911 sc cis residual pressure

[ahh911]

"What i dont understand is that when i disconnect the WUR to FD line after 1 day there will be not even a single drop of fuel although residual pressure is within spec."


1981 us, 911sc. I recently checked pressures and removed the wur several times. After 1 day for my car too I don't remember any fuel coming out of the line after disconnecting the wur to the fd line at the wur side. I have removed it under these conditions several times. I don't think there is anything wrong with my car at startup. Maybe others have more info as it's my only 911/cis.

Edit: Now that I think about it, the first time I removed it there was a little fuel but I can't remember how long it sat, usually 12 hours later and no fuel really, if I shook the line of course some would come out, maybe not having any drip out is not a bad sign on the FD side? Does it mean that it is well sealed?

Phil

[Schulisco]

Control pressure is far too low. Don't think it will make a difference when the gauge being connected over night. The fuel pump refills the system quite quick unless there are no pressure leaks or a faulty fuel pressure accumulator. From the very beginning the CP must (depending on the ambient/engine temp) not lower than 1bar/10°C (14,5psi/50°F) and must then climb up to 2,9-3,2bar (engine warmed up) depending on the particlular engine and it's CIS equipment. The CP should arise the upper/warm pressure value in 2-4 minutes. Not longer and not shorter time.

Thomas

[Schulisco]

Originally Posted by leonardo911sc

What i dont understand is that when i disconnect the WUR to FD line after 1 day there will be not even a single drop of fuel although residual pressure is within spec.

That's weird! Never seen before. I always had fuel there. Of course more with reisdual pressure and less. if the RP is already gone. That's would explain why it took that much time to get a control pressure being measured...!
Did you checked the fuel lines being correct connected? The middle output/input of the FD supplies the WUR. And the WUR is being connected to the fuel return lines. I don't expect that it qould make a difference if the WUR is in wrong direction, but who knows? Of the FD has a problem that it won't supply the WUR with enough fuel...

Here's a good fuel flow diagram: https://jimsbasementworkshop.com/CIS/pages/fuel_flow.html

Remember: I have a new original Bosch fuel pump with a new check valve mounted. The check valve is far more important than most people expect...it works very closely together with the fuel pressure accumulator.

Thomas

[boyt911sc]

George,

Take a look at Ginger77 recent post about his fuel accumulator test. You might as well perform a similar test to verify the condition of your FA. If it tested OK, no harm done.

Tony

[leonardo911sc]

Good day Everyone!

We have narrowed the problem.

As Thomas said the control pressure builds up very slowly. I guess that while control pressure is trying to build up to push the plunger down (lean the mixture) a large amount of fuel is injected into the cylinders. See below video from morning cold start with gauge connected from last night (Phil's suggestion).

CHECK THE WHITE SMOKE after start indicating a very rich condition.

https://youtu.be/GPXHHIFnsUw

Also set the basic mixture last night. My adjustment was only 1/2 off. So i dont think this is a problem. Didn't make any impact.

Tony i will do the accumulator test although it is a brand new Bosch unit. Just to confirm.

Thanks

[Schulisco]

Good morning!

Very good progress!

The white smoke is not that much. it's quite common I suppose...depending on every car and ambient temperature. Don't forget - the K-Jetronic is not that precise as modern injection systems are.

The video shows a tiny little bit of surge due to the rich mixture, but as long the engine kept idling currently no issue, this is fine tuning afterwards... We have to find out why the pressure/the fuel at the WUR drains that much...if there's no leak where fuel is getting out of the engine, it must drain internally. As already mentioned either a bad check valve, fuel accumulator, injector, fuel distributor, fuel pressure regulator valve at FD.

I would also make a cross check: Let the fuel gauge connected. Next morning let the fuel pump run without cranking the engine until it fills up the lines and the CP at the WUR is showing at least more than 1bar CP. Be aware, that the WUR will be electricially heated at the same time when ignition is on. But as the pressure of it is still that low no problem until you let the ignition on not more than 30secs...
When control pressure is high enough then try to start then engine. I suppose it will fire up right away...

Good idea to test the fuel accumulator itself again. I posted yesterday methods to test it:
https://forums.pelicanparts.com/porsche-911-technical-forum/1119829-accumulator-pump-check-valve.html

One last thought: just in case if those tests above won't bring us further - I would open the testwise the fuel filler cap after running the engine and before starting it the next morning...just to assure that we're not facing a gas tank ventilation problem....

Good luck!

Thomas

[boyt911sc]

George,

After our telephone conversation, I went to the garage to start the two (2) cars sitting in the garage for more than 2 months without being started.

• The ‘89 Carrera 3.2 on jack stands needs a new clutch saver........started after a slightly extended 1st. attempt.
• The ‘78 SC on the lift needs newly rebuilt brake calipers........started after the 2nd. attempt.

Twelve (12) hours later today, I repeated the tests:

• The ‘89 Carrera 3.2 started after a normal 1st. attempt and slightly shorter than yesterday.
• The ‘78 SC started immediately at 1st. attempt. Quicker than the Carrera 3.2.

