E-pump inlet pressure
E-pump inlet pressure
Note #2: Bubbles can also be gasses pulled out of solution by high suction. A plugged filter can cause high suction in the intake line, this pulled gasses out of solution. The gasses don't readily go back into solution, even under some pressure.
Both of these have happened to me. When the tank strainer got plugged the lines appeared to be full of air bubbles. They didn't come from outside the system, but rather from the fuel itself. Just make sure that the line to the tank is free flowing.
If the lines empty overnight, then you have a problem with air leaks.
Ah, something I hadn't thought of: The system you are working on was designed for a gas vehicle. The return line may not be into the bottom of the tank (as it is on my diesels). If so, this would allow the flow to reverse after the engine is turned off.
The check valve does fail. But it's purpose is really to maintain a minimum pressure in the IP. It is a plastic valve with a spring closer. The plastic element in mine failed and I replaced it with a bored orifice (1mm, IIRC), thus relying on the constant pressure function of the lift pump to keep the IP pressurized. No problems with that for the last 100K miles.
Note #1: Air is smaller then diesel fuel. It can get sucked into openings that the fuel won't easily leak out of. Also, o-rings can distort differently when under pressure from inside or under pressure from outside (suction inside the system).
Note #2: Bubbles can also be gasses pulled out of solution by high suction. A plugged filter can cause high suction in the intake line, this pulled gasses out of solution. The gasses don't readily go back into solution, even under some pressure.
Both of these have happened to me. When the tank strainer got plugged the lines appeared to be full of air bubbles. They didn't come from outside the system, but rather from the fuel itself. Just make sure that the line to the tank is free flowing.
If the lines empty overnight, then you have a problem with air leaks.
Ah, something I hadn't thought of: The system you are working on was designed for a gas vehicle. The return line may not be into the bottom of the tank (as it is on my diesels). If so, this would allow the flow to reverse after the engine is turned off.
The check valve does fail. But it's purpose is really to maintain a minimum pressure in the IP. It is a plastic valve with a spring closer. The plastic element in mine failed and I replaced it with a bored orifice (1mm, IIRC), thus relying on the constant pressure function of the lift pump to keep the IP pressurized. No problems with that for the last 100K miles.
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