Fuel system question.

[cfiatzph]

Why when I have the fuel selector off can I not drain the redisdual gas out of the gascolator to see if fuel is getting by the fuel selector in the off position? Is this some sort of hydrolock?

Thanks

[Bruce Fenstermacher]

Yes it is. The fact that you can't drain the gascolator is a good sign things are OK in the fuel valve.

[sphillips]

Fuel selector off, no head pressure to push fuel.

[cfiatzph]

Duh!!! I removed the glass and turned the fuel selector off no drips turned it on flows good then off no drips. Just was testing the annual rebuild of the fuel selector.

Thanks guys.

[bradbrady]

Cfiatzph,
Thats part of an anual inspection, Turn the gas off, remove the screen, check for junk, check for leaks with the gas off! Looks like every thing is OK.
brad

[Green Bean]

Actual, head pressure has nothing to do with it. It is all about the vent. If the tank wasn't vented, the fuel would not flow. A leak, with the fuel selector valve in the off position, indicates a vent leak caused by an "O" ring or gasket or line leak, in the system.

The same is true when you turn a gas can upside down and the flow is restricted. I that case, some liquid will flow, but not at the max rate allowed by the diameter of the line, until the vent is open.

This was the reason in the 70's, that vented fuel caps, or other alternate systems, were "AD" to aircraft, because the manufactured vent did not always insure a vented system. Sometimes a full fuel tank, had no fuel to the engine, even though it had head pressure, because the vent was blocked.

[GAHorn]

Actual, head pressure has nothing to do with it. It is all about the vent. If the tank wasn't vented, the fuel would not flow. A leak, with the fuel selector valve in the off position, indicates a vent leak caused by an "O" ring or gasket or line leak, in the system.

The same is true when you turn a gas can upside down and the flow is restricted. I that case, some liquid will flow, but not at the max rate allowed by the diameter of the line, until the vent is open.

This was the reason in the 70's, that vented fuel caps, or other alternate systems, were "AD" to aircraft, because the manufactured vent did not always insure a vented system. Sometimes a full fuel tank, had no fuel to the engine, even though it had head pressure, because the vent was blocked.

Actually, head pressure has everything to do with it. But with no ventilation to the tank, a vacuum is created and therefore head pressure is negated. (No flow results because head pressure is reduced to zero due to the lack of vent.)
A leak with the valve off, does not mean a leak in the vent system. It means a leak in the valve. However a condition of no/low fuel flow with the valve on does indicate a defective/plugged vent system.
(We're saying the same thing, just saying it differently.... except that any flow/or drips of fuel with the valve off ...is a defective valve ...(not a defective vent.)

[Green Bean]

Your right, you did say what I was saying. Except, in a system that uses head pressure as a measurement, it will have a way to read that pressure, but that is not required in a gravity system of the Cessna 170.

The bottom line is if it leaks, fix it.