Residual fuel pressure loss.............

[boyt911sc]

When people are having some problem starting with their warmed engine, we always point our fingers to these two most common culprits: defective FP check valve or fuel accumulator. There is another culprit to consider, and that's the PPR (primary pressure relief) valve in the fuel distributor (FD). At the delivery side there is the FP check valve that prevents the fuel from going back to the tank when the FP is shutdown. At the return side, the PPR, aside from maintaining the system pressure during operation has another built-in function. It works as a check valve to prevent the back flow of fuel to the gas tank after the FP stopped operating.

Let me start from the beginning at cold start: When the engine is cold, the bimetallic metal strip pushes down the spring resulting to lower control pressure (cold). As the engine run, the heated bi-metallic spring relaxes and allowing the spring to go upward reducing the diaphragm valve opening resulting to higher control pressure (warm).



Next picture is the warm control pressure state, where the control pressure is totally controlled by the spring resistance. Please take note that the diaphragm valve is a restrictor type valve and not a shut-off valve. It allows continous flow of fuel while in operation (cold or hot).



After you switched off the engine/FP, the system fuel pressure would slowly decrease. The fuel under pressure inside the FD would leak back to the tank. As the fuel exit from the FD via the return line to the tank, the PPR would seal the remaining system pressure and control pressure that eventually becomes the residual pressure. See attached diagram.



'78 SC FD with two (2) return lines:


'81 SC FD with three (3) return lines. The 3rd line is from the FV (frequency valve)


The PPR for '78 & '81 side-by-side. Although very similar in function, an identifying difference is the size of the tip end of the valve. The PPR has 2 rubber seals that need replacement when leaking fuel is experienced. This is another source of fuel leak that affects the residual pressure.





This investigation was initiated by T77911S (Ty) and I just expounded the topic. What we need to add to this thread are the '73.5 - '77 FD description of the other PPR's. We like to make the investigation of the other FD's but we don't have any on hand. So any one who has some information to add and share with us is highly appreciated.

Summary:

The residual pressure is directly affected by fuel leak/s. Three well known culprits are: FP check valve, defective fuel accumulator, and primary pressure relief valve. Presence of any fuel leak in fuel line system has to be checked too.

Tony

[E Sully]

Here is a picture of one from a 1973. 0438 100 004.

[boyt911sc]

Ed,

Thanks for providing the picture. Since the 73.5, '74, and '75 have FP running before start, I was hoping one like it. So here is what I have so far for PPR:

'73.5.............single spring PPR (provided by E. Sully).
'74................single spring PPR (provided by manbridge 74)
'75................single spring PPR (provided by Walter_Middie)
'76................????
'77................????
'78-'79..........PPR plus secondary check valve for control pressure.
'80-'83..........PPR plus secondary check valve for control pressure with bigger tip end.

Mid car owners please chime in. Thanks.

Tony

[manbridge 74]

My 74 has single spring PPR. Looks just like 73 pictured above.

Also have another FD ending in 017. Also looks the same.


Kudos for an easy to understand post about what is going on with residual pressure.

I think its clear that workshop manual is misleading about WURs leaking back. At least for 74s.

[CharlieB]

Finally!! I've been posting about the PPR for some time now. My old-time Porsche mechanic pointed it out to me and it solved my SC hot start problem in a jiffy. Great post and pics. I always suggest checking and cleaning the PPR before getting down to the FP check valve and the Accum. So easy even us cavemen can do it!!

[Walter_Middie]

My stock 75 has the single spring PPR valve. It also looks just like 73 pictured above.

Once you remove the cover nut and spring, how do you extract the PPR valve?

[T77911S]

nice job tony,
this would explain why when my brother replaced the WUR he said his hot start problems got better......now its back. residual pressure was leaking back through the WUR, to the tank.

i have not actually looked at my 77, but i did reach around to feel the PPR, (sounds kinda kinky), and i did not feel the allen head on the "cap" that screws in for the (what i call) the WUR check valve, it felt more like the one sulley posted, so it looks like they added this feature in 78, to help maintain residual pressure.

