Quote:
Originally Posted by timmy2
Have you fixed the residual fuel pressure immediately going to zero by replacing the accumulator?
|
Up until last night I had been more or less ignoring the residual pressure issue because my fuel pump runs when the ignition is turned on.
However, because I'm running out of options, I checked the fuel accumulator last night. The bottom fitting holds vacuum without any leaking. I think this means that the accumulator checks out.
So I next checked residual pressure with the valve on my pressure tester closed. It holds right at the system pressure of 70 psi.
I did some more reading and then pulled out the "primary pressure valve" in the fuel distributor. It was immaculately clean. More importantly, it is of the "single spring" type.
At this point I realized I'm chasing my tail. The WUR outlet goes directly back to the fuel tank. There are no check valves or regulators AFTER the WUR.
The only possible source for residual pressure loss would be the WUR itself.
The fuel pump check valve, the fuel accumulator, and the PPV are all
upstream of the WUR.
Sure enough - the 911 workshop manual confirms this - if the residual pressure does not drop when the valve is closed (position 3 in the manual) the problem can be in the [warm up regulator] or control pressure regulator. Since my car does not have a control pressure regulator, that leaves the WUR.
The manual says the test for the WUR is to disconnect the WUR outlet (with the fuel pump shut off). If it leaks with a residual pressure between 1.5 and 2.4 bar, the WUR has failed. Of course, if the residual pressure is zero, that would suggest the same thing.
I also see that around 1978 Porsche added a check valve to the PPV and routed the WUR outlet to that check valve. Apparently they found that the WUR doesn't do a great job when it is acting as a check valve. This actually makes sense, since prior to '76 the residual pressure shouldn't have been much of an issue, but once they added the microswitch for the fuel pump, the retained pressure became a more significant problem.
Unfortunately, none of this solves my problem. However, it is one more thing to cross off the list.
Of course, it does raise a few other questions - for example - why on earth was my control pressure zero before I cleaned out the WUR?
Also - why is the WUR not retaining any residual pressure?
I have a theory on this - I have my cold control pressure set at 11 psi. If I'm understanding the system correctly, this would mean that the pressure valve is only exerting a small amount of pressure on the fuel flow. Put another way, the WUR is permitting the vast majority of fuel to flow through the WUR and back to the tank. A pressure drop is expected after the pump is shut off, so maybe with a warm WUR I would have some residual pressure.
Anyway, sorry for the long post. I'm still looking for advice, hints, or tips.
One question - is there a "baseline" setting for the mixture screw? My car obviously had its fuel injection messed with in a past life, and I have no idea where I'm working from with some of these settings.