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Walt Fricke's Avatar
 
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CIS system pressure fluctuation

My system pressure (1982 SC) fluctuates between 4.9 and 5.1. My guess is that I am seeing the pressure valve at work, opening and closing a bit. Might this be it? Is this right? I don't think this has an effect on the proper running of the system, but don't know this. Anyone else observed something like this? I don't recall it from previous use of the pressure testing instrument.

Old 10-05-2015, 09:55 PM
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Also, I've been thinking of installing a pressure sensor so I will always be able to read the system pressure. I am supposing I can read that pressure up front just after the fuel pump if I want to. Won't the system pressure set by the pressure valve in the FD also serve to set it back to the fuel pump? Flow will increase with the amount of fuel being injected, but the pressure will stay the same. (Not talking about control pressure here)

Or do I have that wrong, and I'd need to take it somewhere else, like from say the cold start valve line which is on the controlled side of the system pressure system, and once it has done its short term squirting should be at SP?
Old 10-05-2015, 10:06 PM
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In case it helps, my 82 SC has a very constant pressure (no fluctuation) but a fluctuating idle (very slight, but noticeable.) I don't think system pressure is supposed to fluctuate, but maybe it is and that's what's causing my wandering idle?
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Old 10-06-2015, 06:10 AM
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i was seein gthe same thing on my 930.

checked and even swapped rear fuel pump. checked flow to WUR. all good

ended up sending FD off to be rebuilt. i suspect pressure reg doing it but could not confirm even though i did mess around with it quite a bit.
actually had the FD rebuilt twice but have not checked it since i got it back.

so unfortunately no real answer to help you, although i will check the sys pressure soon and let you know.

how does it run
do you see the CP fluctuate
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
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Old 10-06-2015, 06:14 AM
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How much fluctuation? Does the needle 'flutter'? My experience with my '83 is that system pressure is pretty stable. First thought is that the variation would be coming from the fuel pump. If you could get the fuel pressure gauge on in such a way to isolate the fuel pump, that might help you zero in. Of course, as you said, I'm not sure that indicates a problem. Sign of a fuel pump that will fail in the future?
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Old 10-06-2015, 07:10 AM
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i dont remember the 911 fuel head as well since i have gone 930, but on the 930 i can connect to the output to the CSV, that is system pressure taken right off the "top" of the pressure reg. i was comparing the sys pres there to what was at the WUR. (2 sets of gauges)
i have an old 911 FD at home but i would have to dig it out.

fluctuation is not normal. i would start with relacing Orings in the pres reg.

one other thing, i also found that one of my CIS gauge sets read differently than the other ONLY when reading sys pres,. i just dont remember if they both fluctuated. sucks having memory issues but then i have done so much with the CIS on my car...
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 10-06-2015, 09:29 AM
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Jason,

Your fluctuating idle may be caused by a too rich mixture, which may be due to it being set too rich or a WUR slightly out of spec.
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Old 10-06-2015, 10:12 AM
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On a properly tuned system, the lamda system will ever so slightly cause a fluctuating idle, which is normal. As Paul said, could be rich, try turning your mixture screw counter clockwise a bit, like 1/8 to 1/4 turn, and see if that helps.
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Old 10-06-2015, 12:34 PM
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But the OP's question is regarding system pressure and whether a fluctuating system pressure could be a problem. When you're measuring system pressure, you're measuring the fuel pressure up to the WUR (valve closed). So I suspect the answer is that it depends on how the WUR behaves. Does the control pressure fluctuate too?
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Old 10-06-2015, 03:55 PM
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System fuel pressure.........

Quote:
Originally Posted by Walt Fricke View Post
My system pressure (1982 SC) fluctuates between 4.9 and 5.1. My guess is that I am seeing the pressure valve at work, opening and closing a bit. Might this be it? Is this right? I don't think this has an effect on the proper running of the system, but don't know this. Anyone else observed something like this? I don't recall it from previous use of the pressure testing instrument.



