80SC Fuel pressures results

[Clockmike]

I have a 1980 911SC which starts well when cold, runs well, idles well but unfortunately it will not start again after it’s hot. It can only be started after the engine cools down completely. I decided to do my first fuel pressures tests. Here are the results. Temp was 64 deg.
System pressure 7.65 bar
Cold control pressure 3.25 bar
Warm control pressure 3.7 bar - 4.0 bar over 10 minutes
Residual pressure 1.4 bar that drops to zero in 4 minutes.

It appears to me that both the warm control pressure and residual pressure show an issue. What’s the next step?
Thanks

[Sailkane]

Appears the residual pressure is the issue…..1 st symptom of a bad fuel accumulator

[Clockmike]

That would be welcome news if it turns out to be the fuel accumulator since I was thinking it was looking like a WUR issue.

[mike sampsel]

Get your system up to pressure (as you would for a residual test) shut off the engine and remove the hose on the bottom of the FA, if it's leaking there, it's bad, if it's not dripping than your poor residual might be something else.

The bottom hose attaches to the return line to the fuel tank. Use two wrenches to get the bottom line of the FA.

[PeteKz]

First: How many miles on your car and what previous CIS work have you done?

Second: Likely that you need a new FA. If it's original, replace it even if it's still "good." At this age, they are failing with regularity (probably accelerated by the ethanol contamination in current fuel). To test, disconnect the the bottom (drain) connector from the FA, plug the line, and run the pump. If you get fuel out the bottom of the FA, it's bad. Do this before chasing the other components.

Other stuff: Your system pressure is too high. Should be 4.5-5.2 bar. Your CCP also is too high, what is the number on the WUR? The CCP should be around 2 bar at that temp. WCP should be about 3.5 bar. You can search for the Temp/ Pressure charts for the various WURs on this site, and compare yours.

Become familiar with this Jim's Basement Workshop CIS Primer (now maintained by rarlyl8). You will find the pressures and other specs there:
https://cis911primer.com/home.html

[boyt911sc]

Originally Posted by Clockmike

I have a 1980 911SC which starts well when cold, runs well, idles well but unfortunately it will not start again after it’s hot. It can only be started after the engine cools down completely. I decided to do my first fuel pressures tests. Here are the results. Temp was 64 deg.
System pressure 7.65 bar
Cold control pressure 3.25 bar
Warm control pressure 3.7 bar - 4.0 bar over 10 minutes
Residual pressure 1.4 bar that drops to zero in 4 minutes.

It appears to me that both the warm control pressure and residual pressure show an issue. What’s the next step?
Thanks

Mike,

• The fuel distributor has a very high SYSTEM PRESSURE.
• The WUR is out of spec.
• Residual pressure is going down too quick.
• Do not replace part unless tested and confirmed defective.

(1). Adjust the system pressure to 5 bar.
(2). Rebuilt/calibrate the WUR to deliver the correct CP and WCP.
(3). The three (3) most common culprits to cause excessive residual pressure loss are:

• Defective fuel pump check valve.
• Defective fuel accumulator.
• Defective primary pressure check valve.

Tony

[Clockmike]

Here’s a bit of background. Full top to bottom engine rebuild 4 years ago. Recently I checked back with engine rebuild mechanic to ask about hot start issue and was told FA was replaced during rebuild but there’s always a possibility that it was defective or failed. It was suggested I do the fuel pressures tests.

This week before I did the Fuel Pressures test, I replaced the FP check valve. This did not seem to help. In addition, although the car runs fine except for the Hot Start issue, I have always suspected it was running rich from the smell of the exhaust.
Engine starts easy when cold and idle starts high but slowly drops down to 950-1000 RPM when warmed up.

My WUR is part number 0 438 140 072

[PeteKz]

Okay, more info.

FA was new 4 years ago. Check it for fuel leaking as I described. FP check valve is new, but check it anyway. Do this by removing the hose from the top of the fuel filter and plugging the top of the filter. run the pump to pressurize the FA. Wait a half hour. Then open a connection to the FA or unplug the top of the FF. You should get several tablespoons of fuel squirt out. if you do, then both your FP check valve and FA are good.