I will investigate the fuel line in between the WUR and FD later today after 9 hours rest and keep you posted.

Tony

[leonardo911sc]

Thanks Guys.

Will wait for test results Tony.

Will also test my Accumulator by plugging the fuel return line going to the bottom port and start the fuel pump.

[boyt911sc]

George,

A cold motor depends on a CSV to start and this is one of the basic principles for CIS. The spray pattern of a CSV is always good even for very old ones. They hardly fail but the fuel injectors are different. You need an atomized mist-like V-shape spray patterns for the injectors. Test and confirm.

I have NOT replaced or fixed anything in my ‘78 CIS except for a broken “Pope boot” years ago due to cracks for a long long time. And it ran using a 5-year old stagnant gasoline. Do not use old gasoline. I did it just for experiment and not a good practice. A properly set-up CIS will run if you have fuel, ignition, and no significant air leak. Plus a fully charged battery.

Tony

[leonardo911sc]

Hi Tony

Injectors are new. Old ones were crap. Last night i removed one new injector from the intake to perform the basic mixture setting. Spray pattern was proper. V shaped umbrella with good atomization.

Gasoline is premium 100 octane only as this is a high compression engine. I put gasoline every week as this car is now my testbed. I drive it every day...

[Schulisco]

Originally Posted by boyt911sc

George,

A cold motor depends on a CSV to start and this is one of the basic principles for CIS. The spray pattern of a CSV is always good even for very old ones. They hardly fail but the fuel injectors are different. You need an atomized mist-like V-shape spray patterns for the injectors. Test and confirm.

I have NOT replaced or fixed anything in my ‘78 CIS except for a broken “Pope boot” years ago due to cracks for a long long time. And it ran using a 5-year old stagnant gasoline. Do not use old gasoline. I did it just for experiment and not a good practice. A properly set-up CIS will run if you have fuel, ignition, and no significant air leak.

Tony

Tony, please don't get me wrong, but now I'm confused about your last posting, because I think it distracts from the important things we're facing now ...

I don't understand why you're now talking again about the CSV again...as George in the beginning of this discussion mentioned - with the CSV his engine won't start, without the CSV it would. So conclusion is pretty clear to me - the mixture is too rich to start the cold engine with the CSV. Nevertheless - the CSV is, as you wrote correctly, required for a proper start. But this also requires the rest of the system works also well. And this is the point - it is not.

With the CSV it gets even way more richer and this prevents the start of the engine. Now we're getting closer as George this morning figured out, that his control pressure doesn't really exists until he cranked the engine several times, clearly seen in the video.
This means totally clear that the FD enrichens the mixture far beyond because of lack of fuel in the hose. This is a inacceptable situation antd the root cause of the problem. With the upcoming control pressure above 1,3bar the mixture is getting lean enough that the engine will start finally - with the CSV connected. So he has to figure out why and where the control pressure and the fuel at this place is drawning away ...

The CSV in Georges car works as it should. But it's not the culprit.

Beside that - you're absolutely right about the spray pattern and so on...but this is IMHO currently not helpful in finding out, why Georges engine drawns the WUR hose from fuel as he figrured out. Do you have an idea where and why the fuel could going from there?

No offense.

Thomas

[ahh911]

Continue on as planned, but did you guys notice:

The last video the test equipment fuel lines were primed from the night before, the control pressure came up twice as fast and it didn't start.

The video before this with the test equipment not primed : therefore air in the control line at startup, the control pressure was near zero for many seconds, and... it started.

Just one data point that puts uncertainty in any particular theory.


George, do you have a sensor plate switch? It might be a good idea if it's unplugged for now to make sure it's not playing games and reducing possibilities as it would shut off the fuel pump intermitantly even on startup if it was switching on an off for some reason, if I understand the wiring diagram correctly. It's very tough to get to, maybe someone else has a way of disconnecting it that's easy.

Phil

[boyt911sc]

Originally Posted by leonardo911sc

Hi Tony

Injectors are new. Old ones were crap. Last night i removed one new injector from the intake to perform the basic mixture setting. Spray pattern was proper. V shaped umbrella with good atomization.

Gasoline is premium 100 octane only as this is a high compression engine. I put gasoline every week as this car is now my testbed. I drive it every day...

George,

93 octane fuel is more than sufficient specially now that you are troubleshooting. How consistent could you make the motor to start at FIRST attempt? If you test run the motor once a week, it should start and run all the time. If you could not achieve this GOAL, then you have some work to do and to solve this annoying problem.

Cold start for CIS is much easier than Warm start because the CSV is utilized. You are getting closer to solving this mystery. Keep us posted.

Tony

[Schulisco]

Originally Posted by ahh911

The last video the test equipment fuel lines were primed from the night before, the control pressure came up twice as fast and it didn't start.

The video before this with the test equipment not primed : air in the control line at startup, the control pressure was near zero for many seconds, and... it started.

