Air in diesel supply - for 12 years!

[KellysEye]

>Well, that's certainly not my experience with a keel tank and CAV-type filters (which incidentally seem to have the inlet and outlet higher than the top of the filter element).

It doesn't matter if the outlet is higher. The lift pump on the engine sucks the fuel up from the tank blow and thus the system is pressurised, so leaks/air in can appear anywhere even if the outlets are higher.

 

[pvb]

>Well, that's certainly not my experience with a keel tank and CAV-type filters (which incidentally seem to have the inlet and outlet higher than the top of the filter element).

It doesn't matter if the outlet is higher. The lift pump on the engine sucks the fuel up from the tank blow and thus the system is pressurised, so leaks/air in can appear anywhere even if the outlets are higher.

When the engine's running, the fuel in the pipe is under vacuum, not pressure, so won't leak out, although air can leak in.

When the engine's off, gravity will cause the fuel in the pipe to be under a slight vacuum, so again if air can leak in there won't be an escape of diesel.

Depending on the plumbing of the fuel pipes, it's possible that there might be low spots where fuel could escape in some circumstances. In the OP's case, with all the attention the system has received so far, I think that fuel escaping from a leaky joint would have been spotted by now.

 

[dratsea]

Racor 120?

If so might have an answer.

J

 

[pvb]

Racor 120?

If so might have an answer.

The OP said "I have changed from a CAV filter/water separator to a Racor filter – mostly because of the built in pump function." Don't think the 120 has a built-in pump, does it?

 

[KellysEye]

>When the engine's running, the fuel in the pipe is under vacuum, not pressure, so won't leak out, although air can leak in.

Vacuum is no pressure as in space or pressure below atmospheric pressure.

>When the engine's off, gravity will cause the fuel in the pipe to be under a slight vacuum, so again if air can leak in there won't be an escape of diesel.

The engine fuel pump sucks fuel from the tank, equivalent to reverse pressure. You might call it a vacuum but it is full of dissel from the injectors to the tank. Once it goes through the pump it is still pressurised to ensure the injectors spray diesel. If there was little pressure the diesel would drip through the injectors with an obvious result.

Any microscopic hole will let diesel out and air in. Are you speaking from experience of having a leak?

 

[pvb]

>When the engine's running, the fuel in the pipe is under vacuum, not pressure, so won't leak out, although air can leak in.

Vacuum is no pressure as in space or pressure below atmospheric pressure.

>When the engine's off, gravity will cause the fuel in the pipe to be under a slight vacuum, so again if air can leak in there won't be an escape of diesel.

The engine fuel pump sucks fuel from the tank, equivalent to reverse pressure. You might call it a vacuum but it is full of dissel from the injectors to the tank. Once it goes through the pump it is still pressurised to ensure the injectors spray diesel. If there was little pressure the diesel would drip through the injectors with an obvious result.

Any microscopic hole will let diesel out and air in. Are you speaking from experience of having a leak?

I can't decide whether you're trolling or not.

 

[KellysEye]

>I can't decide whether you're trolling or not.

Not at all, I think I mentioned we had many leaks when we had copper piping fuel lines, which I changed. What I've said is based on extensive experience of air in/diesel leaks.

I noticed you didn't answer the question, are you going to?

 

[Wino]

Check non-return valve on fuel pump.

 

[pvb]

Not at all, I think I mentioned we had many leaks when we had copper piping fuel lines, which I changed. What I've said is based on extensive experience of air in/diesel leaks.

I noticed you didn't answer the question, are you going to?

Certainly, in the past I've had 2 difficult instances of air leaking in to diesel lines, causing starting difficulties. In each case, there was no detectable leak of diesel from the pipework. Using both tissues and talcum powder, diesel wasn't found. Eventually, remaking the joints cured the problem.

When the diesel tank is below the engine, diesel lines are subject to vacuum. You might call it "reverse pressure" or "pressure below atmospheric pressure", but it's still there, and it can suck air in through fittings, whilst not allowing diesel to leak out. That's my experience; your experience may be different.