Otherwise, it appears the SP check valve is the likely problem. Since the check valve and SP regulator are on the FD, those problems could be related. If the test of the FA is good, then remove and clean out the FD with carb cleaner spray. Use the info from Jim's Basement CIS Primer to remove and clean the SP regulator, check valve, and plunger.

At this point, I'll guess that some crud or corrosion is preventing the check valve from sealing and preventing the regulator from working correctly.

[Clockmike]

Thank you PeteKz and Tony for your knowledge and guidance. I’m looking forward to digging in on these tasks very soon.
While staring at the various parts on my engine last night I noticed something on the WUR. It appears that at some point it was rebuilt and modified so it is now adjustable. That seems it would be a good thing?

[PeteKz]

Yes that's an adjustment screw on the WUR. That's also not the stock electrical connector on the WUR.

[Clockmike]

I disconnected the fuel filter since I planned to change it out then I removed the FA.
Once removed, I pulled a vacuum on FA and it would not hold vacuum at all.
I just installed a new FA and Fuel filter. Need a stupid updated fitting for fuel filter before I can test. ��

[boyt911sc]

Your fuel distributor and warm-up regulator are out of spec. (too high fuel pressures) and you are losing residual pressure too fast. Correct these problems if you want your CIS to run well. After replacing the FA was there any improvement in the residual pressure? And the primary pressure valve in the FD? Why did you replace the FP check valve? Was it defective or you just decided to replace it? Keep us posted. Thanks.

Tony

[Clockmike]

Once I receive the new line fitting for the Bosch fuel filter, I’ll have all the fuel system fittings buttoned up. Then I’ll do all the pressure tests again. I replaced the FP check valve out of an abundance of caution and it looked very old.

[boyt911sc]

Take your CIS Reference Manual out and set the fuel pressures correctly. The system pressure (7.6 bar) you have is too excessive for normally aspirated CIS. That is even greater than most 930 motors have. Normal system pressure for your CIS: 5 bar/72 psi.

Tony

[Clockmike]

Tony, If I have a mild webcam, bigger pistons and more compression, would the system pressure need to be slightly higher for the CIS to run well?

[boyt911sc]

Originally Posted by Clockmike

Tony, If I have a mild webcam, bigger pistons and more compression, would the system pressure need to be slightly higher for the CIS to run well?

Mike,

The system pressure spec for your fuel distributor is 65~75 psi. NOT 110 psi.! That’s 50% more than needed. Most older 911 Turbos use less than 100 psi. system pressure with 2 fuel pumps. Why are you using 110 psi? If you need more fuel, increase the flow rates.

Tony

[Clockmike]

Just to be clear, I have not set pressures myself. The mechanic that rebuilt the engine is the one that set up the pressure and WUR. I only recently got interested in chasing down the hot start issue because I had some time. Now I must admit that I’m enjoying the process. Especially since my mechanic is two hours away and very busy.

[boyt911sc]

Originally Posted by Clockmike

Just to be clear, I have not set pressures myself. The mechanic that rebuilt the engine is the one that set up the pressure and WUR. I only recently got interested in chasing down the hot start issue because I had some time. Now I must admit that I’m enjoying the process. Especially since my mechanic is two hours away and very busy.

Mike,

Your mechanic could be good and honest, but has a flawed understanding about CIS. If you can’t find someone knowledgable, then you are stuck with this fellow. Or do the CIS troubleshooting yourself. You have a better chance of fixing the problems with the help from people in this forum than any where else. And it’s FREE.

Tony

[mike sampsel]

Originally Posted by Clockmike

Tony, If I have a mild webcam, bigger pistons and more compression, would the system pressure need to be slightly higher for the CIS to run well?

I have webcam 20/21, 3.2 cylinders and 9.5 compression. My FP is 70 psi.
No need for 110 here. Car runs and starts great.

[Clockmike]

After replacing the Fuel filter and the Fuel accumulator, I rechecked the residual pressure this morning. It was 1.5 bar after 30 minutes. Looks like I’m back in business. Plan to redo all 4 of the pressure​​​​​​ tests again.