Walt,

The system fuel pressure in the CIS FD is nothing but a pressure relief valve. Thus the pressure reading is stable and steady. If you are experiencing a fluctuating pressure reading for the system pressure, I would investigate the following:
a). Fuel pressure gauge. I have three (3) sets of CIS gauges that one time, I installed all at the same time to compare the system pressure reading during one of FD rebuilds. Three (3) is over kill, two (2) would be sufficient. All three gauges read very identical pressure values. So I use two (2) gauges now for calibration purposes.
b). Fuel pump. FP could be delivering erratic delivery fuel pressures.
c). Defective FD return valve.
d). Defective FD primary valve.

But I would place my bet on the fuel pressure gauge as the culprit. Keep us posted.

Tony

Last edited by boyt911sc; 10-06-2015 at 07:37 PM..
Old 10-06-2015, 03:59 PM
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Thanks, guys

I'll try Tony's trick of replacing the tester gauge. Gauges aren't expensive, though it is nice to have the one I have, which is marked in PSI and bar (which is the one I use, as the specs are all in bars).

I am a bit dubious about this, though because, as someone else asked, the control pressure doesn't fluctuate.

Also, just so everyone knows, the WUR has nothing to do with the system pressure - at least not with the measured system pressure. Because to measure system pressure you cut the WUR out of the whole system. Which is why it is a bit tricky to measure system pressure on a running engine - fairly shortly after you turn the lever to the system pressure setting the control pressure starts rising until it reaches system pressure, because the WUR is what basically sets control pressure and it isn't doing its job of knocking the pressure at the top of the plunger rod down. So the engine is going to go way lean.

This year has the frequency valve, and it has an effect on the fuel pressure in the injector lines by varying the pressure on the bottom side of the individual fuel injection line diaphragms, which can richen and lean the fuel delivery. I have disconnected the air fuel sensor, so the frequency valve system operates at all times on its default settings, which are varied only by throttle position. Shouldn't affect system pressure as measured.

When I have things set right, the car runs well (very poorly when not right), which suggests that the fuel pump, which is only about two years old, is working fine. When tested for volume delivery over time it checked out. And my experience with fuel pump failures is that usually they start making noise, and when they fail, they do catastrophically, so to speak, and without warning, and you have zero pressure - or at least not enough to cause fuel to squirt out of the injector you pull out of a manifold after you have confirmed you have spark in the dark of a winter's night in some parking lot.

The suggestion that the little O ring in the FD pressure setting system is going bad is a good one. I have been told these can go bad and lead to all sorts of hard to diagnose problems. The FD is a loaner from Tony from his stock of FDs he hasn't had a chance to rebuild, so I'll have to aks him is i can pull the pressure setting stuff out and inspect, and perhaps replace, that O ring. I've got one from a FD rebuild kit.

Tony - what is a PD primary valve? The return valve I take to be the system pressure setting valve, which bleeds off excess pressure from the pump back to the gas tank, and allows the pressure regulated fuel to enter the FD. But primary? PD?

My CIS system has other, more bothersome or perhaps more primary, issues. But I'll address those in a different discussion to keep this one more or less on the subject of fluctuating (though not by a lot) system pressure at the gauge.

I'll keep posting when I learn something more about this.

Walt
Old 10-06-2015, 06:09 PM
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Quote:
Originally Posted by Walt Fricke View Post
to keep this one more or less on the subject of fluctuating (though not by a lot) system pressure at the gauge.
Just for fun, waiting for Tony to respond. Tony turned me on to this diagram.

It's the fuel accumulator (below)

Only reason I am posting is I have worked on a 70's V-12 Jag that had two fuel regulators with diaphragms. Each bank had one. One regulator was swinging wildly, +10 and -10 around it's 30psi spec point.

The 911 accumulator is supposed to allow maximum throughput with the engine running and then squeeze on downstream pressure to hold residual.

It it roughly similar (maybe?) to the Jag (Bosch) fuel pressure regulators.

Here is the accumulator diagram:



Here is the Jag regulator:

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Old 10-06-2015, 06:42 PM
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I suppose the fuel pressure accumulator could cause fluctuations. But on my car, I replaced what I think was a perfectly good accumulator with a brand new one. So not high on my list of suspects.

My fuel accumulator has a 3d port - on the bottom, so to speak, which drains any fuel which gets past the diaphragm back into the return line to the fuel tank. A kind of fail safe if the diaphragm fails and starts to leak.
Old 10-06-2015, 07:12 PM
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Fuel accumulator..........

