CIS Warm Control Pressure at Full Flow
 

Should a WUR maintain the set warm control pressure when the metering plate is full up and injectors spraying at max?

My warm control pressure is set to 2.9 bar with system pressure at 4.8 bar. As I raise the metering plate past ~ 3/4 (engine off / fuel pump running / injectors in bottles), the warm control pressure starts falling and stops at ~ 2.5 bar. Low to middle metering plate positions give me normal control pressure.

The car is a '74 911. I don't have any running issues and AFRs are as expected throughout RPM / load range. This is with the original 001 WUR and throttle position pressure valve in place. Just makes me wonder if I have a weak or failing fuel pump.

Thanks for any input.

Interesting. If you close the valve on your CIS gauge set to measure System Pressure and raise the metering plate, does the SP also drop? I agree, that would suggest a marginal fuel pump that can't maintain pressure at higher flow rate or maybe an obstruction in the fuel supply.

If that's the case, check the fuel pump output for pressure and flow.

Your 74 as well as my 1973.5 CIS have a throttle position regulator in addition to the WUR. The TPR is designed to have a slightly richer mixture at idle, lean out a bit at mid range, and richen the mixture at full throttle.

http://forums.pelicanparts.com/uploa...1709334409.jpg

Good points. I will run the same test with the WUR valve closed to see the effect on system pressure. I'll also try it with the throttle in the midrange and report back. Thanks for the help!

Another thing to consider: The air metering plate does not move up in actual operation nearly as much as you can push it with your fingers. You may be raising it farther than the fuel pump's delivery capacity.

However, start with a pump delivery test.

Those early WURs are pretty sweet, as in you can easily adjust them with the hex screw on the bottom.

I did a quick test with the pressure gauge valve closed to register system pressure (4.8 bar). Metering plate at full up dropped the pressure to 4 bar. I briefly pinched off the return line to the tank to get an idea of total pump pressure and the system pressure climbed to ~ 5.7 bar. Not sure if that's a valid test of anything.

PeteKz has a good point about actual metering plate height at WOT in a running engine. I was just curious about my observation.

The 001 WUR's adjustability is pretty nice. IIRC it only changes cold control pressure but I haven't messed with it for years. Speaking of adjustable WURs, I recently bought a UTCIS-V on a whim because I've wanted to convert my 001 WUR and throttle position regulator to a vacuum assisted setup. I'm well aware of the horror stories, but couldn't resist. Steve gave me a really good deal, so I figured what the heck. I have the unit in hand and I'm pretty impressed so far. The electronics are self-contained in the aluminum block and it's basically plug and play. More to come when I get it installed. So far Unwired Tools have been great to work with.

It sure sounds like a restriction in the fuel supply. You demonstrated the pump can produce spec pressure when dead-headed. The fact that it can't maintain pressure at high flow is consistent with a having a partial blockage somewhere in the supply side. Ideally you would insert a pressure gauge at various points in the supply path to see where the restriction is, but no one has the adapters that would require. I think you're going to have to test a few things piecemeal.

How's your fuel filter? Do you know it's history?

How does the pump sound? If the screen in the tank (a '74 has one, right?) is clogged the pump will cavitate and make a noise like it's grinding rocks.

There is a flow spec (cc per minute) for the pump. You could break the connections at various points (filter inlet, filter outlet, accumulator in/out, FP outlet, etc.) to see if the max flow changes significantly at any point. If you do this, remember you're handling gasoline and take appropriate precautions.

Good luck.

All of the pressure tests are done with the engine off, and air flow meter plate in the down position. Testing Control pressure on engines with the Throttle position regulator are done by opening the throttle body, but the air flow plate is in the down position. I have not been able to find any pressure results with the plate raised as you have mentioned, or while the engine is running. The only one on the forum that might have an idea on that might be Pelican Parts Forums - View Profile: boyt911sc Maybe he will read the thread and comment.
To get an idea of the fuel pumps supply ability after verifying System pressure, you need to disconnect the return line to the gas tank and put it in a container, then run the fuel pump for 30 seconds. This test is done with the electrical to the WUR disconnected.
My '73.5 2.4l with the 001 WUR should get 750cc's. The chart for later '74 2.7l on with the 008, 009, 017,and 033 WUR shows 850cc for 30 seconds. If there is a restriction it will be less than 850cc. If you've never checked it, I would take the filter screen in the bottom of the gas tank out and clean or replace it. Of course, to do this the tank should be as close to empty as possible, since it is at the lowest point in the tank. Pinching off the return line should raise the pressure as it restricts gas flow, unless it was already suffering from blockage.

http://forums.pelicanparts.com/uploa...1709477766.jpg

Appreciate the responses. The pump is new within the last two years. It's a universal Bosch unit and still mounted in the rear. No strange noises or hiccups.

I changed the fuel filter and accumulator a few months ago. The filter was new but at least 20 years old. I still have the original '74 filter setup and stashed a few filters while they were still around. Fuel tank screen was replaced at the same time. The old one was perfectly clear too.

I tested the pump flow according to the manual when I put the new filters in and had more than enough, but will do it again just to verify.

I mentioned in my original post that I'm not having any starting or running issues. This was just an observation as I was double checking my current pressures in preparation for the UTCIS install. I thought the same thing as ESully about the test specs all being with the metering plate in the down position. I don't recall any posts about CIS pressures in a running engine or with the FD flowing to the injectors.

I'll see if I can do a flow test later today. Thanks again for the input!

Before you consider UTCIS, read this page:
https://forums.pelicanparts.com/porsche-911-technical-forum/1147010-digital-wur-utcis-v.html

Then remember this: If it ain't broke, don't fix it!.

Thanks PeteKz. I know, I know. I love the simplicity of the original stuff. There's some piece of me that want's this to work though. I hate the idea of adding electronics to the mix, but I've never liked the idea of the 74-75 setup with the throttle position compensator leaning out mixture based on throttle position vs engine load. I was contemplating switching out to a 033 WUR and thermotime valve but the cost was just annoying. The UTCIS was about the same cost as a typical WUR rebuild so I figured I'd give it a chance. If it turns out to be a dud I'll at least have an interesting paperweight and maybe a potential backup. The original design that had fuel running to the control box was downright dangerous. The current iteration has all of the pressurized components contained in a single machined block of aluminum. Fingers crossed. Thanks again for the input.

Then check with Tony for a replacement WUR that does not use the throttle regulator.