Fuel Management

My CIS pressure gauges finally arrived so I set off on the quest for an optimal setup per the CIS manual on this sites member page. Car in question is a Euro 6.9 #923 (early 1977,maybe even 1976). In the course of doing so I ran into a snag, leading to the immediate question here. Depending on the responses - if any! - I'll post the rest of the story.

Here's the story: the car starts fine and was running reasonably well, but room for improvement in sheer power and idle was a bit rough. In the course of testing the various CIS fuel pressures I discovered a couple issues:

1. Primary fuel pressure was very low, about 4.5 bar. The regulator didn't have a single shim on it.
2. At the same time, the hot control temperature was far too high, virtually the same as the primary pressure.
3. I fixed primary pressure by replacing the regulator body, o-ring and playing around with the shims. This brought primary pressure up to spec (5.4 bar).
4. After fixing primary pressure, hot pressure went up to more or less the same as the primary pressure. Car will barely run in this state.

In other words, WUR doesn't seem to regulating fuel pressure. It's almost as if, at some point in the past, the WUR stopped working and somebody decided to reduce the warm control pressure by reducing the primary system pressure. I also noted that the WUR "adjustment" post seemed pretty far down, roughly 4-5mm below the top of the WUR body. Tapping the post out to reduce hot pressure had no effect. Not surprising I guess, cause it apparently wasn't regulating before either.

Here's the main question at this point:
I took the WUR apart, cleaned, recleaned a couple times and made an observation. This WUR (part 0438140010) has TWO diaphragms under the plate inside. The one closest to the o-ring has a single small hole in addition to the screw holes. Then there's the solid diaphragm that's shown in all the various WUR pictures and diagrams. But I've never seen a WUR picture with the second diaphragm with the hole. Can this be the cause of the pressure issue? Anybody ever see a 2 diaphragm WUR?

Comments and suggestions welcome, as always. Thanks!

Anyway,



Dave

Detail edit:
It was obvious that the WUR had been worked on in the past. The heads of the 4 screws holding the diaphragm, o-ring etc all had some scarring and had obviously been removed and fiddled with at some point. Inside of the WUR is in good shape though, no corrosion, diaphragms very clean and shiny, no vacuum leak and the vacuum-operated diaphragm that the springs sit on works, no leaks.

Dave

OK, the post you refer to driving up or downwards sets the cold control pressure, not the warm control pressure.

The correct sequence of events is :
a) set and correct the system pressure (as you have done)
b) verify ignition timing
c) set and correct the warm control pressure
d) set and correct the cold control pressure
e) set CO levels

A too-high control pressure (ie one that follows system pressure) usually is a blockage in the line from the FD to the WUR, or the line from the WUR back to the FD, or in the pressure damper can next to the FD (the 450's always have this can, can't remember for the 6.9's).

As a first step in your case I would remove the lines in question, blow them through with carb cleaner. Also blow out the damper can with carb cleaner, into paper towels so that you can see what comes out (probably rust). Next, you also need to verify that the inlet screen to the WUR is not clogged.

The pressure regulating component of the WUR is the black cylindrical unit with the two hose fittings. Normally, on the underside, when disassembled, you see two holes in the block, one central and one between the two chrome faced rings. Each of the two holes leads to one of the external ports. The dimensions of the O ring a re critical in affecting pressures. Normally then there is one ?stainless? diaphragm disc, not two.

The way the hot control pressure is adjusted is by driving this entire assembly up or down in the housing, as this affects how hard it presses onto the vertical pushrod. But don't mess with this until you've verified the lines, damper and screen.

There is more than this to driving the cylindrical block up or down, as doing so also affects the acceleration enrichment.

---

116.036 - trusty rusty
116.036 - Rooikat
116.024 - the merc
116.028 - Penelope
116.032 - Esmeralda
126.037 - Scheckter
126.037
107.046

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I have also seen a blockage in the return line to the tank, at the fuel tank itself, cause back pressure that increased control pressure to the point that the car would not run.

Chris

Chris, you are correct, I see that from time to time as well, but a blockage there usually manifests itself as a very high system and control pressure ~8bar, or whatever the pump can manage to do.

The fact that he managed to shim the system pressure up and down leads me to believe the return line is not blocked.

However, I agree that it would be good practice to blow the return line through and verify that it is clear.

---

116.036 - trusty rusty
116.036 - Rooikat
116.024 - the merc
116.028 - Penelope
116.032 - Esmeralda
126.037 - Scheckter
126.037
107.046

---

---

quote:

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Originally posted by S class

Chris, you are correct, I see that from time to time as well, but a blockage there usually manifests itself as a very high system and control pressure ~8bar, or whatever the pump can manage to do.

The fact that he managed to shim the system pressure up and down leads me to believe the return line is not blocked.

However, I agree that it would be good practice to blow the return line through and verify that it is clear.

