CIS problem: system pressure = control pressure. Both wrong.

[worn]

Time has passed; mostly out of the garage working on the house.
In last episode my system pressure was holding at about 2.5 - 3 bar, with similar pressure in the control pressure circuit. I had taken apart the WUR cleaned it and put it back together.
The fuel pump delivered more than 2 liters per minute, as measured by disconnecting the fuel line from the accumulator. So that was without restriction, and not proper according the the Bosch booklets that show testing at the return port from the accumulator. I just retired from academic research, and am used to ruling in or out possible hypotheses, including the hypothesis that I am a dumb-ass.

The system pressure could be low due to:
A) Input pump unable to keep up with a wide open return to the tank via the frequency valve.
B) Fuel regulator not working (but it had been fine before I rebuilt the motor).
C) Pump not able to keep up with the demand. My idea was that it could generate proper pressure only if outflow was highly restricted.

So what I did was to pull the fuel distributor out of the car, but still connected to the airplate. I hooked the fuel line from the pump to the inlet side of the regulator and put a hose on the return fitting so I could measure flow and collect extra gas. I left the pressure gauge in the normal spot at the top of the regulator. I then capped off the WUR return port, the frequency valve port as well as 4 of the 6 injector lines. I used really short 8 mm bolts with copper washers.
In short, I had fuel input from the pump, unrestricted return, and blocked everything else. I left two injector lines attached to observe flow.

The outcome was 2.5 bar system pressure. Fuel flow was 1.4 liters per minute via the return port, which is a bit low. Then I squeezed the return line with pliers and system pressure went readily to 4-5 bar. At that pressure fuel came out of the injector lines as I raised the airplate.

Cool. Although the fuel pump is a bit wimpy, I am thinking the fuel pressure regulator is the likely suspect and could use some help. The regulator has one shim. Is there a place where you can buy shims? I see kits for about $125 that have O-rings, a spring and the push valve. I am thinking about fabricating shims, but would prefer to just get a few from a vendor.

Any thoughts about the fuel pump?

[boyt911sc]

Originally Posted by worn

Time has passed; mostly out of the garage working on the house.
In last episode my system pressure was holding at about 2.5 - 3 bar, with similar pressure in the control pressure circuit. I had taken apart the WUR cleaned it and put it back together.
The fuel pump delivered more than 2 liters per minute, as measured by disconnecting the fuel line from the accumulator. So that was without restriction, and not proper according the the Bosch booklets that show testing at the return port from the accumulator. I just retired from academic research, and am used to ruling in or out possible hypotheses, including the hypothesis that I am a dumb-ass.

The system pressure could be low due to:
A) Input pump unable to keep up with a wide open return to the tank via the frequency valve.
B) Fuel regulator not working (but it had been fine before I rebuilt the motor).
C) Pump not able to keep up with the demand. My idea was that it could generate proper pressure only if outflow was highly restricted.

So what I did was to pull the fuel distributor out of the car, but still connected to the airplate. I hooked the fuel line from the pump to the inlet side of the regulator and put a hose on the return fitting so I could measure flow and collect extra gas. I left the pressure gauge in the normal spot at the top of the regulator. I then capped off the WUR return port, the frequency valve port as well as 4 of the 6 injector lines. I used really short 8 mm bolts with copper washers.
In short, I had fuel input from the pump, unrestricted return, and blocked everything else. I left two injector lines attached to observe flow.

The outcome was 2.5 bar system pressure. Fuel flow was 1.4 liters per minute via the return port, which is a bit low. Then I squeezed the return line with pliers and system pressure went readily to 4-5 bar. At that pressure fuel came out of the injector lines as I raised the airplate.

Cool. Although the fuel pump is a bit wimpy, I am thinking the fuel pressure regulator is the likely suspect and could use some help. The regulator has one shim. Is there a place where you can buy shims? I see kits for about $125 that have O-rings, a spring and the push valve. I am thinking about fabricating shims, but would prefer to just get a few from a vendor.

Any thoughts about the fuel pump?

Warren,

I don’t know what Bosch reference manual you have been reading but the test procedures you are doing are totally wrong. Why? Your concept and understanding how your CIS works is flawed. I offered to test your fuel distributor weeks ago for FREE in order to help you. You ignored it and refused to listen.

