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D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

I've got my CIS tester hooked up and keep getting system pressure (about 4.8 bar) when activating the fuel pump when the engine is cold instead of the 1.5 or so bar I should have to accomodate a cold start. Is this a sure indicator the WUR is fubared?

I've rebuilt the WUR by replacing the o-ring and shimming the spring inside to give more tension but no good results yet. Still get system pressure instead of regulated pressure.

Anything I'm doing wrong? What else can go wrong on the WUR that would give such high pressure readings?

I've read over the Haynes and the Probst book but can't seem to find any other expanation besides the WUR is bad. But there isn't much in there to go bad that I can see.

Smoothie · Posted: Mon Mar 28, 2005 5:45 am Post subject:

Dumb question but I have to ask for clarification - do you have the valve on the tester opened? System pressure is system pressure all the time and shouldn't change. To read it you close the CIS testers' valve. To read control pressure, you open the valve. Control pressure is the one that gets altered by the WUR according to temperature and causes coldstart enrichment as it's lowered.

If you do have the testers' valve opened and you're seeing system pressure where you should have control pressure, then there's a blockage somewhere - either in the WUR or the lines from/to it.
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"..it's made in Germany. You know the Germans always make good stuff."

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D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

Sorry, yes the valve is open. This is on an '80 na.

I'm getting an incredible amount of fuel when I crack openthe return line to the tank on the fuel distributor. Of course, there's no residual pressure showing on the gauge and the fuel pump hasn't been run for almost two hours now. I just cracked the return line from the WUR to the fuel tank and the fuel comes out at a good pressure and doesn't quit. I was under the impression this was a low, if any, pressure that returns to the tank. And why does it keep flowing?

The other book I have says, if the pressure to the WUR is too high, crack the return line and see if the pressure drops to where it should be. If it does, then there's a line restriction leading back to the tank. That's what I'm trying to check right now but the fuel seems to have an endless flow to it.

Any suggestions?

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

IIRC the return line the book should be referencing is from the WUR that goes into the side of the fuel distributor. Also, you may want to check the relief/bypass valve in the distributor to make sure it isn't stuck open and the o rings are in good shape.

If you are using a jumper on the 30 and 87 pins of the fuel pump relay connector, did you disconnect it before opening the return line?

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

I may have solved the return line pressure problem. I opened up the fuel tank at the filler neck and relieved the pressure. Now I don't get pressure coming back to the fuel distributor via the tank return line and the fuel no longer gushes out of the tank return line. Is the tank supposed to be under that much pressure? I know it's been like that since I bought the car over 18 years ago.

So I cracked the return line where it's connected to the fuel distributor and ran the pump. I still get a high reading on the gauge that is equivalent to system pressure. When I close the valve on the gauge, the pressure remains the same, about 4.8 bar. That's with the tank return line disconnected at the fuel distributor so that should rule out any restrictions in the tank return line. Guess I'll order a new WUR.

On a side note, can someone tell me what this is:

I'm talking about the piece that is in the background, behind the banjo bolt and the fuel lines.

The small pipe leads to the frequency valve.
The large pipe is the return line to the tank from the fuel distributor/WUR.
Behind those is a small solenoid valve that has a yellow/red and a brown/white wire running to it. I looked it up in the workshop manual schematic and it says it is a "supplementary start valve". The wiring diagram shows it tied to the cold start valve. Can someone else look at their manual and see if I'm making this up? I don't find a reference to it in any of the manuals I have. There is no fuel line going to it and it's mounted on the bottom of a bracket that is on the passenger side (left) of the fuel distributor, as you can see. I'm baffled. I think someone else posted about this before, not too long ago. I'm just wondering what it's function is and if it's tied to my pressure problem at all. Thanks!

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

It sounds like the wires going to the air flow sensor.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

924RACR · Posted: Mon Mar 28, 2005 9:22 am Post subject:

I'm thinking you're looking at the hot-start valve. Yes, sounds like your WUR is shot - BTDT. Car works a lot better with a good WUR!
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dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Vaughan,

Is the hot start valve the same as the supplemental start valve shown in the wiring diagram?

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

That makes sense.

Anyone have an idea if the fuel tank should be pressurized like it is? Seems strange.

Roger · Joined: 06 Jan 2003 Posts: 1235 Location: Cordova, TN

Isnt t e spring in the fuel distributor to control the pressure at wich the fuel is recirculated to the tank? If it has been shimed then is it possible that the system pressure was raised?

My tank makes a wooshing sound when I open the cap, but I always though it was a vacume sucking in air since the fuel system is not vented.