to add to what tony has said, if system pressure is 5 bar, and WCP is 3.4 bar, as soon as the FP is turned off and the primary pressure drops to below 5bar, both valves in the PPR close, blocking the return from the WUR to the fuel tank, thus giving a higher starting point for residual pressure. if you have an earlier car without this PPR, once the FP is turned off, the residual pressure drops to what ever the control pressure is, which is much lower than the 5 bar of system pressure. this does not mean your residual pressure ahould be 5bar, the system still needs to equalize, it is just giving you a higher starting point.

for the early cars,as the WUR cools, the control pressure will decrease, so the residual pressure should never be more than control pressure.
what this updated PPR is doing is shuting off the return path for fuel from the WUR to the tank, taking the WUR's lower (than system) presure out of the equation to allow the system to start with a higher residual pressure.

so far the easiest way i know of to test to see if the WUR/PPR is the cause of bad residual pressure is to first do the test with the valve on the gages closed, then test it with it open.


question:
since my residual pressure is bad, i cant test it, what i would like to know, is how long can ANY pressure be maintained in the system, for both systems, with the updated PPR and the old style. IE, what is your residual pressure 24 hours later.

[boyt911sc]

Originally Posted by Walter_Middie

My stock 75 has the single spring PPR valve. It also looks just like 73 pictured above.

Once you remove the cover nut and spring, how do you extract the PPR valve?

Walter,

The PPR slide valve usually stays inside after you removed the nut and spring assembly. If the valve won't slide out easily or won't come out, remove the main return line connector to the FD. It will expose the valve and have access to slide it out. HTH.

Tony

[prebordao]

Originally Posted by manbridge 74

I think its clear that workshop manual is misleading about WURs leaking back. At least for 74s.

Nope. The manual is right. Why else do you think they added an extra push valve in the primary pressure regulator, a few years later ?

To finally put a stop to leaks from the WUR. I think Tony forgot to mention that in the modified design, the output from the WUR connects to the push valve in the FD, and only then, fuel gets back to the tank.

As the push valve is completely closed with engine off, WUR leaks stop. Completely.

But not for a 74...

[manbridge 74]

Once you remove the cover nut and spring, how do you extract the PPR valve?[/QUOTE]

Correct me if I can't recall, but I read in Haynes manual that you can use a tapered wooden dowel to fit inside PPR valve to withdraw it.

@ T77911S,

You did well to correct my "failure prone" comment in another thread. I should have stated it, "corrosion prone."

I too am of the CIS faith and believe it trouble free if exercised regularly.

@prebordao,

Okay, so how about one of us early mids trying a later updated PPR? Or is length different, not allowing use in, say, a 1974 FD?

[prebordao]

You need a new fd not just the ppr. There are additional lines going in and out.

[prebordao]

this gives you a more complete picture:

[T77911S]

i dont think leaky WUR's is or has been a big issue, if it was, it would also top the list of residual pressure problems, even though my brother claims it helped when he had the car.
i have brought this issue up in the past and it has more or less been ignored or dissmissed, until prebordao brought up the leaky WUR's and i got tony on board.


the PPR check valve was/also added to maintain residual pressure over longer periods of time, to also help with cold starts. with the early cars and the FP running as soon as the key is "on", this was not as much of a problem when cold. my FP runs all the time so i dont see any problems with cold starts due to lack of pressure.....although it might be why it was wired on

the WUR's valve will open with anything above control pressure. so that means that as the WUR cools down, the control pressure goes down and so does residual/system pressure, on the early cars.

i am planning on plugging the output of my WUR and see if my residual pressure gets better, i also want to pull the PPR and replace the Oring on the plunger. i am also going to pull the bottom line off the FA to check for fuel.

[boyt911sc]

Originally Posted by prebordao

Nope. The manual is right. Why else do you think they added an extra push valve in the primary pressure regulator, a few years later ?

To finally put a stop to leaks from the WUR. I think Tony forgot to mention that in the modified design, the output from the WUR connects to the push valve in the FD, and only then, fuel gets back to the tank.

As the push valve is completely closed with engine off, WUR leaks stop. Completely.

But not for a 74...

PJR,

The '73.5 to '75 CIS won't have much trouble with residual fuel pressure because these systems are completely different for the later CIS ('76-'83). The '73-'75 have the FP running when ignition SW is @ ON position (not start). So you have a time lag to pressurized the system. Unlike the later group, the FP does not run before the 'start phase'.

