CIS Fuel pressure dropping off quickly

[Paul325321]

Hello, I have a 1977, 911 2.7 which I purchased as a stalled restoration. The engine had been rebuilt by the previous owner and I’m currently trying to make sure it runs well. I have the engine in a home made test bed, it runs but blows black smoke occasionally when accelerating. It’s a hobby, I’m not a mechanic but love mechanical tinkering and learning about how things work.

I have been reading threads and manuals to become familiar with CIS, I noticed that many of the auxiliary cold start systems were not connected to power on the engine, despite this it seemed to start ok. I decided to start testing the CIS system and components without making any adjustments. So far I discovered the Air Valve on the circuit from manifold vacuum to WUR is missing, the Thermotime sensor needed replacement, the Aux Air Slide works ok and most of the system pressures seem to be within spec except : here is my current question

The system pressure Fuel Distributor to WUR seems to be in spec (test valve on gauge line to WUR open and closed) - pressure also seems to increase within spec when the heating element in the WUR is powered up. But as soon as I shut down the Fuel Pump with the test valve to the WUR open, the system looses pressure almost immediately. If I close the test valve on the gauge, shutting off the flow from the WUR and power up and then shut down the pump, the system holds pressure, gradually reducing over time.

I’m not sure where to look for a return valve failure, somewhere in the circuit there must be a valve which is allowing the pressure to bleed quickly - given that the pressure holds when the valve on the test equipment is closed - I’m struggling to whether this eliminates the fuel pump and accumulator return valves. Sorry fir being so long winded, I have worked on a 912 but never a CIS, this is new to me.

[Paul325321]

I also noticed the injectors seem to be proud of the manifold, I wonder if this is a contributing factor.

[Niels911SC]

Your injector sleeves sit too high (the orange bits), they should be completely inside the intake runners

[boyt911sc]

Paul,

It would help if you could provide some numbers regarding your fuel pressures. What are your fuel pressures (system, control, and residual) in psi. or bar. When the FP is running:

• When the shut off valve for the pressure gauge is opened, you are reading the control fuel pressure.
• When the shut off valve for the gauge is closed, you are measuring the system pressure.

How long (minutes) does it take the control pressure to drop below 14.5 psi. after turning off the FP? How long does it take for the system pressure to go down below 14.5 psi. after you switched off the FP? These 2 tests are for your residual pressure tests.

Could you post a picture of the motor driver side? Keep us posted. Thanks.

Tony

[Paul325321]

Neils, thanks for the confirmation, I will reinstall the injector sleeves to the correct height. Seems the previous owner did not remove the peened over stops when he removed the old sleeves and the new sleeves are hung up on these burrs.

Tony,
System pressure was 4.8 bar, the temperature in the warehouse was 7 degrees C
Control fuel pressure with WUR disconnected 1.25 Bar

Control pressure dropped almost instantaneously when I shut off the Fuel Pump.
System pressure: After 10 mins I had 1.5 Bar (22 psi)
After 20 mins I had 1.4 Bar (20 psi)

I don’t seem to have good pictures of the driver side on hand - I will take some more pictures diring the weekend or early next week. The WUR last part number digits are 033 which seems correct for the model year.

[Paul325321]

I think I found a description in the Porsche Workshop Manual for K-Jetronic. Ihave to check if I have a push valve on the fuel distributor but I think not for 1977 ( I think these were added in 1978)

[Superman]

In the Bentley manual for the 911SC (1978 - 1983), the procedure is to first check fuel pressure with the engine off and the FP running. (4.5 to 5.2 bar) Then turn the engine on for at least three minutes, then turn the engine off. After 10 minutes you should have at least 1.3 bar. After 30 minutes you should have at least 1.1 bar.

[prschmn]

If system pressure is good and the loss stops with the tester valve closed check the o-ring in I think they called it the "push" valve.
It's removed by the allen in the center of the pressure regulating valve.
Early CIS did not thave them so the warmup regulator has an open path to fuel return line.

[boyt911sc]

Originally Posted by Paul325321

Tony,
System pressure was 4.8 bar, the temperature in the warehouse was 7 degrees C
Control fuel pressure with WUR disconnected 1.25 Bar

Control pressure dropped almost instantaneously when I shut off the Fuel Pump.
System pressure: After 10 mins I had 1.5 Bar (22 psi)
After 20 mins I had 1.4 Bar (20 psi)

Paul,

Your fuel pump check valve, fuel accumulator, and fuel distributor’s primary pressure valve are not causing any system pressure loss. But the WUR-033 is not holding up the residual pressure. The WUR-033 is losing the residual pressure too quick due to leak. Either replace the leaking WUR with good unit or rebuild the leaking WUR to solve the residual pressure problem.

Tony

[Charles Freeborn]

Double check fuel accumulator as well.

[prschmn]

Warm up regulators will ALWAYS bleed fuel-that's why there is a check valve
inside the pressure regulator valve in the fuel distributor.

[boyt911sc]

Originally Posted by Charles Freeborn

Double check fuel accumulator as well.

