911 sc cis residual pressure

[leonardo911sc]

Good day to All!

Working on my best mans car. 1981 SC EURO no lambda.

Having cold start issue. Will fire after 10-15 sec cranking.

1. WUR 089 cold / warm / residual pressures within spec.
2. New pump / check valve / genuine accumulator (3 port) and FD pressure relief valve
3. New cold start valve. Tested for voltage supply when cranking. All good. This also confirms thermotime switch working properly.
4. New injectors
5. Smoke tested. No leaks detected
6. Thermo valve working. No vacuum at cold start. Will open about 5-10 sec later. Ambient temp during test 20 celcius

The problem is when first crank in the morning she will fire but will not start. It seems that cold start valve is doing its job but no sufficient fuel to keep it running. After prolonged cranking she will fire, will hunt a little and all normal afterwards. Steady idle at 1200 rpm, reducing to 1000 and settles to steady 900.

All the above leads me to conclude that she is starving for fuel, so i let the engine cool down over night and next morning i cracked open the WUR inlet hose from the FD. NOTHING! Not even a drop of gasoline. I then removed the FD relief valve. Only a couple of drops.

FACTS: Eventually after some hours the residual pressure is 0 bar.

MY QUESTION is. After 24 hours of stand still the lines are supposed to be empty???

Can someone please do this test and revert with feedback or share his knoledge?

Thanks
George

[7783911]

I recently changed my accumulator to a later (larger 80ml) unit and when pulling the old one out I had plenty of fuel come out of ALL of the hoses

ps: will be posting the original accumulator in classified soon

[leonardo911sc]

Thanks for the info.

Just to confirm. Was this fuel spill after long rest period? Lets say overnight?

[T77911S]

residual pressure usually effects hot start the most.

you really need gauges to check RP.
something to ponder. if there is no RP, then where is the CSV getting fuel pressure from to fire.

hunting = too rich.

you can remove the connector on the top rear of the AFM. this will run the FP with engine off.
when cold, turn key to on for 2-3 sec then start.
if all is well then it is probably RP issue.
start with the check valve.

[leonardo911sc]

No issues with hot start. Just to clarify, hot start is considered the period where engine is still hot above 45c where thermotime switch will not activate the CSV.

With the above conditions engine will start flawlessly.

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

Hunting=rich OR also means that FD is empty of fuel and tries to balance control pressure thus hunting rpm due to unstable pressure?

Morning time i push the FD flap by hand and there is no resistance at all. Is this normal for 24 hour interval or should some resistance be present?

I jumper the pump relay and let the ignition ON for a couple of sec. Flap has resistance and i can hear injectors spray.

Still result is the same.

The Cold and warm pressures are within spec but on the rich side. More specific
Ambient 21c
Cold pressure 1.8 bar
Warm 2.8 bar
With vacuum connected and engine working 3.6 bar
See below graph.




Last but not least i have to mention that after sitting for about 10 hours yesterday, it wouldn't start, but once i disconnected the ground wire from thermotime switch it fired immediately.

Over rich condition?

[leonardo911sc]

i am not new to CIS but this is a new challenge.

Shall i increase the cold pressure to upper limit and test? 2 bar instead of 1.8 @ 21c
Will this make any significant impact according to your experience?

Will also test components individually.
Accumulator disconnect, plug fuel return line and check for leaks at bottom port
Have a new FP check valve somewhere. Will replace also

Something i forget?

For reference i also checked ignition timing. My pulley has only total advance notch 25 degrees. Adjusted with vacuum line disconnected. Idle seems to be about 7-10 degrees.

[T77911S]

NO
dont change anything until you have done more tests.

pull the connector on the back and run the pump then start it.

hot starts are usually an issue about 20-30 min after shutting down.

you can check CCP after sitting to see how quickly the pressure rises if you are concerned about the WUR being empty.

how are the plugs cap and rotor.

you can also pull down on the sensor plate when cold and hunting to "lean it out" as a test

have you messed with the mixture


dont over think it.

[leonardo911sc]

Thanks for pulling the brakes on my thoughts.

Will follow your advise and do these tests.

Rotor and cap good condition.

CO adjusted to 2.5 % idle.

No warm start problem based on your statement (20-30 minutes after shut down)
Engine will start first crank after 2-3 hours.

[SkiVT]

What is your system my pressure? If that is spec it’s hard to imagine the FD being short of fuel volume.

Did you check the AAR for full open at cold start? Typically that would raise your initially rpm to 1600+ rpm by adding air. You mention your cold start only goes to 1200. AFR is very rich at startup so the added air seemingly helps smooth it out.

[leonardo911sc]

Good day

System pressure 5.1 bar

1200 rpm was when ambient temp was 30celcius.
Last night there was a storm and today temp was 18 Celsius. Cold idle 1600 rpm.

AAR was adjusted (altering pin position) a while ago.

Please reply to the following question guys as it is the first time i notice this.

The 089 WUR cold pressure is set by applying vacuum???

For so long time i have been adjusting cold pressure without vacuum and had no problems. See below diagram.

[boyt911sc]

Control fuel pressure is measured as:

• Cold control pressure.
• Warm control pressure.

The above tests could be measured with or without vacuum applied (16” Hg). Without running the motor, you need a vacuum tool to measure the CONTROL FUEL PRESSURE under vacuum.

Tony

[SkiVT]

Notice the different pressures below the graph. The diff with vs without vacuum is around .7 bar. So you can construct the same CCP graph, roughly, w/o vacuum by moving everything by that amount. Much better to just use vacuum.