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Thanks to all for the comments. I've done some more digging. It's becoming somewhat frustrating! Here's a little more info:

1. There definitely IS something weird about the return line. I hadn't checked it originally but did so yesterday and it was indeed blocked. I eventually managed to blow it out but it was very difficult. What's weird is that it seemed to reclog itself. Interestingly I was able to gravity-siphon gas through the return line out of the tank and the flow was good. I'm at a loss as to what's going on and not sure what to do except keep on blowing.

Also, where does the return line enter the fuel tank? I can't find it and searches didn't turn up anything.

2. Because of questionable fuel return I bypassed the fuel return line altogether with a hose from the FD to the gas tank. The flow is strong and steady when pump is running.

3. The lines to/from the pressure damper are clear. It's possible to blow carb cleaner or air through it from both directions. I didn't see anything come out (rust, chunks etc). Perhaps it's a good idea to replace the damper anyway? Looks like the original.

4. Timing is OK.

5. Even with the fuel return bypass, the WUR pressure closely tracks the primary pressure minus about 0.3 bar. I tried primary pressure at several levels with the same result. I also swapped a WUR from a known and well running vehicle and it did the same thing as well. So, it would seem the the problem(s) don't lie with the WUR.

6. A further problem was discovered during the static fuel pressure test. After shutting off the pump the pressure drops to about 2.5 bar and initially holds steady BUT drops after only about 3 minutes. Presumably the fuel accumulator is bad? I'd think a leaking diaphragm wouldn't hold pressure at all and you'd see an immediate decline, even if slow, instead of a delay. It's clear that the accumulator is also very old, much older than the other components on the "fuel cluster" under the car. I guess it won't hurt to replace it. Regardless, it's not clear to me how the accumulator would account for the pressure problems.

7. I noticed that manifold vacuum seems rather on the low side, about 28 cm Hg. I'm at 5000ft altitude, so that would account for some of it but it still seems low.

Hoping for some more ideas here. I'm not ready to visit my mechanic quite yet! Thanks,



Dave

You are doing well so far.

WHen the engine is shut off, its normal for control pressure to drop immediately to approx 2.5 bar, because that's the opening pressure of the injectors. If it decays rapidly from that point down, it means their is a leak in the system. This leak could be :
a) dribbling cold start injectors
b) dribbling main injectors
c) fuel distributor leaking fuel past the metering piston into the inlet manifold
d) fuel pump outlet check valve is not closing properly
e) accumulator diaphragm torn.

By far, the most common of the above possibilities is the accumulator. Its easy to test. Disconnect the 6mm ID leak-off line from the back of the accumulator where it goes back to the damper can. Plug the connection to the damper can (otherwise the tank will empty itself on the ground). You should see some fuel coming out of the back of the accumulator. Have an assistant start the engine, or cheat the pump to run by means of depressing the air meter flap. You should then see an initial squirt of fuel out of the accumulator as the diaphragm deflects with pressure, but if fuel continues to squirt out, then the diaphragm is torn. I've seen everything from gentle flows out ofthe accumulator (ie small tear in diaphragm) up to powerful jets of fuel (complete failure of diaphragm. Either way, you need to replace it then.

---

116.036 - trusty rusty
116.036 - Rooikat
116.024 - the merc
116.028 - Penelope
116.032 - Esmeralda
126.037 - Scheckter
126.037
107.046

---

---

The return line enters the tank back at the lower left hand side of the tank. There are two short rubber hoses connecting two ports on the tank to the hard lines under the car. They are about 100mm long, one is 8mm ID, the other is 9.5mm / 10mm ID. One is the return line, the other is the tank breather.

If you disconnect there, you will then be able to determine if your return line problems are the hard line, or the entry into the tank, or even just the rubber part itself.

That stub return into the tank goes in at the base of the tank, and inside the tank, it runs horizontally towards the centre of the tank, where it has a plastic jet affair that directs the return flow directly into the entrance of the discharge swirlpot. The reason for that, is so that when the tank level is getting extremely low, the car can still run until literaly zero fuel is in the tank.

---

116.036 - trusty rusty
116.036 - Rooikat
116.024 - the merc
116.028 - Penelope
116.032 - Esmeralda
126.037 - Scheckter
126.037
107.046

---

---

quote:

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Originally posted by S class

You are doing well so far.

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LOL, thanks for the encouragement! I need it at this point.[xx(]

quote:

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Originally posted by S class

WHen the engine is shut off, its normal for control pressure to drop immediately to approx 2.5 bar, because that's the opening pressure of the injectors. If it decays rapidly from that point down, it means their is a leak in the system. This leak could be :
a) dribbling cold start injectors
b) dribbling main injectors
c) fuel distributor leaking fuel past the metering piston into the inlet manifold
d) fuel pump outlet check valve is not closing properly
e) accumulator diaphragm torn.

