WUR Rebuild?

[echrisconnor]

Is there a good previous post, or does anyone have instructions on rebuilding/refurbishing a WUR?

I've had a hot start problem which I believe I have finally tracked down to the WUR not holding pressure on the return to the tank and allowing fuel pressure to drop in the system over time. Somewhere in the past I've seen an article which I can't find through searching which detailed pulling the WUR apart, flipping the metal disc over and cleaning things up.

Thanks all!

chris

[RarlyL8]

I don't beleive the WUR has anything to do with holding pressure.
There are a couple of fuel check valves and the fuel accumulator that serve the function. The types and location may be specific for the year and type of engine you have. Pelican may have a schematic for a '76 Carrera in their archives.

[echrisconnor]

I've heard conflicting things here on this, but I've done a lot of research here on CIS and determined that this has to be the problem (although I'm still open to suggestions). There are two sides of the fuel system which need to maintain pressure. There's the input side and the return side. If you look at a diagram, the WUR is the only thing keeping return side fuel pressure contained, since you've got pressure on one side, and an open return to the tank on the other. Typicaly its the input pressure side which leaks.

The factory manual suggests you check things in this order:

1. Fuel pump (check valve)
2. Fuel distributor (pressure regulator valve O ring)
3. Fuel injector leakage
4. Cold start injector leakage.

If, you close off your CIS pressure gauge, so that fuel does not travel through the WUR, and the leak problem goes away, then it is the WUR leaking the fuel pressure.

I've checked and or replaced all of the list above (and the accumulator as well), and it appears now that I only loose system pressure when the gauge is open and pressure can reach the WUR. I have not yet opened the return line to see if fuel dribbles out of the fitting during leakdown, but I suspect that's what I'll find.

So, I'm curious if this is the case whether the WUR can just be easliy cleaned/repaired, or whether I need to spring for a new one.

c

[cmonref]

With the pump running, fuel can "escape" thru the injectors (main and starting) or thru the WUR or the system pressure regulator bypass on the distributor. With the pump running, the system pressure regulator always has fuel going thru the bypass, since it wants to reduce the pressure from the pump (85 psi or so) to the desired pressure in the system (65-74).

Asuming normal operation, the starting injector flows fuel only when electrically powered, and the main injectors flow fuel when the pressure to them exceeds their closing pressure value.

When the engine and pump stop, the system regulator closes off its bypass, and the main injectors close for lack of pressure. That leaves only the WUR to continue to bleed off pressure. It will continue to bleed until all of the pressure is gone.

As the pressure reduces, the WUR bleeds fuel at a slower rate. Within a short few minutes, it reaches the 20 psi or so that the accumulator holds, then continues to bleed based on the pressure in the accumulator. This bleeding and falling pressure continues until the pressure is all gone.

The bleed process creates two phases: one where there is still subtantial pressure in the accumulator, which is necessary to do a hot-start. The second phase is when the pressure is gone (an hour or more?), and the engine is ready for a cold-start. At shutdown, the accumulator is holding just enough pressure and fuel volume to create this two-phase process while the WUR bleeds merrily away.

So, it is highly unlikely that your WUR is the cause of your hot-start problem. If it were bleeding down fast enough to cause a problem, you would have serious running problems that would occupyyour time.

So we are reminded that the accumulator or the fuel pump check valves have been well-established, time and time again, as the two reasons for a hot-start problem that is traced to lack of residual pressure.

[echrisconnor]

Brian,

Thanks for the great writeup! So, my hot start problem, is after an hour with a very hot engine. I see the fast drop in fuel pressure as things depressurize to about 20 24psi, then I hold pressure for close to, but not quite factory specs for the first 10-20 minutes. However, after 30-40 minutes, I have zero pressure.

After an hour, when I turn the pump back on, I can hear bubbles in the fuel system working through. In order to re-start, I just need to lift the air meter plate, and quickly feel no pressure, build up to strong resistance. Then, a second later, the injectors start to sing and I'm ready to start.