[leonardo911sc]

Thank you all for the input.

Welcome Tony!

Alright then. Let me adjust the cold pressure by use of vacuum gauge and see what happens.

Will first apply vacuum to my current cold setting (1.8 bar @21c) to see how off spec my WUR is.

Reverting...

[leonardo911sc]

Mr SkiVT, sorry i dont know your name. I am George

Based on your observation +0.7 bar my cold control pressure with vacuum should be 1.8+0.7=2.5 bar. Way out of spec to the lean side. Let me check.


Thanks

[SkiVT]

Yes you are on the right track. Remember your TTV holds off vacuum for 30-45 seconds at cold start so in real world, you were starting your engine at 1.8bar. Still theoretically lean but maybe not as bad as the 2.5bar math you state.

Mike

[leonardo911sc]

In order to present facts here is a video of cold start after 9 hours car was parked.

https://youtu.be/oddh3p_FES8

When i disconnect ground wire from tts engine will start.

It leads me to over rich condition with tts connected. But the fact is that wur cold pressure is set to lean.

Tomorrow morning with engine stone cold i will adjust cold pressure under vacuum.

Reverting with photos and video.

[leonardo911sc]

Good day

Job done.

Cold pressure with vacuum was 2.3 bar @ 20c ambient.
Adjusted to 1.8 bar. Engine sounds more happy. Picked also 100 rpm more at cold start.

Will let it cool down and give it a try.

[SkiVT]

I like the custom wur adjustment tools :-)

[leonardo911sc]

Good day!

After setting the cold start pressure i went out of town for a job.

Car stayed stopped for 3 days.

Started with first crank and all went smoothly.

The main problem has not been resolved though. After 9 hours stopped the engine will not start unless i disconnect the ground cable from TTS, thus disabling CSV.

Although TTS has been thoroughly tested i will change with a known good working unit from my car. I dont really think this is the problem though. After 9 hours the engine temp is about 23 c so CSV should be activated.

Any thoughts on how to proceed?

[Schulisco]

George,

you wrote, that you were new to CIS...did you already made/checked the basic setting of your CIS? The pressures seems ok to me. Don't be so concerned about the cold pressure of the WUR. Porsche claimed it to be in an interval as shown in the diagram...
When the CSV is spraying gas while cranking and the engine won't start (guess mixture is way too rich) and it will start wihtout CSV, then I guess there could be two problems:
1. too less air
2. too rich mixture in general

You wrote that you already checked the AAR if it opens up while engine is cold and getting closed by heating up? Is it opened up when engine is cold? You should check if the AAR is moving the cover? If it's moving, then probably the whole CIS is misconfigured. Remember: You can manipulate a CIS in that way to correct out / cover faulty parts and you never get an idea that sth could be wrong...

I have a popup valve in my CIS airbox, so I can see if the CSV is spraing gas. Did you checked that in person on your car? Will say - did you saw the CSV is sparing only while cranking?

If all those things are fine, then sth else is still wrong we still don't know unfortunately. In those cases I tend to make a basic setting of the CIS as Porsche recommends:
1. check meticulous the correct height of the sensor plate like Porsche says, to do this correct the spring or the screw underneath depending what your car has
2. turn the CO screw fully close (lean it complete out), then switch on fuel pump (ignition on, remove plug from the sensor plate housing), and start turning the screw cw to "richen" the mixture until you can hear the injectors spraying (squeaking noise), then turn it half turn back, done. (be aware that the injectors are flooding the cylindersif you're too slow!!). Plug back in, ignition off.
3. Turn the idle screw completely in so that's fully closed, then turn it out 2 fully rotations.

Now you're done. Then - if everything works as designed - the engine should start right away on the very first crank. Check idle of the engine (esp. when warmed up), then check the CO setting right away.

The height of the sensor plate impacts heavily the mixture over the whole speed range, and you can balance it out with idle and CO screw so that the normal driver won't recognize it until he requires full power or wondering why the mpg decreases that much...
If this basic setting won't ever work, then you still got another problem to figure out, most likely false air / vacuum leak, which is the hardest enemy on the CIS...

Oh, one addition: Concering the residual pressure and the fuel accumulator....
I had a fuel accumulator which was in spec as the Porsche specs given. BUT: It wasn't as good as it should be. The residual pressure is only one side of the medal! The whole world thinks the fuel accumulator is only for the hot starts. But it isn't.
Together with the check valve it assures that the whole fuel system is fully loaded with fuel, not only for hot start (here the fuel accumulator assures that the fuel remains on pressure to prevent bubbles), but it also assures a proper cold start by assuring the whole CIS system is filled with gas/fuel (otherwise the fuel pump has to refill and repressurize the system while cranking), and it suppresses any pressure pulsations while the engine is running, especially when you're pushing the accelarator quickly, then the suddenly increased amount of fuel is filled for the very first moment from the fuel accumulator, and then afterwards it will be loaded up again by the fuel pump. Also when going on idle it cushes any pulsations given by a dropping sensor plate. So conclusion: The fuel accumulator is important on every operation condition for a smooth and powerrful CIS engine!!It's all about the pressures on a CIS engine....

After replacing the old fuel accumulator I could see, that the residual pressure was far more than 2bar/30psi, and it took much much longer to drop the fuel pressure (now several hours instead only half an hour before as the specs says!!!). Now it takes the whole night to drop completely the residual pressure!!

And the thing is - a very good working fuel accumulator can cover a leaky / faulty check valve close the fuel pump and vice versa a very good check valve can cover/hide a faulty fuel accumulator...

HTH.

Thomas