The Bosch WUR's (0-438-140-xxx) do not stop fuel flow (cold or hot) and that's contrary to what you believe. Look closely at the diagrams or dissect a WUR and see it for yourself. You do not know this for a fact!!!!!! and it is your opinion. Facts are much powerful than opinion in a technical forum!!!!

How many WUR's have you seen or tested in your life to support you claim? The fact that you could not even fix your CIS problem simply shows how much you know about the subject. The WUR connection you mentioned (highligthed above) is a perfect example of your ignorance about the subject. When did you learn to measure control fuel pressure? Last July 2011?

All this discussion precipitated when you claimed that the WUR's that allow fuel to pass through are defective and referred the Porsche manual as your source of information. I have that book too. And it was not easy to go public and contradict a factory shop manual. One way to get to the bottom of this, was to research and performed numerous tests. Good working WUR's are not capable of stopping the flow of control fuel pressure unless it clogged/blocked by foreign debris in the line. What stops the flow of control fuel is the loss of system fuel pressure resulting to the PPR valve closing and only then the secondary valve (FD with secondary push valve) comes into action. The return line is always open and continuously sending fuel back to the tank during operation (FP running).

The push valve for the return line is always open when system pressure is present in FD. And PPR (with secondary valve) will only come into action when the system pressure drops below the pressure set by the PPR. After you shut-off the engine/FP, there are still residual system pressure and control fuel pressures in the system. What happens when you disconnect WUR's return line immediately after shut-down? There are still fuel pressures inside the FD & WUR and fuel will leak if you break open the line!!!! What the factory manual says is that if fuel leaks when you disconnect the return line, the WUR is defective and should be replaced (As per manual). But all good working WUR's will leak because the diaphragm valve does not completely close and it never closed (cold or hot) so fuel under pressure will seep out. What's so difficult to understand about this topic?

Tony

[prebordao]

Well tony, you should have read my posts more carefully...

So in the enf you keep your opinion and i'll keep mine and my results.

And yes, i can read pressures (an exceedingly difficult task !)

Lastly, what's the purpose of the wur return line going to the push valve in the fd, cos you got me confused...

[boyt911sc]

Originally Posted by T77911S

i am planning on plugging the output of my WUR and see if my residual pressure gets better, i also want to pull the PPR and replace the Oring on the plunger. i am also going to pull the bottom line off the FA to check for fuel.

Ty,

If you try to do what your are planning, I'll wish you LUCK!!!!! Are you aware what you are planning to do (blocking the WUR's return line) is similar to measuring the system fuel pressure? The engine won't start!!!!!! When you install a fuel gauge kit to check the fuel pressures, you have valve open and it gives you the control fuel pressure. And if you closed the valve, it would register the system fuel pressure. That's exactly what you are planning to do. Do you realize it now?

When you do the test, hook up a fuel gauge kit so you won't be surprised why you can't start/run the engine. What you are planning to do is making the control and system fuel pressure equal. CIS does not operate in that kind of condition.

If I may suggest, before doing your test of blocking the return line (bad idea), hook up the gauge. Then start the car. Turn it off and close the valve (similar to system pressure check) and try to re-start the car. BTW, you won't have residual fuel pressure because you won't be able to run the engine. Prove that I'm wrong and I'll be very grateful and thankful to buy you dinner for two.

Keep us posted.

Tony

[CharlieB]

I use a small pencil magnet to remove the piston from the PPR hole. I have used a small dowel as well. Clean the hole out with a Q-tip and brake clean. You can add shims to the PPR assembly which will raise the system pressure. So many PSI per shim. You can buy complete new assemblies for the PPR on Ebay!

[Vereeken]

Tony and others,

Does the PPR valve have any effect on cold starts on a 82 SC?

What is the failure mode of this valve? It works or it does not work? Or can it also work intermittently? From the components I can see how it can get stuck but I see nothing else that would make it fail apart from a bad sealing O-ring taht lets fuel out?

Michel

[Vereeken]

This is a picture of what I believe the valve is. As you can or can not see it has two shims on it. If I read the above correctly that would be to increase pressure? Why would you do that ? I do not think it came like that from the factory?

Michel

[T77911S]

the system pressure has a spec. it may have been shimmed to meet that spec.


i think the check valve in the PPR WILL help both hot and cold starts due to it blocking the return line from the WUR so that not only residual pressure can not bleed off through the WUR but, it also provides for a HIGHER initial residual pressure.