Phil,

you're correct. And this seems to be confusing. Honestly I also do not have a clear explanation for this. Only that, that the ambient temperatures and other conditions also play in the game. Even the CP comes far quicker up than in the second video.

Video #1: https://www.youtube.com/shorts/GIcySvwJhXo
Video #2: https://www.youtube.com/watch?v=GPXHHIFnsUw

BUT: I'm pretty sure, that the CP must be far quicker on the 1,5bar from the beginning when the engine is cranked while the fuel pump runs when putting the ignition key in position "start". That's on my SC the case. Anyway - trhe problem is the fuel pressure which is too low in the beginning and - as George earlier wrote - the hose for the control pressure was pretty empty with no fuel. That's weird and quite unusual:

Originally Posted by leonardo911sc

What i dont understand is that when i disconnect the WUR to FD line after 1 day there will be not even a single drop of fuel although residual pressure is within spec.

Thomas

[leonardo911sc]

Hi Guys!

My opinion is that Thomas is right. Although i can't prove it i think that control pressure is raising very slow. Only when it gets to 1.2 bar (correct setting for current ambient temp) the engine will start, BUT it gets more complicated as i have already primed the system many times prior cold start without any success as part of my troubleshooting.

If you notice the engine WILL fire from the CSV function but wont start.

Please ignore the first video. The second is based on the correct conditions. Cold start in the morning, gauge installed the night before and system primed.

Tony if i insist cranking for about 10 sec the engine will start, RPM will fluctuate, fuel smell from the exhaust. As per my tests, this is the time where CSV is deactivated due to excessive cranking. This is the reason i started disconnecting the CSV during cold start and things improved.

[ahh911]

"Checked RP. Will keep 1.3 bar for more than 1 hour."


George, you mentioned that you primed the system many times before the last starting video. Earlier you wrote what is above in quotes but the video shows no pressure. Shoulnd't the gauge show some pressure early in the start cycle from the just prior to start fuel pressure tests? Weird that it holds RP on earlier tests but for this start test it goes to zero, unless something is wrong or I don't understand something, which is very possible.

But, if the information you've collected is correct it seems to say that with a start condition and the csv activated the residual pressure drops to zero? That would mean a start with the csv not plugged in and after priming the system would show pressure if you took another similar start video? Does this make sense?

Phil

[leonardo911sc]

Hi Phil.

Apologies for my inability to explain thoroughly.

1. I did prime the system when i installed the gauge last night. Actually started the engine and let it run for about 5 minutes so to check if any leaks present from the gauge fittings. Overnight pressure will decay. Next morning will start from 0 bar. This way i simulate a normal cold start and observe the gauge. This is what you see in the last video.

2. Past weeks i was priming the fuel system by jumper the fuel relay prior cold start in order to check if my starting problem is related to residual pressure loss. Although the system was primed the engine wouldn't start easily unless CSV was deactivated. All residual tests have shown that residual pressure is holding.

Hope the above are clear now.

Since i set the basic mixture last night I am thinking to adjust the mixture allen screw to the lean side and see if it starts easier. If it does i will take it to the gas analyzer and check CO%

Thanks

[ahh911]

George,
Thanks, makes sense.

You do have a sensor plate switch? That thing is not false triggering the fuel pump?

Zero control pressure, the plate moves much higher on startup opening the switch with authority but runs very rich. (first video)
Some control pressure, the plate moves less and partially trips the switch for some reason and it activates and deactivates the fuel pump briefly during startup without you knowing? (second video)

Phil

[leonardo911sc]

Have checked switch operation by slightly lifting the plate. It works every time.
Besides that i have disconnected the plug during past troubleshooting without any positive results.

Thanks

[Schulisco]

Guys,

during cranking the engine, the fuel pump is powered through terminal #50 (1). After the engine fired up and is running, the fuel pump relais is powered by terminal #15 in a self holding mechanism until the engine and terminal #15 is switched off (2):



The situation shown in the diagram is when the engine already runs with connection to terminal #87a. That's why the relay is shown as it is. The resting position is on the right to terminal #87.

So - even if the sensor plate not really good adjusted, with that circuit the powering of the fuel pump is assured at anytime.

To be sure, that it runs, you can remove the plug from the sensor plate housing (this is the security circuit to shut off the fuel pump in case of stalled engine at a car crash to prevent fire the whole car on the hot engine). With the plug removed the fuel pump will run from switching on the ignition.

Again - this is not our problem.

Oh, and a little annotation concerning the thermo time switch:
If I'm right then George already replaced it with a new one. Bosch brought some years ago a new fully electronic version of them. The old one were half mechanic half electric (not electronic!) with a bimetall strip inside being warmed up with a resistance. The new one are fully electronic and they should not fail any more. The difference between both is: The new ones always powers the CSV for a second on every start of the engine! The older ones did not, if either the engine or the bimetallic strip were hot enough. Then the contact is interrupted as long the temperature is high enough. We had such a discussion in a german forum years ago:
https://www-elfertreff-de.translate.goog/showpost.php?p=213944&postcount=7&_x_tr_sch=http&_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp

Thomas