Have you done a fuel supply test by measuring the output of the return line?
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John Brown · Joined: 07 Nov 2002 Posts: 903 Location: Leesburg VA

The solonoid valve in the picture is a 'warm start' control pressure reduction valve. Powered through a temperature switch in the coolant pipe that lays back below the distributor and warmup/fuel pressure regulator. Reduces control pressure for 'warm starts'. Assuming it's off when it's supposed to be off it will not have any effect on running control pressure. If it did it would be to lower same.

For a positive check of the fuel control pressure regulator (I don't like the warm-up terminology because it really is a full time pressure regulator) first crack open the line running TO the regulator From the fuel distributor. Conrol pressure should drop; if not line is plugged. The crack open the return line FROM the regulator TO the tank return; if pressure drops then regulator is working (at least some) and line is plugged. Do this at the fittings on the back of the regulator and you will have a 100% check of the regulator isolated from the lines to and from.

The bi-metallic heated strip in the regulator is apparently deceptively simple. I have never been able to repair one in the times I tried. What the failure mode is I'm not sure; but you probably will need to buy a new regulator.
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John
80 931 - #931 44Cup
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D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

Thanks John. I did both of those tests and found the lines were indeed clear.

Ordered a rebuilt one today from Perf. Prod. They have the least expensive prices on them that I found out of all the places I checked, about $76 plus shipping.

I need to go back and study this some more because I'm slightly confused about a couple things:

1. When the regulator reaches operating temp, it should cause a rise in pressure on the guage, right? From about 1.5 bar to around 2.5 ish, depending on ambient temperature. (Don't have my book in front of me and don't recall the exact pressures.)

2. Should all fuel stop flowing through the regulator when it's warmed up or should you still see some flow back to the tank? It seems there would still be flow back to the tank, even though the diagram in the manual shows the flow to be blocked off completely.

3. If there is flow all the time through the regulator, does that mean the spring inside the regulator is doing all the controlling of the pressure?

4. System pressure is what's coming out of the top of the fuel distributor and going INTO the control pressure regulator. So when you close the valve on the gauge, that gives you system pressure readings, right? And opening that same valve on the gauge should give you the control pressure produced by the control pressure regulator. And that pressure should be manipulated by the amount of tension on the spring inside the control pressure regulator, right?

(The system pressure is up to spec so I don't think it's anything to do with the fuel distributor or the pressure reflief spring inside the fuel distributor.)

So does the spring inside the control pressure regulator play a large part in deciding the cold/hot control pressure? And if so, as a spring ages, I would thing it gets weaker. Assuming there is no blockage on the line coming INTO the control pressure regulator, and seeing that the system pressure is correctly reading at 4.8 bar, I wonder why the reading would be TOO HIGH instead of TOO LOW. Unless the spring has somehow been taken out of play. But with no spring, there would just be a straight through shot in and out of the control pressure regulator and, assuming no blockage on the return line, would create very little pressure except from things like elbows and turns in the pipes and the basic way the control pressure regulator is configured.

I'm wondering what fails inside these things that would cause an INCREASE in control pressure instead of a decrease, though the book does say a decrease is possible too and you must replace the unit if it shows a decrease.

I could see how the decrease could happen but not sure how the increase can come about. As the bimetallic strip heats up, it moves TOWARDS the inlets, releasing the spring and allowing the spring to apply pressure on the thin metal plate, which in turn restricts how much fuel can pass from one hole to the other. When the bimetallic strip cools, it restricts the tesion of the spring, releasing the thin metal plate and allows more fuel to pass through, resulting in a richer mixture to allow for cold starts. Isn't that correct? So over time, the spring will weaken, giving less tension on the thin metal plate and resulting in less restriction instead of more and lowers the pressure you see on the guages, right?

I need to do some more testing on my old one. Please correct me if I misunderstand how these things work. Thanks.

Smoothie · Posted: Tue Mar 29, 2005 8:25 am Post subject:

By what you said, you have a good understanding of how it works.
The bi-metal spring controls the pressure through the warm-up period, then once fully warmed the other spring takes over. The metal diaphragm is always opened to some extent, allowing fuel to flow. Yours would have to be blocked - either varnished shut, corroded shut (if that's possible), or some junk got in to block it.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

Yeah, that's where I'm heading next with this. Take it off and check the orifices for blockage. I know both lines are clear but I'll double check when I take it off.
thanks

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

The regulator won't and can't cause system pressure to be higher than what is generated by the pump, and regulated by the little plunger on the side of the fuel distributor. It sounds as if you have your head wrapped around things OK.

In simple terms, you can think of the control pressure regulator as sort of a pressure relief valve.The same fluid mass that acts on the top of the plunger is what goes INTO the regulator. When the regulator is cold it "bleeds off" more fuel causing a lower control pressure. As it warms up, the diaphragm partially blocks off the return fuel, and this results in a higher control pressure. Low control pressure = richer mixture, higher control pressure = leaner mixture.
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'79 Eurospec 931