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My 1980 911SC always starts well when cold, idles well, runs well but unfortunately it will not start after engine is hot. Once hot it can only be started after engine cools down completely. I decided to do my first fuel pressures tests. All tests done at 64 deg temp. Here are the results.
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
"Residual pressure 1.4 bar that drops to zero in 4 minutes"
This is the cause of the hot start issue. Pressure is what holds the fuel compressed as a liquid so it will not vaporize in the lines when it sits on a hot engine. The fuel accumulator has always been pointed to in the past, but the system has several check valves that could be leaking. I bet you could analyze the curve on your pressure drop and figure if the accumulator is functioning or not. A working accumulator should show a steep drop till it reaches the spring pressure, then a stable drop.
I have found that CIS is very reliable. My engine had about 12 things that were working incorrectly or not at all, and the engine still ran for the most part. I had 25 years of maintenance boo-boos to correct. Fixing the residual pressure should get you running again but I bet you have a variety of small problems you will discover.
I agree. The residual pressure is a big problem. I did the test twice because it was so dramatic. I decided to time it per minute.
1 min 1.4 bar
2 min 1.25 bar
3 min 1.1
3.5 min drops straight to zero!
I have changed my mind on the behavior of the accumulator, since it also acts as pressure buffer, it would be designed to never reach full spring pressure. I bet yours is working and the drop at 3.5min is when the accumulator hits the starting point of a preloaded spring pressure.
The Bentley manual says to check the residual pressure with the gauge valve closed and open. "If it is OK with the valve closed but drops with the valve open, check the line pressure regulator. If no faults there, check the control pressure regulator for leaks."
Did you check residual pressure with the valve closed?
If it is bad with the valve closed, it looks like it could be at 10 different points being the fuel-pump-check-valve, the cold start injector, the accumulator, 6 port injectors, or the frequency valve/solenoid.
You can pull out the line pressure regulator and inspect it. This is also where you will need to replace shims to adjust the system pressure later, so you can count how many are in there and measure their thicknesses.
While you are testing, you can also try to test your accumulator using the bleeder valve on the test gauge. Right after shutting off the pump with the valve closed, press the relief for the gauge, and drain the gasoline into a container. A functioning accumulator will drain it guts into your container. A non-functioning accumulator is only going to relieve the line pressure. You will have to use your best judgement to determine what these volumes would be unless someone on here knows.
I removed the hose from the top of the fuel filter and plugging the top of the filter. Pressurized the fuel system and then cracked the lower fitting on the FA. Fuel dumped out.
Then I removed the FA and pulled a vacuum on 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. 😤
I removed the hose from the top of the fuel filter and plugging the top of the filter. Pressurized the fuel system and then cracked the lower fitting on the FA. Fuel dumped out.
Then I removed the FA and pulled a vacuum on 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. 😤
updated fitting for fuel filter ???? what is this voodoo of which you speak ?
Apparently the Bosch replacement fuel filter has been updated to a new part number. One of the end connections no longer works with my car. So you have to buy a new line fitting from filter to line on outlet side.
I had problems sourcing the right filter for my 1981. Parts stores would try and sell you the wrong part. I think there was a change from 1980-1981.
The accumulator not holding vacuum sounds like a problem with your test setup. If it was leaking air under vacuum, then it would be leaking fuel onto the ground during operation. There is a sealed chamber of air inside that is protected by a rubber membrane and the spring. If the rubber leaks, the spring has no effect, but fuel is still contained within the metal housing.
I watched a video on putting a vacuum to the lower fitting of a FA, when it’s removed, to determine if part is defective. They used a good FA and a suspected defective FA. FA’s were three port versions like mine.
They used a hand pump vacuum which pulled 10 bar of vacuum to bottom port. The good FA held vacuum and the suspected defective FA was unable to hold vacuum and dropped quickly. I had the same results when I tested my new one and then the suspected defective one.
Apparently the Bosch replacement fuel filter has been updated to a new part number. One of the end connections no longer works with my car. So you have to buy a new line fitting from filter to line on outlet side.
I have a 78 . guess i should keep an eye out for this ,, many years ago i bought like 5 fuel filters to change every few years ( which is really pointless with todays quality fuels anyway ) . But im out and next filter i purchase i will have to watch for this.
I watched a video on putting a vacuum to the lower fitting of a FA, when it’s removed, to determine if part is defective. They used a good FA and a suspected defective FA. FA’s were three port versions like mine.
They used a hand pump vacuum which pulled 10 bar of vacuum to bottom port. The good FA held vacuum and the suspected defective FA was unable to hold vacuum and dropped quickly. I had the same results when I tested my new one and then the suspected defective one.
well that sounds very encouraging.
I hear those things aren't cheap so i hope you fixed and you are on your way soon.
Tool price is not bad at harbor freight. I plan to also use it to help bleed the brake lines. I should be up and running tomorrow.
I am getting grief from the fact my system pressure is far too high from what I’ve read and heard. I’ll see if trying to decrease it hopefully doesn’t screw up how the CIS is or was running before hot start issue.
Tool price is not bad at harbor freight. I plan to also use it to help bleed the brake lines. I should be up and running tomorrow.
I am getting grief from the fact my system pressure is far too high from what I’ve read and heard. I’ll see if trying to decrease it hopefully doesn’t screw up how the CIS is or was running before hot start issue.
i meant Accumulator is expensive..
If its a factory pump system pressure should be in spec unless maybe you have a blockage in the return line, that would be very unusual.
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.
10-27-2023, 02:57 PM
80SC Fuel pressures results