By far, the most common of the above possibilities is the accumulator. Its easy to test. Disconnect the 6mm ID leak-off line from the back of the accumulator where it goes back to the damper can. Plug the connection to the damper can (otherwise the tank will empty itself on the ground). You should see some fuel coming out of the back of the accumulator. Have an assistant start the engine, or cheat the pump to run by means of depressing the air meter flap. You should then see an initial squirt of fuel out of the accumulator as the diaphragm deflects with pressure, but if fuel continues to squirt out, then the diaphragm is torn. I've seen everything from gentle flows out ofthe accumulator (ie small tear in diaphragm) up to powerful jets of fuel (complete failure of diaphragm. Either way, you need to replace it then.

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Thanks, very helpful checklist. To be clear, the pressure holds perfectly for a few minutes and then declines abruptly. Anyway, I'm glad that accumulator is most common cause of pressure drop, cause I just ordered one, and for a good price. The existing unit is clearly very old, almost certainly original, so I chalk this up to maintenance. I'm in an extremely hot desert environment so hot start problems are best avoided. Dealer price is about $600US!! Thanks,

Dave

quote:

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Originally posted by S class

The return line enters the tank back at the lower left hand side of the tank. There are two short rubber hoses connecting two ports on the tank to the hard lines under the car. They are about 100mm long, one is 8mm ID, the other is 9.5mm / 10mm ID. One is the return line, the other is the tank breather.

If you disconnect there, you will then be able to determine if your return line problems are the hard line, or the entry into the tank, or even just the rubber part itself.

That stub return into the tank goes in at the base of the tank, and inside the tank, it runs horizontally towards the centre of the tank, where it has a plastic jet affair that directs the return flow directly into the entrance of the discharge swirlpot. The reason for that, is so that when the tank level is getting extremely low, the car can still run until literaly zero fuel is in the tank.

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Many thanks! Will play with this stuff immediately.

Yet I'm still not sure how much closer I am to solving the underlying pressure problem. To recap it's this:
[control pressure] = [primary system pressure] - [~0.3 bar] regardless of system pressure.

Is it possible that the FD could be blocked somepace, specifically around the primary pressure regulator where the WUR return line goes? The return line itself is clear. I'm going to play with this.... Cheers,

Dave

OK, back again. Symptoms are the same per previous post:

[control pressure] = [[primary system pressure] - [~0.3 bar]] regardless of system pressure. When system pressure is set as low as possible (no shims) the car runs pretty well. A casual passenger probably wouldn't notice anything amiss. But it's most definitely not optimized. Per above here are my most recent efforts:

• blew out fuel return line. It's now 100% clear.


• Replaced short hose stub from return line to tank. It was in very bad shape.


• Blew out tank vent, which seemed to be clear.


• Triple checked that all lines to/from WUR are clear. They are.


• Flushed and blew out fuel damper behind FD. It was clear and unobstructed.


• Timing is good.

None of this had any effect. Also, to repeat, the pressure problem occurs with 2 (two!) WURs in exactly the same way. One is a rebuilt unit and the other was on the car and has been cleaned etc by me.

Now this really is getting frustrating and I'm really at a loss. [8][8][8]
Any further ideas would be very welcome, even if long shots. Seems like the easy and obvious have been exhausted. Thanks,

Dave

1977 450SEL 6.9, No.923

Possibly related to your issue, this weekend I had a euro 280SL 107 come in with fueling issues. I put the K-jet gauges on, and noticed that the car started fine, but then the system pressure steadily climbed up to about 12 bar, with the control pressure following it closely. After various experiemnts we determined that the steel return line under the car was partially blocked. Blowing it out with compressed air seemed to suggest that it was OK, but it appears it was constricted such that flow was insufficient.

We confirmed the problem by disconnecting the return line at the fuel distributor, and then connecting a temporary line from the FD return port into a fuel can. THen the car ran happily and pressures remained correct.

I then took out the return line from under the car, and cut it open, to find it full of rust flakes.

---

116.036 - trusty rusty
116.036 - Rooikat
116.024 - the merc
116.028 - Penelope
116.032 - Esmeralda
126.037 - Scheckter
126.037
107.046

---

---

quote:

---

Originally posted by S class

Possibly related to your issue, this weekend I had a euro 280SL 107 come in with fueling issues. I put the K-jet gauges on, and noticed that the car started fine, but then the system pressure steadily climbed up to about 12 bar, with the control pressure following it closely. After various experiemnts we determined that the steel return line under the car was partially blocked. Blowing it out with compressed air seemed to suggest that it was OK, but it appears it was constricted such that flow was insufficient.

We confirmed the problem by disconnecting the return line at the fuel distributor, and then connecting a temporary line from the FD return port into a fuel can. THen the car ran happily and pressures remained correct.

I then took out the return line from under the car, and cut it open, to find it full of rust flakes.

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Great minds think alike. I did the same thing early on, but without success. Since then I checked and rechecked all other lines, blew out the damper, etc, so I can't say with certainty that there weren't multiple causes. I'm going to try it again after installing the new fuel pressure accumulator that just arrived. I don't know what difference that would make, but it can't hurt.

Dave

1977 450SEL 6.9, No.923