Luis_M

I've just finished putting my fuel system and intake back together on my 81 Euro S. This included cleaning out the WUR inlet filter screens with Carb cleaner (they were completely plugged) and replacing the WUR internal O-Ring. I've got the CIS gauge hooked up. System pressure is a perfect 5.4 bar but the control pressures are way too high. I tested the following with the fuel pump relay pins jumpered and the engine stopped. Warm pressure was measured after about 7 minutes after powering the WUR resistive element to heat the bimetallic strip.

Cold control pressure (with 500mbar vacuum)
ambient garage temp 68F/20C
0 min: 2.6-2.7 bar (should be 1.4-1.8 bar per WSM chart for M28.11 engine)

1-7 min warm-up with vacuum: 3.3, 3.7, 4.1, 4.2, 4.35, 4.4, 4.4
(stabilized 6-7 minutes after powering heating element)

Final "Warm" pressure with 500mbar vacuum:
4.4 bar (should be 3.4-3.8 bar)

Warm pressure without vacuum:
4.2 bar (should be 2.8-3.2 bar)

Since the whole range of pressures is high by about 1 bar at every point above, I'm considering a few ways to lower control pressure:
1. knock the main inlet/outlet iron assembly out further from the casing.
2. Adjusting the screw on the vacuum diaphragm under the bottom plug.
3. Removing one or more of the inlet filter screens (although they seem to be held in by a metal ring I can't easily remove)
4. adding small washers between the upper and middle casing pieces to effectively increase the distance between the bottom spring bases and the inlet/outlet assembly (should be same as 1). I've got a larger rubber seal in place of the original casing seal that should be able to accomodate about 0.5 mm of increase and still be effective.

Also, seems I'm getting less of a bump after applying vacuum than expected. Diaphragm holds vacuum.

BTW, I have not yet checked the return line to confirm that there is no obstruction to flow there, which would probably be a good thing to do.

Any ideas welcome...

jpitman2

Almost certainly there is a blockage in the WUR circuit somewhere still. After cleaning the WUR screens, were you able to blow air through it readily? In cases like this, Watson recommends something like the following, with CARE, and an extinguisher handy - preferably engine NOT running, but pump relay jumpered, showing the high control pressure:-
- crack open the INLEt to the WUR - if fuel sprays, and pressure drops, problem is further along. If pressure DOESNT drop, problem is in the line back to distributor.
- crack open OUTLET of WUR - if pressure drops, problem is further along; if little fuel emerges, and pressure doesnt drop, problem is in WUR.
- crack open return line outlet at distributor - same comments as above.
- crack open return line entry to tank gauge head, apply same comments as above.

In my brief experience with playing with WUR O rings, you would have to be lucky to get EXACTLY the right one to work , without needing to recalibrate the settings. I have tried 2 sizes (14x1.5 and 15x1.5), and got significantly different hot control pressures .
There is a thread on a Mercedes site about this ... http://www.benzworld.org/forums/w126...libration.html
check it out and see if its useful.
jp 83 Euro S AT 53k

karl ruiter

When I had trouble with mine, I pulled the whole thing apart so that just the iron inlet, outlet assembly was in place with nothing pressing on the back, and still my pressure were too high. Then I pulled off the outlet side hose and pressurized the inlet side. Only a trickle coming through. Then I pulled off the back side of the inlet outlet assembly so the metal diaphram was not in place and pressureized the inlet. Still only a trickle. Conclusion: The metal inlet screen. I could not figure out a way to replace or clean it.

jpitman2

I found recently on ebay a tool described as a wur screen remover - ~$4, so what the hel!. It looks like something that would just chew the cr@p out of the filters - small radial cutting edges on the front face.
I washed one in brake cleaner a few times and then blew compressed air through , and it cleared ok . Somepeople have had success with loading the gas with berryman's, and bridging the pump relay for long periods to help to dissolve crud and take it to the filter....
jp 83 Euro S AT 53k

Luis_M

jpitman2: I was able to blow air easily through the inlet. I guess I'll check again to see if more crud has accumulated on the screens since I put everything back together. I used a 14x1.5mm o-ring. Thanks for those links. Did the bigger O-ring give you higher or lower pressures? Will look into pinpointing point of blockage as suggested. I bought one of those $4 screen removers, too. I am equally puzzled about how to use it.

karl: interesting idea about checking the pressure with the bottom half detached. I guess this gives you the lowest possible CP. The MB link above says pressure should be 0.5 bar in this setting. Hopefully same is true for the 928.

thanks for the input.

jpitman2

Luis, its not just that the inlet should be clear, but also the outlet must be clean and open. If fuel has dried in here it can leave deposits that can block things.
The essence of the mercedes site stuff is - remove the bimetallic strip assembly, re-assemble with springs and pin, apply fuel pressure. This will give what should be hot control - adjust position of cast iron part to achieve desired result. Re-assemble with bimetallic, note temperature, apply pressure - if too high for cold control at current temp, tap pin into WUR gently until desired pressure reached. Apply power to heater , and pressure should rise to hot control previously set . If it doesnt ???
Also first test mentioned above should be done first - apply pressure without bottom half of body fitted at all, and pressure must be as low as lowest cold control - if its higher, there is a problem inside the regulator.
I have made a test rig, and have been able to get the hot and cold set, but when I power the heater, the hot control is not being reached, and I suspect the O-ring I have used - more testing to do when time permits. I am using kero for safety, but I doubt that will affect the behaviour.
jp 83 Euro S AT 53k