Questions on the accumulator. Mine doesnt' leak, and if you lift the air meter plate with the engine off, you can watch the fuel pressure rise as you lift it, and drop as you lower it. This, I take is the spring in the accumulator. The accumulator doesn't "hold" pressure does it? It simply dampens spikes in pressure, and the spring will take pressurized fuel and retain a certain amount of pressure by pressing against the present fuel with it's spring and taking up lost volume that may slowly leave the system. There is no check valve or uni-directional capacity to it.

So, is your hypothesis that with hot starting, if it isn't the check valve, it's got to be the accumulator? Or, if the WUR leaks down too quickly, and the engine stays hot for a long time, can't this still make for hot starting issues?

[vash]

chris, you described perfectly the symptoms of my 75 car. pressure drops until zero. flip the knob on the pressure gauge, isolate the WUR, it holds press. new WUR fixed it.

[Tim Hancock]

Chris yes you can take your WUR apart, I did. Maybe you have some crap in it? If the diaphram was ripped, I THINK you would see fuel leaking from the vent.

Did you say that the pressure holds positively when you close the CIS tester's line to the WUR?



You did clean the pressure regulator orifice (spray out with carb cleaner)when you changed the o-ring right?

[Westy]

OMG, We need a WUR thread. All of the sudden everyone has WUR issues. I will try and start one tomorrow. Us WUR's really need a thread with good infor that's contained in just the one thread (if that's posible). I am WUR crazy!!!! But of course, my WUR sucks!!!!!!

[OldTee]

You might consider a smoke test. It involves putting smoke into the intake and seeing where it leaks out. Did that on my SC and found 5 leaks. Two injector seals, the popoff valve, a crack in the air box and one leaking intake hose. I got my test done in the Washington DC area by Stuttgart Performance Engineering in Gaithersburg.

[echrisconnor]

Maybe we need two threads; one for the WUR and one for the Accumulator.

On the accumulator, my assumption has been that if the thing isn't leaking, it's good. Anyone know if they can have other issues? Can the spring break or loose its rebound over time?

On the WUR. From what I understand the little metal disc in it at the base of the fuel lines is what deflects and allows fuel to pass through with regulated pressure. My assumption is that if this disc is even slightly deflected, or has a little goober in it, then the WUR may leak. This low pressure leak may, or may not, affect the warm and cold pressure values.

So, I guess I have two tests...

1. Swap accumulators from a friend's car and see if the problem goes away.

2. Take apart and try to R&R the WUR. Still, anyone have a good chronicle of what's involved?

[echrisconnor]

I found this picture of the inside of a WUR, anyone have a picture of the metal disc up by the fuel lines?

c

[echrisconnor]

oops, here it is.

[Tim Hancock]

Originally posted by Tim Hancock
Chris yes you can take your WUR apart, I did. Maybe you have some crap in it? If the diaphram was ripped, I THINK you would see fuel leaking from the vent.

Did you say that the pressure holds positively when you close the CIS tester's line to the WUR?



You did clean the pressure regulator orifice (spray out with carb cleaner)when you changed the o-ring right?

Chris, did you do the above? Just curious.

[echrisconnor]

Tim,

Sorry for not responding directly to your comments earlier.

It appears that pressure holds for much longer when I close the valve to the WUR.

When I cleaned out the pressure regulator, it was spotless in there, but I still cleaned it out with carb cleaner at a Q tip.

[Tim Hancock]

When you close the valve effectively taking the WUR out of the system, the pressure should remain relatively high for a pretty long time.

Here is a photo of some text addressing the testing of residual pressure. When mine was a problem, I had had the spring/o-ring assy out during an earlier troubleshooting attempt and I positively cleaned the parts before reinstalling, yet the pc of crap was still in there as I found out later after pounding my head against the wall for a few days.