I will let you alone and have your way. But if for some strange miracle you come to your senses, you know where to contact me. Wish you the best. Keep us posted.

Tony

[worn]

Hi Tony,
Thanks for all of your help and kind offers. I sent you a PM. If I wait till I come to my senses, it could be a long pull. My wits are generally scattered far to seek.

[worn]

Originally Posted by boyt911sc

Warren,
Your concept and understanding how your CIS works is flawed.
Wish you the best. Keep us posted.
Tony

I am trying my best to understand CIS. I am impressed that a fuel map could be developed by a hydraulic analog computer.



Here is what I think I know - subject to change.. I hope you can help me learn more Tony. The image has an attempt to color code things and I hope it is clear.
1. Fuel comes from the pump, through the accumulator and filter and a T to the cold start valve reaching the FD where I have a blue arrow. (Color does not represent actual fuel color) It enters a hollow cylinder with a piston and slits where it can be delivered to the injectors.
2. Fuel leaves the cylinder following the green arrow. The fuel pushes against a spring loaded piston in order to get out - this regulates system pressure.
3. System pressure enters the lower set of pressure regulators through an orifice. (purple arrow). Each lower chamber provides a reference pressure for the regulator valves in the upper half of the FD. All six chambers are connected by drillings.
4. The frequency distributor can modify lower chamber pressure via the drilling indicated by the red arrow. I now recognize that it is unlikely that the frequency valve could cause my problems.
5. The WUR modifies the movement of the piston by regulating pressure at the top of the piston. Pressure goes from the lower chamber through a tiny orifice in the diaphragm that covers a hole in the lower FD indicated by the orange-brown arrow. When the engine first starts the WUR allows gas to leave the chamber above the piston, flow is limited by the tiny hole in the diaphragm so pressure drops above this piston, so at a given load the piston uncovers more slit area, fuel flows faster into the injectors and you get cold start enrichment.

There is a tiny drilling indicated by the yellow wire going from the lower part of the cylinder of the bottom part of the FD casting. I cannot remember off the top of my head what that drilling does, but I suspect it may dampen piston movement. I will look it up.

The photo is of a spare FD that I bought from ebay years ago because I thought I might rebuild one someday. It looked remarkably clean on disassembly - which is a great reason to not take them apart. No really. I just couldn't resist seeing the innards in person.

Warren

[worn]

Here is the upper half:


I am not showing the well-known piston, but it rides in the cylinder at left. Such dazzling machining. Better than any bling.
The cylinder crosses between the upper and lower castings. It has a hollow inner chamber where gas enters through super fine nylon screen (1).

Within that lovely cylinder are six minute slits only 200 microns wide (2). They line up with drillings that reach each upper chamber (3). The O-rings mean that each chamber is sealed to its slit. Movement of the airplate will raise and lower the polished piston that cuts off or progressively reveals the slits. Basic CIS stuff. More air more fuel.
Within each upper chamber is an orifice linked to a fuel injector (7).

Covering the orifice is a thin steel diaphragm (not in my picture). This is pushed towards the orifice by lower chamber pressure, but also pushed away from the orifice by upper chamber pressure. The upper and lower chamber pressures are fed by the same pump, so that ends up as a wash. But a spring in each upper chamber biases the diaphragm towards opening, reducing pressure in the upper chamber (4).

This is brilliant because no matter how much your battery changes during the day or the pump varies, the pressure drop across the slits is always constant. Flow then is only affected by piston position from the airplate, and not disturbed by pump variation.

The springs ride on spring perches surrounding the orifices (5). They support little caps which push against the diaphragm. In our 911s, the springs can be adjusted by changing the position of the spring perches. The perches ride on set screws, one set screw per chamber.

[boyt911sc]

Warren,

It is good that you went to the details of a fuel distributor. Forget for the moment about your post because you are getting too far ahead. What you need now is to learn and be able to measure fuel pressures (control, system, and residual). You probably know it by now.

Test and evaluate the fuel pump. It has to deliver at least 1 liter per 30 sec. or 2 liters per min. Measure the flow rates before the fuel accumulator.

Connect a pressure gauge to the main fuel deliver line and measure the max. pressure (head pressure) the FP could deliver. No need to run the FP too long. A 3-sec. test should be enough to get the pressure reading (shut-off valve closed). Hope you are not using a HF pressure gauge kit.

