C, system pressure is ~4.9 bar. But with the tester open (i.e., reading control), the gauge still sits at ~4.8–4.9 bar. Residual is decent: about 2.7 bar right after pump-off, 2.2 5 min, ~2.0 10–20 min, and 1.8 60 min.

To rule out the car’s return plumbing, I pulled the WUR return to a bottle. With the pump running and the tester open there is no flow at all—not a drop. If I crack the WUR inlet banjo, fuel does arrive there. I opened the WUR: inside is clean and dry, diaphragm/gasket look good. With the diaphragm removed I can blow through inlet→outlet; once it’s assembled, it’s shut. On a previous look, the little pivot/pin felt loose.

Drive symptoms line up with this: cold starts usually take a second go and idle hangs around 400–500 rpm for a few minutes before it improves. Hot restarts are fine. (A separate fuel smell turned out to be banjo washers; fixed.)

My read: control ≈ system + zero flow to bottle points to a mechanically stuck-closed WUR (pivot/valve/lever issue) rather than a measurement glitch. CSV probably gets it lit; then it’s lean through warm-up.

Before I ship the WUR out, am I missing any obvious 924-specific gotcha? Any quick bench check worth trying that doesn’t involve touching the calibration plug? I also have a line on a rebuilt 0 438 140 073; it will bolt up, but I know its stock curve isn’t 924. Anyone running a 073 that’s been calibrated to 924 numbers with good results?

Happy to post photos of the FD T, gauge routing, or the inside of the WUR if that helps.
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924 - 79'

[coup85]

Update, as an extreme measure, I removed the filter on the intake of the WUR, and pressure returned to normal (1.8 bar/3.8 bar/4.9 bar). The weird thing is, the cold start is still bad... And now I need to find some new filters!
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924 - 79'

[Beartooth]

I'm not sure it's the same thing going on, but when I got my FD rebuilt, frequency valve working the way it should, and pressures set, it ran well briefly, then it got hard to start and would stumble and barely had enough power to move itself. I found the control pressure sky high (close to system pressure), and had to flush out the WUR. Despite flushing all the injector and metering lines, there was still enough gunk to block the WUR a couple times before it all worked out.

On the WUR, maybe yours is different, but all I remember is a fine metal mesh at the inlet, which you can just flush out with brake cleaner and compressed air from the backside if you take it apart. Is this filter you took out internal to the WUR, or external? I'd think you need some kind of filter in that circuit, but maybe you still have the screen in the WUR? Unfortunately, of the several filters in the FD and between the WUR, frequency valve, etc, I don't know where you can find new replacements, or if such a thing is made anymore... The good news is, so far I've been able to clean and reuse all of them.
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1980 931 diamond in the rough

[Raize]

Taking the fuel cell apart is not scary, I've done it on 4 WUR so far and one even had corrosion inside (this will also cause the high pressure issue if the diaphgram pieces are stuck by corrosion) that I cleaned off with some sandpaper and then polished with super fine sandpaper. They all worked afterward and didn't leak.

I've never actually had the clogged filter problem myself but it would be easy to back flush once the diaphgrams are removed from the fuel cell. You just have to be careful and maintain very high cleanliness.

[coup85]

My focus now is on the distributor. My system isn't holding pressure as it should after I 'opened' the WUR line back. Sadly, a fuel leak on the filter delayed my testing plans! I hope that's solved by today so I can continue testing. I need to find the pressure leak –crossing fingers is not the accumulator, or I'd need to sell my house for it
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924 - 79'

[Raize]

[coup85]

Interesting, are you saying is normal that the system can’t hold pressure longer than 5-6 minutes in early models?
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924 - 79'

[morghen]

My 85 also didnt and worked ok.
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Supercharger and EFI kits
https://www.the924.com

[Riggard]

Even with the return going directly to the tank the WUR should hold pressure for approx 20 minutes. I have an older car where the WUR bleeds directly to the fuel tank and residual pressure is fine for 20 mins. The wur and accumulator take care of this.

If it bleeds faster, you have an issue somewhere. Easiest way to pinpoint is start clamping hoses after you shut down the engine, start directly after the fuel pump (before the accumulator) and see if that fixes things; if so, it's the check valve, if not, clamp after the accumulator and so forth.

Accumulators are expensive, but with some creativity you can alap use ones from other cars, which are often cheaper. The only variables to check is the cc's they hold, pressure and fittings, there's a few models that come close to the 924 in this regard and can be fitted using different fuel line fittings.

[coup85]

Thanks, that's my plan for today. Still, the easiest explanation for the loss of pressure was the leak in the filter. The thing is, the system held pressure perfectly fine when the WUR was blocked, so that might reduce what to check in case the filter wasn't the issue.

PS: I might also need some guidance in case I need to fix the accumulator –I think I heard some 'drops' inside of it on my last test, after shooting down the pump.
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924 - 79'

[Raize]

[Riggard]

Agree with all of that raize.
The point I was trying to make is that even tho the design is a bit flawed and was updated later/in other cis systems, a well functioning system should not have issues. It will bleed when cold, but hot start issues never arise when cold.

As to the dropping fuel pressure because of the WUR that the topic starter is experiencing. The fact that it didn't with a clogged wur indeed gives a clue. I'd skip some steps and like raize suggested; make sure the WUR does not cool down when testing residual pressures. Either because the engine is hot, or because you wire the WUR to 12v directly. All other ways of testing will produce flawed results IMHO.

[coup85]

Quick update. Filter leak sorted, and system holds pressure as expected- On the residual check –valve closed, it holds: 2.2 bar at key-off → 2.0 @10 min → 1.9 @20 → 1.8 @60, so pump check and accumulator look fine.

Where it goes wrong: with the WUR cold –valve open, it dumps to 0 bar in about a minute and I get a sharp hiss from the bay. If I pinch the main return hose in the bay, pressure stays at 1.8 bar for 20 minutes and the hiss disappears. That pretty much tells me the leak path is into the return before that pinch point. WUR leaking pressure or FD return valve, I suppose???
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924 - 79'

[Raize]

WUR fuel cell therefore has a tiny through flow leakage. A grain of dirt is all it takes. Your first test 100% confirms it.

Time to open it up and clean whatever is stopping it from closing fully, there is an o-ring you can replace in there too that may be perished or otherwise damaged.

[coup85]

Thanks Raize –as always, I really appreciate your help. I'm going to clean it up but I suppose is time for a repair kit
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924 - 79'