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Today I started car again to see how it reacted . Outside temp was 34°C / 93°F.
Starting was good but first 10 secs the idle was very slow rising from 400 to 1000 rpm. Pressure was 1.5 bar / 21 PSI .
After warming up for about 2 to 3 minutes, my pressure went to 2 bar / 29 PSI
So then I start driving the car, acceleration was good but once I kept accelerating above 60 Mph, it hesitated when I shifted up as if I lacked power
I checked my pressure again : still 2 bar. This is not good.
I also measured my mixture via color tune spark plug and it was nice blue when accelerating so combustion should be ok, not ?
I don't understand where my problem is, except that my pressure should be 3.4 bar instead of 2... so the only culprite that I can see is my WUR
If your control pressure is in the range like it looks like it is, it does seem that the WUR is the issue.
Is there a test you can do on the WUR where you remove the vacuum line and the 12v and see how it reacts. Maybe jpitman can chime in on what to look for here.
Your cold start seems a little off too though. I assume your cold start injector is working correctly since the car fires right up? My idle on a cold start is much higher, like 1,600 rpm then falls gradually to where I have it set, which is around 900 rpm, but it takes a few minutes to get there. That is also controlled by the WUR so it does seem like your WUR could be the culprit.
I believe that the bimetal is working, since it rises my control pressure with 0,5 bar, but the control pressure itself should become much higher
once engine is fully warmed up (range should be 3.2 - 3,8 bar while mine is only 2 - 2.2 bar)
Control pressure cold with ambient temp = 25°C/77°F should be 2.0 bar instead of 1.5 bar when checking the curve of 0438.140.036 or 0438.140.086
So pressure warm and cold are much too low.
It looks like the membrane inside the WUR is not tight enough to make the pressure.
I believe that the bimetal is working, since it rises my control pressure with 0,5 bar, but the control pressure itself should become much higher
once engine is fully warmed up (range should be 3.2 - 3,8 bar while mine is only 2 - 2.2 bar)
Control pressure cold with ambient temp = 25°C/77°F should be 2.0 bar instead of 1.5 bar when checking the curve of 0438.140.036 or 0438.140.086
So pressure warm and cold are much too low.
It looks like the membrane inside the WUR is not tight enough to make the pressure.
I believe control pressure is measured on the input line to the WUR from the Fuel Distributor, and with the engine not running and the fuel pump jumpered. If your control pressure is too low, I think the problem is in the fuel distributor or the fuel pump, filter, line system.
I think you need to work with the shims on the fuel distributor pressure screw until you get the right control pressure. You can rebaseline the mixture screw by turning it all the way CCW then with the fuel pump jumpered start turning it CW until you hear fuel squeaking or squealing in the air measuring shoe assembly, then back it off CCW again about 3/4 turn. That should get you close on mixture. If the control pressure is wrong, you can mess things up with the mixture screw and think it's running right but really isn't. Step 1 is to get the control pressure right.
I was just forced to learn a bunch of this on my own car, so not an expert by any means, but learning - so sharing, and having an AFR gauge was very helpful to be able to see what was going on.
I believe control pressure is measured on the input line to the WUR from the Fuel Distributor, and with the engine not running and the fuel pump jumpered. If your control pressure is too low, I think the problem is in the fuel distributor or the fuel pump, filter, line system.
I think you need to work with the shims on the fuel distributor pressure screw until you get the right control pressure. You can rebaseline the mixture screw by turning it all the way CCW then with the fuel pump jumpered start turning it CW until you hear fuel squeaking or squealing in the air measuring shoe assembly, then back it off CCW again about 3/4 turn. That should get you close on mixture. If the control pressure is wrong, you can mess things up with the mixture screw and think it's running right but really isn't. Step 1 is to get the control pressure right.
I was just forced to learn a bunch of this on my own car, so not an expert by any means, but learning - so sharing, and having an AFR gauge was very helpful to be able to see what was going on.
The FD pressure screw (=fuel system pressure regulator) is for setting the system pressure. This is correct , I am measuring 5.8 bar. It is the control pressure measured at the input of my WUR that is not correct. This is a kind of counter pressure against my system pressure, regulated via the membrane inside
the WUR.
But I definitely hear my fuel squeal and the setup of mixture via color tune sparkplug from Gunson is accurate. So mixture seems correct at idle.
Ofcourse, once you are driving it is another situation...and there my problem comes to the surface while accelerating heavily...it stumbles each time my gear shift up, as if I am lacking the correct amount of fuel.
So to rule-out my WUR, I've just sent it back for checkup to the overhaul company since it was overhauled 2 months ago.
Perhaps internally there is an issue ? But I cannot open the WUR as it is still under warranty,
I'll keep you posted with the result...
Problem has been found by my Kjet specialist Allan ( super thanks !) : Brown dirt under the membrane of the WUR,
also the shell of the WUR was rusted inside which indicates water in the fuel.
How is this possible and how can this be prevented ?
Are there special additives for the fuel?
So are you now going well? Glad to hear a cause has been found. I dont think there is anything that can remove water from the fuel - maybe try hanging some bags of stuff (dessicant) that extracts water from the air - dehydrating crystals? Can you get a sediment bowl suitable for the pressures involved? Is the in-tank strainer in good condition ? If not, replace, and the filter as well.
Do what aircraft operators do - try to keep the tank as full as possible to minimize the volume of air that might have moisture in it, and store it where the temperature change is minimal - might be difficult where you are in winter?
jp 83 Euro S AT 57k
So are you now going well? Glad to hear a cause has been found. I dont think there is anything that can remove water from the fuel - maybe try hanging some bags of stuff (dessicant) that extracts water from the air - dehydrating crystals? Can you get a sediment bowl suitable for the pressures involved? Is the in-tank strainer in good condition ? If not, replace, and the filter as well.
Do what aircraft operators do - try to keep the tank as full as possible to minimize the volume of air that might have moisture in it, and store it where the temperature change is minimal - might be difficult where you are in winter?
jp 83 Euro S AT 57k
Well, before installing my new WUR, i will first clean
my fuel tank and fuel line together with FD.
Probably because my tank was always only filled 1/4
because my 928 was still in overhaul status.
My fuel pump and filter are new. Perhaps the problem
is injection cleaner I used 2 months ago making rust parts loose in my fuel system.
to keep water out of the fuel tank keep the tank filled,
Ok thanks, seems indeed logic.
Remark : years ago I removed the supplementary fuel
tank under the rear right fender because the fixation points were all rusted away and I also removed the
carbon filter system.
Now the overflow of the filler neck of my fuel tank is open with only a rubber hose hanging downwards.
Is this bad for moister getting into my tank?
Damn, this looks really awful. Well in my case, it could
be that I filled my tank with bad fuel. I heared from a fellow 928 friend of mine that some gas stations add water to their fuel to cut costs....hopefully it was not
my case...😬...i think that in my case it was rust that
came lose by adding injection cleaner to my fuel. Remember that I also had a leaking injector because
of this!
I believe inspection of my fuel distributor and tank would be wise🙂
Generaly , water in fuel come with condensation .
Hot day and cold night for exemple ...
Rust need humidity and oxygène .
When a car don't run for few month , fuel finish in vapor and rest pressure go down .
Water stay in fuel distributor and you know the result ...
The best way for don't have rust , is to drive your car !
Try , with a wood screw , to unplug the little filters in the fuel distributor to see if you have rust .
07-23-2019, 09:09 PM
