Erratic WCP

[Vereeken]

I have a curious case of erratic WCP behaviour on a WUR installed in a Euro SC.

The 089 WUR is Vac assisted.

I measured WCP with engine not running jumpering the Fuel Pump.

(all in bar)
Non vac I came to 3.3 with vac 3.9.
Specs says non vac 2.7/3.1 and 3.4/3.8.
The vac situation was measured with engine running and very hot. So the vac pulled by the engine is correct.

Fine no drama, I turned the WCP down 1/2 turn.

Measured again and all fine non vac dropped to 2.7/2.8 so right in spec.

FFWD 2 days. Drive the car for 10 minutes and what do I get? Yep the old numbers again. WTF?

Aside from chipmunks and Gremlins any idea.

The wur is new-ish and the screens are perfect.

So I suspect the small diameter hard line running away from the WUR. I believe it is called the fuel return line.
But why would it be fine one day and wrong again the other?

Should I not hear fuel returning to the gas tank if you run the FP only and listen at the filler neck?

[boyt911sc]

Originally Posted by Vereeken

I have a curious case of erratic WCP behaviour on a WUR installed in a Euro SC.

The 089 WUR is Vac assisted.

I measured WCP with engine not running jumpering the Fuel Pump.

(all in bar)
Non vac I came to 3.3 with vac 3.9.
Specs says non vac 2.7/3.1 and 3.4/3.8.
The vac situation was measured with engine running and very hot. So the vac pulled by the engine is correct.

Fine no drama, I turned the WCP down 1/2 turn.

Measured again and all fine non vac dropped to 2.7/2.8 so right in spec.

FFWD 2 days. Drive the car for 10 minutes and what do I get? Yep the old numbers again. WTF?

Aside from chipmunks and Gremlins any idea.

The wur is new-ish and the screens are perfect.

So I suspect the small diameter hard line running away from the WUR. I believe it is called the fuel return line.
But why would it be fine one day and wrong again the other?

Should I not hear fuel returning to the gas tank if you run the FP only and listen at the filler neck?

Michel,

What did you mean (highlighted above)? You are using the correct values for the WCP for WUR-089. However, you need a controlled test using the specified vacuum (400 mm Hg). The big question now is what was the actual vac pressure exerted by the engine during the test.

I still at a lost with the “1/2 turn method” to adjust the WCP. Could you elaborate on this. Thanks.

Tony

[Vereeken]

Hi Tony

Engine was giving me 0,48 bar of vac whic is according to me OK (450-550)

The 1/2 turn is the 1/2 of the allen screw at the bottom of the diaprhagm. CCW.
In order to lower the WCP.
That seemed to do thr trick but now I am back where I started and touched nothing.

I am just working on a WCP check with the bimetal removed so there can be zero interference.

Michel

[T77911S]

that allen is NOT for adjusting CP.

this is from memory and not looking at one in quite a while.

that allen adjusts the height of a "plate" on the diaphragm.
there is an inner spring that sits on that plate and an outer spring that is on a fixed base. by changing the height of that plate with the allen you change the compression on that spring.
THAT spring along with the outer pushes up on the valve that controls the pressure thus increasing the pressure. when you lose vacuum the diaphragm drops down releasing pressure on the valve thus lower the CP.

now to what you have changed
by changing the pressure on that spring what you have really changed is amount of vacuum needed to hold the diaphragm up.

so, IF you lowered the spring pressure the diaphragm needs LESS vacuum to pull up and raise the CP. what this now does is change the RPM at which your mixture goes rich, BUT will probably drop the CP a tad longer when you accelerate. not necessarily a good trade off.

(I hope I got my ups and downs correct, tony will check me. )

[Vereeken]

You are correct of course.

I am refering to the allen under the diapraghm at the outside of the wur at the bottom underneath that gold looking cap that you need to drill to get to that allen.
Indeed in the WUR there is another allen between the springs. I did not touch that.

I am fairly confident that the allen I touched is the correct one.


I just did the WCP test NO vac and with the bimetal removed.
Straight to 2.5 BAR (too low) so I adjusted the allen with 1/2 turn CW now I am at 2.7 bar WCP with no vac.


I let the FP run for 5 minutes and the WCP holds rock steady.


So why on earth would it move to 3. 9 bar with VAC applied after a while.....


It is not the springs.
It is not a blockage I think otherwise the WCP should have increased with the FP running.


Can the VAC pull increase after a while?

Did I miss something?

Any help appreciated.

[pmax]

Originally Posted by Vereeken

Can the VAC pull increase after a while?

Did I miss something?

Any help appreciated.

I would gather more data.

Measure the vacuum pull to eliminate that variable.

[boyt911sc]

Originally Posted by T77911S

that allen is NOT for adjusting CP.

this is from memory and not looking at one in quite a while.

that allen adjusts the height of a "plate" on the diaphragm.
there is an inner spring that sits on that plate and an outer spring that is on a fixed base. by changing the height of that plate with the allen you change the compression on that spring.
THAT spring along with the outer pushes up on the valve that controls the pressure thus increasing the pressure. when you lose vacuum the diaphragm drops down releasing pressure on the valve thus lower the CP.

now to what you have changed
by changing the pressure on that spring what you have really changed is amount of vacuum needed to hold the diaphragm up.

so, IF you lowered the spring pressure the diaphragm needs LESS vacuum to pull up and raise the CP. what this now does is change the RPM at which your mixture goes rich, BUT will probably drop the CP a tad longer when you accelerate. not necessarily a good trade off.

(I hope I got my ups and downs correct, tony will check me. )

Ty,

You are indeed correct. However, what you meant by (high lighted above)?



Michel,

What is the heater resistance (Ohms) value of your WUR-089? 30, 36 Ohms or something? You are not alone in observing this ‘strange’ behavior of the vacuum diaphragm. I had encountered this problem a few times and still could not figure out the culprit. I did Kemo’s WUR this summer and took me several hours of work to calibrate the WCP with vacuum. Actually it took me days to get it to work only after replacing the section with the diaphragm.

Tony

[T77911S]

tony,
I have very very little testing on this but you could help out here. like the 930 WUR that can be adjusted (when modified) to change how much boost is needed to lower the CP, the 911 WUR can be adjusted to lower the CP at a certain vacuum.

here is how I would test it.
pull a vaccum on the WUR and then slowly release the vacuum and notice at what point the CP starts to drop.
then adjust the allen in the bottom and do the test again and see if the amount of vacuum needed to lower the CP changes.
I would also try it pulling a vacuum to see when the CP goes up.

as you know, as RPMS go up vacuum will go down. there will be a certain RPM where engine vacuum can no longer hold the diaphragm up so then the CP will start to go down.
that allen adjusts the compression on the center spring thus changing the vacuum needed to hold the diaphragm up.

as said I have only done minor testing on this. on my 77s, I noticed I started to go rich around 4k RPM.

[T77911S]

to the OP.
I see your only issue is erratic CP ater messing with the allen,

tony probably wont agree with me but 3.9 is not bad. you are also getting a .6bar drop in CP, ideal would be .8.

the allen WILL change the CP but it also changes your cold CP along with the WOT transition point to going rich. if you lowered the CP, then your car should go richer at a lower RPM.

if the car ran fine like it was, set it back to what it was and leave it alone.