Control Pressure same as System

[wicks]

Hi all - thought I would post here in hopes of casting a wider net on a fuel pressure issue I'm having with my 964 3.6 turbo.

I had a perfectly running car (36K miles) a week ago. Last fuel related issue I had was replacement of the accumulator which solved the hot start issue I'd had.

Right now, I have a car that will start/idle after cranking up the air meter enrichment screw. It will not run without missing and generally acting starved. Very far from driveable. Before I did that, the car would start then stall immediately. So I feel that cranking up the mixture screw has really just hacked the problem to get an idle. I am able to adjust the AFR to 14.5 using this screw though. This happened after a car wash one day after the car drove well all afternoon.

I've rebuilt the fuel head with all new orings and diaphragm, did not fix. I've replaced the system pressure regulator, shimmed with same as original shims. I've cleaned out the air meter and made sure it moves freely. I've checked and rechecked all intake connections for air leaks. I've replaced all vacuum lines.

When I run the system pressure I am in range at 6.7 BAR. When I throw the tester lever to open the WUR side of test line, I get no change in pressure. I.e the system pressure is the same as the control pressure. Not even a slight change on the gauge.

Anyone with fuel pressure knowledge familiar with what piece of the system would cause this? Blocked lines is a cause I've read, but I can't figure how a line could have become blocked, and I know fuel is making it back to the tank, I can hear it.

[993Speedster]

Sounds a whole lot like you’re tuning around a vacuum/boost leak. You have access to a smoke test machine?

[T77911S]

check the inlet screen to the WUR. if it gets clogged your CP will be hi.
the return side back to the tank could be clogged. I can blow through mine.
possibly an issue with pressure regulator. it closes the return side back to the tank from the WUR to help with residual pressures.

just a word of advise, and to others. you should always check pressures (in this case) and as much as you can before you start making adjustments or spending $$.

[RarlyL8]

^^^
Classic signs of a blocked WUR. Other things such as a blocked return line or blown regulator o-ring may cause similar symptoms but this is most common. Best course of action when something changes suddenly is to NOT change any of the tuning settings to compensate. Makes it harder to find the problem and to get back to where it was once the problem is solved.

[wicks]

Thanks all - I removed the WUR, have not taken apart. The WUR is definitely "closed" but I don't know how they operate...takes a certain pressure to open it I imagine. Air at 20 psi won't pass through. Is there a bench test I can do to see if it is basically working?

[flightlead404]

Originally Posted by wicks

Thanks all - I removed the WUR, have not taken apart. The WUR is definitely "closed" but I don't know how they operate...takes a certain pressure to open it I imagine. Air at 20 psi won't pass through. Is there a bench test I can do to see if it is basically working?

Air doesn't pass through it, changes in air pressure operate on a diaphragm. One side goes to port (III?) on the front RHS of throttle body to sense manifold pressure, the other side simply vents the back of the diaphragm.

The WUR alters control pressure based on the movement of the diaphragm and also a bimetallic strip that is heated by a coil powered off the fuel pump(?) circuit.

[T77911S]

there is an inlet screen for fuel that gets clogged
its the fuel line from the FD

[wicks]

I know it's basic function - just don't know whether air should be able to pass through it when it's sitting on the bench. Through the fuel channel, not the vacuum or vent port.

Anything I can do to test it while not installed?

[T77911S]

clean the inlet screen and stop worrying about air.

[mark houghton]

I don't know 964's, but assume they too run with two fuel pumps. Maybe a stupid question, but do you know for sure that both pumps are running?

[RarlyL8]

You can use compressed air to "back purge" the screen but be careful with the pressure so you don't damage the diaphragm. Put the WUR in the freezer overnight to pull back the bimetallic strip and open the space. Clean the screen then blow both ways. Run pressurized fuel through to further clean and verify no occlusion. Here is a schematic so you can see how this works.

[wicks]

Ahh excellent thanks Rarly. Also, my resistance is 17.3 OHMS, wondering if that's acceptably out of range. This is a 164 WUR.

Yes both fuel pumps are running. The screen was perfectly clear when removing fuel lines - nothing to clean.