Charles,

If the fuel accumulator is defective, the system pressure would fail the residual pressure test. As evident, the residual pressure is more than 1 bar after 20 mins. and passed the test. However, the control pressure (valve open) failed the residual test because the WUR was releasing or leaking the pressure too fast.

Tony

[boyt911sc]

Originally Posted by prschmn

Warm up regulators will ALWAYS bleed fuel-that's why there is a check valve inside the pressure regulator valve in the fuel distributor.

Mark,

There is no push valve pressure regulator in pre-SC fuel distributors. I believe the OP has FD-006 (black label) for his ‘77 CIS. If you look closely at the pictures Paul has posted, the return line for the WUR is NOT attached to the fuel distributor but rather to the main return line. In short, no push valve for this type of fuel distributor. Unless my eyes are failing.

Tony

[boyt911sc]

Paul,

Do you have a picture of your fuel distributor you could share with us? Thanks.

Tony

[prschmn]

Tony-AHHH I stand corrected-I failed to look closely enough at the picture!
I suppose a picture o the pressure regulator on the fuel distributor would verify.
I can't remember when the push valve came in.

[Superman]

Originally Posted by Paul325321

Control pressure dropped almost instantaneously when I shut off the Fuel Pump.
System pressure: After 10 mins I had 1.5 Bar (22 psi)
After 20 mins I had 1.4 Bar (20 psi)

Originally Posted by Superman

In the Bentley manual for the 911SC (1978 - 1983), the procedure is to first check fuel pressure with the engine off and the FP running. (4.5 to 5.2 bar) Then turn the engine on for at least three minutes, then turn the engine off. After 10 minutes you should have at least 1.3 bar. After 30 minutes you should have at least 1.1 bar.

Paul's system appears to "pass" the test in the Bentley manual. At least....for the 911SC model. Paul's system shows 1.5 bar after 10 minutes. Is this not within spec for his 1977?

I guess I would also ask this question: Besides taking measurements, has this engine been operated in the car, and with/without running problems. I am wondering whether we have a problem to fix here.

[Paul325321]

Thank you all very much for your help, the CIS system is new to me. Now that I have a 2.7 I need to get to know how it operates. The experience is great and with your support I’m sure to get this solved. I have ordered a rebuild kit for my WUR - seems the most likely culprit. The engine starts and runs quite well, however the occasional black smoke when I blip the trottle is not right. Seems when I blip the trottle it cleans out the cylinders, pumping out black smoke then blipping again the trottle produces no smoke. However given the cold start and WUR were not receiving 12V I decided to step by step check the CIS system. That‘s when I discovered the fast fuel pressure drop when the fuel pump is turned off and the test valve to the WUR is open. When the test valve to the WUR is closed the pressure drop is slow and within spec. Its a 1977 so I suspect no push valve regulator - I will look to post a picture of the fuel distributor ( picture before rebuild). When the kit arrives I will fit and retest - fingers crossed

[boyt911sc]

Paul,

Do you have a picture of the motor driver-side view (bank 1-2-3)? I believe you are missing the thermo-time valve and the vacuum line from the decel valve. Thanks.

Tony

[Superman]

Originally Posted by Paul325321

....The engine starts and runs quite well.....

While I admire your determination, attention to detail and tendency toward data-driven decisions, I also wondering if you are trying to "fix" something that does not need fixing. We have a saying: "If it ain't broke, don't fix it."

Starting and running well is how a CIS system is supposed to behave. You say that with the WUR valve closed, pressure falls slowly and according to spec. Perhaps you have specifications I have not seen. Again, and for the last time, the CIS system in a 911SC can fall to 1.3 bar in only ten minutes and still be within spec. Your system meets that spec.

These CIS systems have no "accelerator pump" feature that carbs typically do. This is part of the reason for the very poor throttle response of CIS systems. Don't get me wrong, as I reported earlier I admire how well CIS operates when up to spec. My car still has CIS.

Black smoke suggests unburned fuel. There could be various causes. Ignition is a logical candidate. Your engine is out of the car, and I wonder if you are running it while it is not connected to the battery in the same way as if it were in the car. In other words, I wonder if it is running in 12.5 Volts or 14. I assume it has fresh spark plugs. I have the best luck with stock Bosch spark plugs.

Another potential culprit might be a too-rich mixture. But there, according to my hazy recollection, if a fuel pressure problem were creating a too-rich mixture, the pressure would be too low. Control pressure, I think.

Perhaps I am missing something, however.

[E Sully]

Paul,
Here are 2 very good videos on tuning the later 2.7 engines. In the second video he notes how he made a mistake with the vacuum line connections, as they are different in later SC's than 2.7's.
https://youtu.be/slCzVrrST2Q

https://youtu.be/Ii5k773lwKs

Originally Posted by Superman

These CIS systems have no "accelerator pump" feature that carbs typically do. This is part of the reason for the very poor throttle response of CIS systems. Don't get me wrong, as I reported earlier I admire how well CIS operates when up to spec. My car still has CIS.

Superman, take a look at the 6 minutes mark in the second video.
The early CIS like mine had a TCPR on the throttle body that did a mediocre job to the mixture.
I think the main reason for the throttle response is due to the plate in the mixture control unit. Modern fuel injection eliminated the barn doors of the early years.