Bosch CIS control pressure way too high
 

My control pressure is way too high, 75 PSI, basically the same pressure as the main fuel pressure. It is supposed to be 30-45 PSI depending on temperature. The main fuel pressure is fine (about 75PSI). So something is blocking the control pressure.
I took apart the warm up regulator and it was clean and I tested it with with compressed air and it flowed OK, allowing air to flow through at around 35PSI.
I'll checked the pressure regulator in the fuel distributor and it moved freely and the channels through which fuel flows from the warm up regulator back to the fuel tank were clear. Next I'll take the return fuel line off and see if the control pressure drops.
My questions are : Has anyone else had the control pressure way too high (not just a little high or low) ? Is there a channel inside the fuel distributor that allows control pressure to bleed down and perhaps that is clogged , or does the main regulation of control pressure happen with the warm-up regulator ?

Welcome to the forum.

The WUR is regulating the control pressure. Which model year / engine do you have? It would be helpful to post specs and all the part numbers of the components (WUR, fuel distributor, etc).

Assuming you performed the pressure test correctly, I think the most likely situation here is a blockage in the return line to the fuel tank.

Here's a resource that you might find useful:

911 CIS Primer - Index

When I had the exact same thing (SP=CP), the filter in the WUR was clogged. It is a bunch of really small holes drilled into what otherwise would be an undrilled part of a passage. Can't recall if it was in the in or out side. Taking the four screws (slotted!) off to remove that stuff from inside the WUR was a challenge (replaced with Allen bolts, mo bettah).

This symptom has to mean that the WUR system is not able to bleed off system pressure to give control pressure. I'd not think it was in the fuel distributor. As I recall, the system pressure used for control purposes has to pass through a very tiny orifice in the FD. Hard to imagine that getting enlarged enough to overpower the bleeding capacity of the WUR.

As long as the springs in the bottom of the WUR can compress they ought not to cause this.

If the blockage was before the WUR, you'd think you'd not have any (or super low) control pressure. So I'm guessing the problem has to be in the WUR or downstream.

Downstream is just plumbing, so issues there would have to be a pinched line? Racers who thought a leaner mixture was meaner used to pinch the return a bit to raise CP.

But if you have throughput, might be something else.

I had the exact same issues a year ago where my control pressure was identical to my system pressure. After a number of tests etc. it turned out to be a blocked orifice in the fuel distributor that was blocking the return fuel from the wur from returning to the fuel tank. I described in detail the way I was able to solve this in some comments that I made in a thread, discussing this same issue, dated July 5 2019. If you read those and have questions, I will be happy to go into more detail.
Hope this helps.

Dave

Flow restriction........
 

The problem is the presence of a flow restriction in the system. To locate the culprit/s, you need to isolate section per section. Start opening the fitting for the main return line to the gas tank by intake runner #3. And so on until you identify the exact location. Keep the pressure gauge installed and continue to monitor the control pressure while you break open the connection along the main return line to the gas tank. Keep us posted.

Tony

Wow, what a great forum !!!! So generous with your time !
I just received my M12x1.5 fitting and hoses so I can plumb the return line from the FD outside the car and back into the tank. Hopefully, my control pressure will be in the 40s. Last thing I would believe is that a 1/4" pipe to the fuel tank would be blocked...but I hope that is it.

To reply to all your comments:
- WUR: It is not blocked, that was my first thought also but I disassembled and made sure the incoming filter was not clogged. All seemed in great condition. After reassembly, I used compressed air just to verify basic function and it passed air easily around 35 PSI.
- Blocked orifice in FD: I removed he pressure regulator from the FD. I verified that all of the orifices (there are 4) in the removable part of the regulator where the return from the WUR enters and then exits if the small plunger spring is pushed in. Also checked that that small plunger was free and not stuck. Also cleaned it all thoroughly. I will review the July 5, 2019 post
-Loosening fittings to find blockage: I did try loosening the fitting from the WUR where it enters the FD. Lots of fuel sprayed out. But at the time, I was not sure that perhaps it was fuel exiting the FD instead of fuel coming from the WUR. After taking apart the pressure regulator, it looks like fuel should not be able to flow backwards out of the FD. I'll find out today if fuel is free flowing from WUR.
-Model year/Part #s? OK please don't hate me but this is a '82 Rolls Royce - rescued it on it's way to the parts yard. My first (and last) British car.
But I bleed Porsche, presently have ('83 Cab, '64 356, '68 Sportomatic, '08 Cayenne)

I discovered the blockage in my fuel distributor return path by removing the primary pressure regulator from the fuel distributor, I then attached a rubber hose to the detached wur return line to the fuel distributor at the wur end. I then - with the primary pressure valve removed - covered the opening for the valve with my finger and then attempted to blow through the hose. At first I could not but all of a sudden something cleared out and I could blow air through and could even here it bubbling up in the fuel tank. What I determined was that the blockage was not in the primary pressure valve orifices but rather an orifice in the fuel distributor itself. As Tony has stated, the blockage could be any place in the fuel return circuit, mine just happened to be here.

Hope this helps.
Dave
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" My first (and last) British car" and I thought I was the only one that thought this... got rid of a mgb.... goes with fiat as well a 128, worst cars to work on.

Solved !
 

I attached a hose to the Fuel Distributor return line and fuel came pouring out, yet control pressure still 75 (same as main pressure). So the cause is not a blocked line returning to tank. So I removed the line from the WUR to the FD at the FD. Ran the pump and no fuel coming from WUR. Previously I had only loosened this line and fule does spray out. This is because fuel comes OUT of the FD from that line (not sure it is supposed to but save that for later thoguht).
So I started to take lines off WUR and realized that the previous owner had plumbed the lines backwards ! I did not notice before since I was being careful to be sure to replace it the same way it came off, not to mention that this is a tangle of lines, plus air pump hoses on top, plus the WUR is underneath the AC compressor.
I swapped the lines, had to bend one considerably to get it back in line. Ran test and control pressure now at 40 PSI.
I found this RR on Craiglist being sold for parts, but owner said it ran fine while spraying starting fluid into the plenum. So far I've replaced the Pump, Filter, all 8 injectors, rebuilt the FD, opened and checked the WUR. Next I have to replace a belt then hopefully it will run.
Thanks for everyones help !

Big hitter the Llama.

Good find. Good luck with the rest. Hope you get it going soon.

I’d like to hear the follow up. How’s it run, pics, etc...
I always love to hear stories like this where you rescue something that’s almost given away because everyone thinks it junk and it’s coaxed back to life. That car was the shyt when it was new and I’m glad to see it hopefully get a new life.
Tony