Keep us posted. Thanks.

Tony

[worn]

Each of the orifices leading to the fuel injectors has one of these filters. Only 5.5 mm across. No, I didn't drive the screw through the bottom of the filter. They have little nubs in the plastic. My FD had 5 out of 6. No idea what happened to the other one. There is a certain amount of debate about having them there as they tend to clog. I suppose it depends upon how you feel about replacing fuel injectors.

[worn]

It was pretty obvious to me after the fact that my warm up regulator wasn't causing the system pressure to be low. The connection between the top of the FD and the WUR is a Cohline tube about 1 mm in diameter. The flow is very limited and wouldn't likely send my system pressure crashing. This is a regulatory circuit rather than a way to get gas to the intakes. But I hadn't understood that at the time. So I took it apart and cleaned it. Now I will probably have to readjust it or maybe make it adjustable with a screw.



The way this works is there is a passage drilled in the top and bottom castings of the FD allowing flow of fuel to the top of the control piston. However, the steel diaphragm lies between the castings and has only a pinhole for gas to go through to enter this system. Because flow is so limited, anything that allows fuel to escape from the top will reduce the pressure on the piston and affect the A/F mixture. The WUR bleeds off some of this pressure, allowing the piston to rise and enrich the mix. This is controlled by yet another metal diaphragm covering a small orifice. A pin riding on what is called a sombrero transfers pressure from a spring and a bimetallic strip to that diaphragm. As the car warms up the strip pushes against the diaphragm via the pin and shuts off the flow of gas. This raises the pressure, called control pressure, and thus pushes down on the piston in the FD. So as the car warms up the mixture gets leaner.

At one time the heat of the car was considered enough to do this trick, but later a resistance winding was added to heat the strip. Yet a second bimetallic strip with its own windings shuts off electrical heating after the car warms up.

[worn]

This may be where I have trouble. It is the only part of the fuel injection system that is easy to remove.



This regulator has no shims at all, but it the ebay regulator: the one in my car has a single 1 mm shim. And it runs at 2.5 bar system pressure. I wish I could see deeper into the drillings but my working idea is that the small O-ring and larger axial spring are the parts that allow fuel to return to the tank at a given pressure. Thus regulating system pressure.

My other idea aboutb how this works is that the larger O-ring acts to preserve control pressure when the motor turns off. The WUR doesn't send its fuel directly back to the tank. Instead, it comes back to the pressure regulator portion of the FD. This would be the push valve portion.

[worn]

Now in way of an explanation, err...excuse for the test I ran. I used to teach a university course about the cardiovascular system. Pump, high pressure lines and drains. Having determined in the established way that system pressure was low, I tried to understand why. I have hypertension and get drugs aimed at opening up the drain lines for my blood, thus lowering pressure. An alternative treatment to lower pressure is to reduce pumping.
It seemed like I either had gas going in too slow to generate pressure, or was leaving too fast and deflating pressure. In the going in too slow category I has the pump, accumulator and filter to consider.
In the going out too fast I decided to consider any fuel line that headed back to the tank or into the intake.

My test removed several of these possibilities and produced exactly the pressures observed when the FD was installed completely. By running a line directly from where the gas enters the engine compartment I was able to bypass fuel accumulator and filter as possible restrictions. They were unlikely: the filter was new and the accumulator wasn't leaking at the bottom connection.

I didn't connect the WUR. Put a pressure gauge at the top of the FD to measure system pressure and simply blocked the WUR return. Also unlikely but possible was the frequency valve. That port was blocked as well.

A publication entitled
Workshop Manual
Porsche
K-Jetronic
Robert Bosch

Indicated the fuel return line connection as the place to make delivery measurements. I believe that Probst also indicates this. This fit well into my concern that perhaps the pump was unable to deliver against a restriction. I know it isn't a common failure mode, but I was wondering if the filter sock in the tank might be clogged, producing cavitation or some such.

The results were suggestive but not conclusive, in part because this isn't a standard test mode. Still, I discovered that the pump can produce appropriate fuel pressure and injector flow (once pressure came up). Pressure loss could only come from the regulator part of the FD. The other outlets were gone, and they weren't very likely in the first place.

