CIS Experts: No Residual Pressure

[gold911]

1979 911 SC 3.0 stock USA

Hot start issue:
-2 year old fuel pump, BOSCH 69513
-New Fuel accumulator

Attached fuel pressure gauges and 5 bar System pressure but once fuel pump is off, 0 pressure. Same with cold and warm control pressure tests. No Residual Pressure.

Can I assume the FP check valve has stuck open? Is there a quick way to test? Something else I should check before spending $$ on new FP? No smell of fuel.

thanks

[Classic 911]

Hi.

If you have a fuelsystem tester that can block out WUR then you can test system pressure and find out if the check valve is ok or not.


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[gold911]

I assume I do have a fuel system tester that can block out WUR. System Pressure (WUR is blocked out?) goes to 0 immediately after fuel pump is shutdown. Is this definitely the check valve?

[Classic 911]

I would assume your check valve is not doing it’s jobb


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[boyt911sc]

Originally Posted by gold911

1979 911 SC 3.0 stock USA

Hot start issue:
-2 year old fuel pump, BOSCH 69513
-New Fuel accumulator

Attached fuel pressure gauges and 5 bar System pressure but once fuel pump is off, 0 pressure. Same with cold and warm control pressure tests. No Residual Pressure.

Can I assume the FP check valve has stuck open? Is there a quick way to test? Something else I should check before spending $$ on new FP? No smell of fuel.

thanks

For your SC, the three (3) most common culprits causing residual pressure loss:

• Defective FP check valve.
• Defective fuel accumulator.
• Leaking FD primary pressure check valve.

You can test them individually to identify which one is the culprit.

Tony

[gold911]

Thanks Tony

How can I test the FP check valve other then replacing the FP as it is integrated? Also, how does one check the FD valve?

thank you.

[Showdown]

Run the tests outlined on this site;

https://jimsbasementworkshop.com/CIS/

And in this thread:

CIS Troubleshooting for Dummies

Once you do so, you’ll be able to pinpoint the source of the problem and then go about fixing it.

None of parts in Tony’s post are terribly difficult to fix but you have to know which one needs fixing first; if it’s the fuel accumulator, buy a new one or get one from Tony, if it’s the check valve either buy a new one or replace the fuel pump depending on the year of your car and type of fuel pump, if it’s the fuel distributor, you’ll have to service the internal valve- not hard.

The point is that until you know which one is the culprit you’re only going to chase your tail.

Get some pressure gauges, do the tests and the results will lead you to a solution.

[boyt911sc]

Originally Posted by gold911

Thanks Tony

How can I test the FP check valve other then replacing the FP as it is integrated? Also, how does one check the FD valve?

thank you.

Gold,

Install a pressure gauge with shut-off valve between FP and FA using appropriate fuel line fitting to measure the residual press loss. Test run the FP for 2~3 sec. with the shut-off closed and record the pressure drop after 20 mins. A 1.1 bar reading or more after 20 mins. or longer time is good.

Next, remove the fuel accumulator from the car and do a pressure test. You could simply blow air using your mouth while the FA is partially submerged in water. I had posted this test with pictures. Or a fancy test using a hand vacuum pump with the correct fuel line fittings.

The test for the primary pressure check valve used in SC motors should be done after verifying that both FPCV & FA were confirmed good. Test for system fuel pressure and observe for 20 mins. similar to previous tests.

Do not replace CIS components unless tested and confirmed defective. Unless you want to spend unnecessarily.

Tony

[gold911]

One more question. Before this hot start issue, I could hear my fuel pump run on my 79sc with the key in the "run" position before starting. Turn ignition on, fuel pump runs. I no longer hear it run before I turn the key to start like before. Should the fuel pump have been running with ignition on and why would it have stopped? Car runs and starts perfectly cold, just new hot start issue.

thanks

[PeteKz]

I know you say you replaced it, but here's a very quick test for the accumulator: Disconnect the lower (drain) line from the accumulator. Plug that line. Run pump. If you get any fuel coming out of the bottom of accumulator, it is bad. Otherwise, assume it's good. Then you only have two things to check.

Another quick check: Pinch off the rubber return line to the tank. Run the pump briefly, just enough to get up to system pressure (with nowhere to go, the pump will deliver it's maximum pressure, so it could be 150psi--don't let it get that high). Now the entire fuel injection side of the system is pressurized, so if the fuel pressure valve in the fuel distributor is leaking, it won't lose pressure back to the tank. If you are still losing pressure, then it's almost certainly the check valve on the FP. If it holds pressure, the check valve is good. Then you're most likely looking at the pressure regulator.

If bad check valve, you can put another external to the pump.

[gold911]

Petekz

thank you for the tip. Where do you suggest to pinch off the return line. Up next to the fuel pump under the car or is there another location.

this is a good idea as I am having issue locating an adapter for fuel pressure gauge to fuel pump line at rear of car. thanks again for your reply.

[hughc]

Following

[PeteKz]

The return to the tank goes through a metal hard line at the front of the engine, thence to a rubber line in the left front corner of the engine bay, then down to the metal return line through the tunnel. Pinch off the rubber line where it comes off the hard line from the engine. That's the easiest place.

If that rubber line is hard, then it's time to replace it while you're in there.

[targa80]

Since you have a 79SC and we will assume the fuel pump is original and it would have the internal check valve. If or when you decide the internal check valve has failed you can purchase an external check valve and add it in-line with the fuel pump output line. Make sure you purchase the end cap because the line is a banjo fitting. When I did this over 20 tears ago I bought the check valve but it did not come with the end cap. At that time I had to buy it separately. Sorry, I do not remember the part numbers for the parts. At least the picture gives you an idea of what you need.

[gold911]

Thank you for your assistance. I connected my pressure gauge to the fuel pump output and it does not hold pressure. Holds the pressure a little better then when connected between the WUR and the FD. Instead of dropping in 1 second after fuel pump is turned off, pressure now drops in 1.5 seconds. I think I might have another leak at the FD. Ordering replacement pump as check valve is integrated. Once pump is installed, will test again.

thanks again for all the help

[targa80]

Originally Posted by gold911

Thank you for your assistance. I connected my pressure gauge to the fuel pump output and it does not hold pressure. Holds the pressure a little better then when connected between the WUR and the FD. Instead of dropping in 1 second after fuel pump is turned off, pressure now drops in 1.5 seconds. I think I might have another leak at the FD. Ordering replacement pump as check valve is integrated. Once pump is installed, will test again.

thanks again for all the help

Why replace the pump if it is working as far as providing fuel. Buy the external check valve as in the picture I posted earlier and add it to the pumps output. It’s cost less and is easy to install.

[boyt911sc]

Originally Posted by gold911

Thank you for your assistance. I connected my pressure gauge to the fuel pump output and it does not hold pressure. Holds the pressure a little better then when connected between the WUR and the FD. Instead of dropping in 1 second after fuel pump is turned off, pressure now drops in 1.5 seconds. I think I might have another leak at the FD. Ordering replacement pump as check valve is integrated. Once pump is installed, will test again.

thanks again for all the help

Gold,

The fuel pressure reading would immediately drop for all CIS when you stopped the FP from running. Did it drop from 6 bar to ZERO in a second?

Tony