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Old 02-14-2021, 04:19 PM   #41
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Since you have no fuel leaks reported when running and pressurized the question becomes where is the air coming from?. The most obvious source is the fuel return. I suspect a bad check valve is allowing fuel to drain out of the IP side of the system back to the tank and drawing air from the tank.
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Old 02-14-2021, 05:25 PM   #42
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Dear Bloody Snow,
I don't know how I can explain it better - difficult without being in person. The day tank is lower than the engine (and lower than the point in the system where the vacuum gage is located). The engine has to suck the fuel up against this height difference. Even when the flow stops the suction holds - the fuel is trying to run back down into the tank.

To further discuss other people's suggestions - the air has to be able to enter the system somewhere. Possibilities are:
1 return line to the tank - would require reverse leakage of the check valves in both the injector pump and in the lift pump (a total of at least 3 checks). Seems unlikely, but is possible. In the industry I work in we are told never to rely on a check valve totally sealing.

2 a "vacuum" leak between the day tank and the injector pump. This is interesting because from the day tank to the lift pump is always under a (slight) vacuum when the engine is running. This means that the engine would be pulling in air with its fuel. Perhaps this tiny amount of air is able to pass through the engine during running but the engine cannot handle the arrival of the accumulation of air which builds up during long periods when stopped.

3 a "one-way" leak between the lift pump and the injector pump, which only allows air in but not fuel out.

I think that option 2 is the most likely. I have personally seen air leaking into a filter set-up when subjected to a mild vacuum. If the leak is indeed between the day tank and the lift pump it could even be a bidirectional leak and no fuel dripping is seen because this system is always under a slight vacuum. You could try putting pressure on this part of the system and look for fuel leaks. It would be nice to pull a vacuum on the fuel line and watch the vacuum gage, but this would probably take a very long time to show anything. The transparent tubing suggestion sounds good as well but, once again, it would probably take a long time to see. Perhaps put the clear tube between the filter and the lift pump and watch for a slug of air at start up. If you see the air, the problem is between the day tank and the filter.

Nick (suffering from the sun)
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Old 02-14-2021, 09:28 PM   #43
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I should have posted this earlier, but attached is a pic of my problem fuel piping. The day tank is at the very bottom of the pic and you can see various lines and ball valves going in and out. The engine filters and engine pump is about the same height as the Racors. All in all this seems like a pretty standard arrangement.

The Racor pressure gauges are about 3' +- above the fuel level in the day tank. My math says that with the engines off and feed valves open there should be about a 2 inch vacuum at the gauges (the fuel in piping wants to return to tank creating vacuum). Indeed, that is what the gauges are showing in the picture.

During normal operation the added vacuum from the clean filters add another 1-2 inches of vacuum, which seems normal.
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Old 02-14-2021, 10:12 PM   #44
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Bruce. If that manifold with 6 outlets plus drain is involved then a search for a recent thread is warranted as in the end if I recall correctly the manifold was the source of air.
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Old 02-15-2021, 12:19 AM   #45
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Its very simple. If the pressure in the line is lower than the outside ambient (14.7 at sea level) air has the potential to leak into the line. If the fuel tank is below the engine, or there are restrictions in the line the engine fuel pump creates a suction which lowers the pressure line pressure further. A column of fuel of about 30 inches creates about 1 psi pressure at the bottom of the column. To lift a column of fuel above the free surface of the fuel tank takes about 1 psi to lift 30 inches. Flow velocity or screen restrictions will add further to lower the pressure. Without flow the column of is at zero velocity, but there is still a vacuum, so a very small air leak can cause the line to drain back to the tank, which makes starting difficult if you do not have fuel in the line. The other problem over time air will come out of solution in the fuel if there significant vacuum. (pop bottle effect) It is difficult to find these small leaks, every thing in the fuel line is a potential source and requires carefull review of the installation and review the condition of the components in the line.
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Old 02-15-2021, 06:34 AM   #46
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Thanks Dennis and Soo Valley - very much in line with what I was trying to explain - in fact, explained better!

As regards a workaround - There should be a valve right at the outlet from the day tank. Closing this valve whenever the engine is stopped should prevent the drain-back problem.

Nick
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