Quote:
Originally Posted by Walt Fricke View Post
I suppose the fuel pressure accumulator could cause fluctuations. But on my car, I replaced what I think was a perfectly good accumulator with a brand new one. So not high on my list of suspects.

My fuel accumulator has a 3d port - on the bottom, so to speak, which drains any fuel which gets past the diaphragm back into the return line to the fuel tank. A kind of fail safe if the diaphragm fails and starts to leak.


Walt,

Test run the FP and measure the control fuel pressure. Stop the FP. Take a good look at the pressure gauge reading. Notice the pressure change on the gauge? The pressure goes up after you switched off the FP. You know what causing the pressure to go up? That's the effect of the fuel accumulator's contraction (the spring loaded diaphragm inside the FA is compressing the residual fuel). This is one visual test for the fuel accumulator.

BTW, to answer your question about PD, it was a typo. It was meant to be FD. I corrected (edited) the mistake. I don't believe a fuel accumulator is a contributing factor about the fluctuation of the system pressure. Only tests and experiments could give a better explanation.

Tony
Old 10-06-2015, 07:57 PM
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Fuel accumulator and fuel pressure regulator........

Quote:
Originally Posted by Bob Kontak View Post
Just for fun, waiting for Tony to respond. Tony turned me on to this diagram.

It's the fuel accumulator (below)

Only reason I am posting is I have worked on a 70's V-12 Jag that had two fuel regulators with diaphragms. Each bank had one. One regulator was swinging wildly, +10 and -10 around it's 30psi spec point.

The 911 accumulator is supposed to allow maximum throughput with the engine running and then squeeze on downstream pressure to hold residual.

It it roughly similar (maybe?) to the Jag (Bosch) fuel pressure regulators.

Here is the accumulator diagram:



Here is the Jag regulator:



Bob,

The Bosch fuel accumulator has nothing to do about maximum fuel delivery in the CIS. What it does is absorb the sudden flow of fuel when when the FP starts. Some sort of damper or absorber. Since the FP has the capacity to deliver more fuel pressure that the resisting force of the spring behind the diaphragm of the FA, the cavity inside the FA expands when the FP is running. And when the FP stops, and the delivery pressure ceased, the spring pressure takes over and pushes the diaphragm back. You could observe this reaction of the FA at the pressure gauge.

With regards to a pressure regulator, it all depends on the sensitivity of the system. It usually has a lower and upper default (+|-). I am not familiar what's inside the Jag's fuel regulator and how it works. I have no first hand experience about it.

Tony
Old 10-06-2015, 08:21 PM
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Quote:
Originally Posted by boyt911sc View Post
I don't believe a fuel accumulator is a contributing factor about the fluctuation of the system pressure.
Thanks for the input, Tony.

I was just fishing for your response on the issue. The spring loaded diaphragm is meant to do it's work only after the fuel pump kicks off.

I have owned my SC for close to 20 years and only this year, through your description, did I gain a better understanding of how the accumulator works.

Walt, I do have the little Bosch book in pdf format I can send you that contains this (ac diagram and other) info. It's a 25 page book that explains a lot of goodness of the K-Jet with Lambda. Just nice hip pocket info. I know you have it hardcopy, Tony. Did I send the electronic version? Advise via PM and I will "ship" asap.
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Old 10-06-2015, 08:30 PM
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i checked mine last night. still fluctuates a little. i too have concluded it is the gauge. as i said earlier, i have 2 sets and had them both on at the same time BUT, mine read 2 different system pressures. my fuel head has been rebuilt, i have good residual pressure, WUR has been rebuilt, i can blow thru the fuel return line and hear it bubble in the tank.
i have done FP tests along with swapping the rear fuel pump.
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 10-07-2015, 02:39 AM
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It is not uncommon for the push valve in the fuel dist to develope a score on the side of the piston This will also produce a corresponding score in the bore for the push valve.
I usually polish the piston on a lathe with craytex and polish out the bore with scotchbrite>
If there is a failure in the o-ring replace it . Any chattering of the system pressure would dictate to check it first. By the way Porsche still lists the repair kit as 928 n110 920 01.

Old 10-07-2015, 06:12 AM
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