Control pressure testing - STUPID Queston #1

mark houghton · 03-03-2009, 04:35 PM

Ok, I've got the gauge and I've read all the instructions, plus those in my shop manual and other locations, but for the life of me I've got a mental block on one simple part of testing the primary and control pressures:

The obvious: The fuel pumps must run in order to get any readings.
The not so obvious: After installing the gauge, do you run the engine or just run the fuel pumps?
The fear: Won't running the fuel pumps (without the engine running) just dump a bunch of raw fuel through the injectors and into the cylinders? Not a good thing.

So, how about just a little tutorial from someone who has done this himself?

gsmith660 · 03-03-2009, 04:46 PM

CIS will not dump fuel through the injectors unless the plunger on the fuel distributor is actuated, the factory manual has a whole section on testing the control pressures if you know someone close has a copy you can use. If I remember right some is with the car off and some with the car running.

mark houghton · 03-03-2009, 05:36 PM

Quote:

Originally Posted by gsmith660

CIS will not dump fuel through the injectors unless the plunger on the fuel distributor is actuated, the factory manual has a whole section on testing the control pressures if you know someone close has a copy you can use. If I remember right some is with the car off and some with the car running.

Thanks Jerry, I've got the shop manual. I think it instructed to test the cold pressure without the engine running (as well as the warm pressure), but the primary pressure needed the engine running? Whereas boost enrichment with the engine off and pumping something like .7 bar of air pressure into the WUR.
Anyway, had I thought about it just a bit more I would have realized that no fuel gets to the injectors unless control arm is drawn down.
Just looking for someone who's BTDT.

jwasbury · 03-03-2009, 06:05 PM

I've done this test on my car Mark...no worries. Just don't push on that plunger. FYI, I also found that I could test my warm CP without running the engine...just leave the WUR unplugged for the CCP test, then plug it in and watch the CP rise. I did the test a 2nd time but ran the engine and found no difference in the WCP running or not...I think that means the heating element in my WUR is good. Definitely make sure that you are referencing the correct chart in the service manual and match up the part number to your WUR. 1st time I checked it, I thought my CCP was out of spec, but then realized I was looking at specs for a later WUR, not the one in my '79. System pressure cannot be tested with the engine running IIRC, at least not with the gauge hooked up on the fuel line between the WUR and the center of the fuel head.

mark houghton · 03-03-2009, 07:03 PM

Thanks Jacob, it's all becoming much clearer now. Real world experience tells me more than the written word sometimes.,

darling930 · 03-04-2009, 03:03 PM

Hi Mark,
Just to throw my 2cents in. Don't forget when you are checking cold control pressure the engine has to be stone cold and unplug the connection from the WUR before you start as there is a bimetallic strip that starts to change the CP when it gets power (only gets power when the fuel pumps are running). Also do not forget to "burp" the pressure gauge lines as described in the gauge instructions. Be careful the pressures are high. It is also a good idea to check the leak down pressure and the "on boost" enrichment if you have it on your WUR. Good luck

mark houghton · 03-04-2009, 04:16 PM

Quote:

Originally Posted by darling930

Hi Mark,
Just to throw my 2cents in. Don't forget when you are checking cold control pressure the engine has to be stone cold and unplug the connection from the WUR before you start as there is a bimetallic strip that starts to change the CP when it gets power (only gets power when the fuel pumps are running). Also do not forget to "burp" the pressure gauge lines as described in the gauge instructions. Be careful the pressures are high. It is also a good idea to check the leak down pressure and the "on boost" enrichment if you have it on your WUR. Good luck

Yep, I plan on doing the entire enchilada once I get in there. My WUR has the boost enrichment (I thought all 930 WUR's did?)...or at least I assume that's what the vacuum hose leading to the intake manifold just below the butterfly is for. It should pull vacuum when the butterfly is closed (off-throttle) and provide boost when boost is there with the throttle open. So, the internal diaphram in the WUR would respond by leaning the mixture with off-throttle, and enrichening it when on boost. Or so I would assume considering where the line is attached. Anyway, I'll be artificially introducing up to .8bar of boost at that line and see what it does to the control pressure.

jwasbury · 03-04-2009, 07:48 PM

Mark,

In my own testing, I found that the WUR responds only to boost pressure by lowering control pressure. It does not raise control pressure under vacuum. I believe others here (Alan L comes to mind) have had the same result.

mark houghton · 08:17 AM

Quote:

Originally Posted by jwasbury

Mark,

In my own testing, I found that the WUR responds only to boost pressure by lowering control pressure. It does not raise control pressure under vacuum. I believe others here (Alan L comes to mind) have had the same result.

Thanks. I was just operating on logic only - assuming the internal diaphram would move in both directions respective to boost or vacuum. To test it, I sensuously applied my lips to the hose barb, gently blowing and sucking like a street walker, and could hear the diaphram moving in both directions.
I'll just add this little tidbit to my memory banks.

JFairman · 03-05-2009, 09:20 AM

It's also a good idea to wrap a thick absorbant rag like part of an old towel around the air purge button just under the gauge when pressing it to purge any air out of the line, so gas doesn't spray or gurgle all over the place.

If you turn the inline ball valve to cut off fuel flow through the control pressure return line >for checking system pressure the motor should be off.
If the motor is running and you do that it will stall out the motor.

JBL930 · 03-05-2009, 09:36 AM

It does look like the WUR would react to vacuum though doesn't it. Has anyone actually had a gauge on there and added vacuum to it and checked? I would have thought it would raise control pressure with vacuum just as it lowers it with boost. I know this has been discussed but it would be nice to have an answer from someone who has actually done this exact test

JFairman · 03-05-2009, 09:50 AM

Send Brian Leask an email.
He would know the correct answer to that.

Along with making the WUR adjustable he can install a stronger diaphram so it can handle 1.3+ bar if you want.

http://members.cox.net/930wur/

mark houghton · 03-05-2009, 10:03 AM

Quote:

Originally Posted by JBL930

It does look like the WUR would react to vacuum though doesn't it. Has anyone actually had a gauge on there and added vacuum to it and checked? I would have thought it would raise control pressure with vacuum just as it lowers it with boost. I know this has been discussed but it would be nice to have an answer from someone who has actually done this exact test

It should be simple enough to find out. Maybe I'll check it when I test all other control pressures. Just pull a vacuum on the port and see if we get an increase in control pressure as theorized.
BTW, I should soon have in my hands a Leaskified WUR...with the stronger diaphram (just some added assurance against future failure).