Such as it is. Thanks for everyone's patient help.
Warren

[worn]

The pump goes up to 7 bar reliably. The fuel flow where fuel goes to the accumulator is 2.5 liters per minute. The gauge is from HF, but readings across several points in the 0-7 bar range match not only a new FLAPS gauge but also an old one from before the Harbor Freight era.

[boyt911sc]

Originally Posted by worn

The pump goes up to 7 bar reliably. The fuel flow where fuel goes to the accumulator is 2.5 liters per minute. The gauge is from HF, but readings across several points in the 0-7 bar range match not only a new FLAPS gauge but also an old one from before the Harbor Freight era.

Warren,

Well written. The problem could be two (2) fold:
a). Your pressure gauge from HF is notorious for giving false readings.
b). Your PPV (primary pressure valve) needs additional shim/s to produce 70 psi.

If I install your FD in my tester, it will only take several seconds to do the test. And then calculate the needed shims.

I suspected you have a HF brand gauge but had to ask to confirm. Your write up is excellent. Time to apply this knowledge into action and fix your CIS problem. Keep us posted.

Tony

[worn]

Thanks Tony,
I give up. I always like to do it myself if I can. I was once known to make my own gravel with a sledge hammer - dumb, but I needed a wall gone and gravel for another project. I will give you a call for shipping advice.
Warren

[creaturecat]

i like the way you operate. Tony. cheers!

[worn]

Just talked with Tony about sending components. What a generous and warm hearted man!

[gsxrken]

Originally Posted by worn

Just talked with Tony about sending components. What a generous and warm hearted man!

Ive followed along and a few times was shaking my head at your order of operations, but that’s not really too helpful, is it. It’s guys like Tony that have the knowledge and the patience and the generosity to help that make this board so unique. He has my own CIS injectors right now and is checking them for me. I wanted to know that the new ones are good before I lean out or flood my new rebuild.
This forum is lucky to have him. Glad to see you’re on the right track!

[boyt911sc]

Ken,

The 2 sets fuel injectors (new & used) both passed the tests. The old used sets are still acceptable to use but their spray patterns are not as nice and uniform like the new set. Which you expect for buying new fuel injectors. BTW, the injectors are on their way to NY.






Warren,

Did you ship the fuel distributor already? Please advise me of the expected date of delivery. Right now, I have plenty of work to do to keep me too busy for weeks to come. And in between tearing down a motor for cleaning and rebuilding for Bill P.

Everyone stay safe and healthy.

Tony

[Charles Freeborn]

Originally Posted by worn

I note that a Bosch manual says that the fuel flow test point is actually at the outlet from the fuel distributor back to the tank. Did I measure flow wrong again?

Sounds like you've progressed well beyond this and apologies for not reading this entire thread - but.. one mistake I made when testing pressures on my '74 is there's a check valve on one of the banjo fittings that you have to connect the test line to the correct side of.

[gsxrken]

Originally Posted by boyt911sc

Ken,
The 2 sets fuel injectors (new & used) both passed the tests. The old used sets are still acceptable to use but their spray patterns are not as nice and uniform like the new set. Which you expect for buying new fuel injectors. BTW, the injectors are on their way to NY.
Tony

Got them back today. Thanks Tony! Going back in tomorrow.

[boyt911sc]

Warren,

The package containing your parts for testing and evaluation finally arrived this afternoon (October 1). There are two (2) FD-077 and a WUR-072 in the box. You did not label which of the two (2) fuel distributors was in the car that you were working on. Any way, I did my best to get them tested and share my findings with you.

FD-077 #925:
System pressure = 69 psi. Good.
Leak test: All six (6) ports do not leak and no visible fuel seepage around the FD body.

FD-077 #344:
System pressure = 56 psi. Out of specification (65~75 psi.).
Leak test: All six (6) ports do not leak and no visible fuel seepage around the FD body.

Note:
Individual flow rate measurements were not done this time and will probably do it later after I catch up with my work load.

WUR-072 #927:
This will need some work to make it perform properly. The WUR will exhibit the same problem/s Mac-Porsche is experiencing with the cold erratic warm-up idle. It will not prevent the motor from starting/running but a noticeable erratic cold idle during transition will occur.

I will have them ready for shipment back to you. You should consider getting a better pressure gauge kit to help with your pressure tests.





Tony