[echrisconnor]

Tim, so you tried to clean the WUR, but despite your attempts it still wouldn't hold pressure? Was the tear down and re-assembly relatively straighforward?

I've done pretty much all the tests listed in your photo, including pulling injectors and watching for leaks.

[Tim Hancock]

No, mine failed the last part of that test as it was leaking past the pressure regulator that you previously stated you cleaned. I just wanted to make sure you eliminated that possibility. It seemed like you said that when you shut the valve to the WUR, that the pressure bleed was BETTER but this test procedure basically states that the system pressure should POSITIVELY hold, not just bleed off SLOWER when the WUR is removed from the system.

If you still come to the conclusion that your WUR is faulty, do not be afraid to take it apart. I have had one apart and it is no big deal.

Good luck

[echrisconnor]

I think I'm at that point... The WUR may be coming apart, but I need to run the test again and confirm that I have fuel coming out of the return side of the regulator when at low pressure, and double check the fuel pressure drop stops completely when I shut off the stop to the WUR.

The last thing I'm struggling with is whether the fuel accumulator can fail and not leak. I've always read that fuel coming out of the bottom of one of these is the indicator that they're bad. There isn't anything else in there to fail is there? No leak = OK?

[cmonref]

echrisconnor,

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The "hot" and "cold" are defined by the action of the thermoswitch in its control of the starting injector and the WUR mechanicals. For a "cold" start, the thermoswitch is below about 113F. The engine expects zero residual pressure in the system, but no air in the lines. Turn the key, the pump starts on its startup circuit, the starting injector starts the engine about the time the whole fuel system comes up to pressure. Release the key, starting injector closes, pump relay switches to its run circuit.

On the "cold" start, the WUR is also in it's "cold" mode. The engine STARTS on the starting injector's fuel, but the WUR must be properly set to "pickup" the job of providing enriched mixture to the running, but cold, engine. It is in this transition that an improperly set WUR can result in improper mixtures which backfire and break airboxes.

For a "hot" start, the thermowitch is above 113F so the starting injector is inactive during starting. The engine expects some residual pressure in the system to prevent vapor lock and to bring the fuel system pressure up when you begin the start sequence. Without the residual pressure, it will eventually start.

But you say you have air in the lines. That has just about GOT to be from a combination of boiling fuel and a leaking check valve. After shutdown, the system pressure will drop, faster than without the check valve leak., but perhaps no so much faster that you would notice. Additional pressure from hot fuel would prolong the leak, pushing fuel back to the tank past the leaking check valve. That would explain "air in the lines" as you describe.

On a related note, Tim Hancock's reprint from a manual above refers to a "control pressure valve" which may be a third faulty component that causes bleeddown. I think that refers to the "control pressure regulator for throttle valve position" which was on the very early CIS cars but was quickly removed in favor of the vacuum controlled WUR. I am sure this component is NOT on your '76.

Just some thoughts for you to chew on in your quest.

Brian

[Tim Hancock]

I was talking about the system pressure regulator which consists of a plunger, oring, spring and shims. It is retained by a threaded plug/cap
on the side of the fuel distributor. It's function is to regulate SYSTEM pressure (not control pressure which is controlled by the WUR) by diverting excess fuel from the pump (which is capable of approximately 100 psi) back to the fuel tank. When the pump shuts off, it closes once system pressure drops to a certain pressure. If it leaks at the plunger end due to a pc of debris, it will allow your residual pressure to drop.
I am not saying that this is Chris's problem, but I wanted to make sure he knew about it because it happened to me. It was not really mentioned in my Bentley Manual so it took me awhile to figure it out.

Brian I do not know anything about a "control pressure regulator for throttle valve position".

Chris, if your '76 model does not have a SYSTEM pressure regulator valve, I apologize for wasting your time.

(a picture showing this valve is depicted on page 240-12 in the Bentley "SC" service manual)