The International Cessna^® 170 Association

My 48 has a fuel pump that normally runs 4 PSI when the engine is running, when I pull the mixture the engine windmills down, the gauge will go up to 6 PSI. and will read that for days or until the engine is started again. the only way to get it to drop off to zero, is to push the mixture in and pump the throttle a few times.

is this normal?

Tom, not familiar enough with the '48 fuel system to say but I do know there is suppose to be a one way check valve. Sounds like your system has one and it is holding pressure. I'm just not sure it should but suspect so.

Tom
I don't ever recall my 48 acting that way and my first thought was that the check valve might be in backwards but that wouldn't do it. I can take pictures if you need me to and will let you know next time I fly it.
Dan

Bruce Fenstermacher wrote:Tom, not familiar enough with the '48 fuel system to say but I do know there is suppose to be a one way check valve. Sounds like your system has one and it is holding pressure. I'm just not sure it should but suspect so.

There is one, and its in there the right way.

Mine falls to just above zero right at at shutdown.

48RagwingPilot wrote:Mine falls to just above zero right at at shutdown.

So, who has the discrepancy? me or you?

Dumb question, but does putting the fuel selector to off do anything? I'm assuming the 1948 has left, right, both, and off?

Ryan Smith wrote:Dumb question, but does putting the fuel selector to off do anything? I'm assuming the 1948 has left, right, both, and off?

Yes the 48 has a fuel selector, it controls fuel to the carb and fuel pump, shutting it off will of course make the engine quit when the fuel in the carb bowl runs out, and the pump pressure will drop as a result.

As I see the system, there are 3 ways to get fuel out of the pump loop. 1 the needle valve in the carb, as fuel enters the carb. 2 a leaking check valve, as fuel will leak back into the delivery line of the selector valve. 3 a leaking out put check valve of the pump.

I believe after I pull the mixture to idle cut off, the needle valve in the carb closes, stopping fuel from entering the carb bowl, the check valve is closed, and the pump continues to pump until the engine rotation stops, thus raising the fuel pressure to the maximum of 6 pounds. and the pump out put check valve will then close to reverse flow trapping the 6 pounds in the loop.

Pumping the throttle will cause a flow thru the accelerator pump, dropping the fuel level in the carb bowl, that opens the needle valve, which bleeds off the 6 pounds.

I think this just happens to be the tightest system I've seen. scary

Color me embarrassed. As sure as I was that mine fell to zero after shutdown, here I am at a neighboring field for fuel looking at a 3.25 psi on my gauge approximately 20 minutes after shutdown. Sorry for the error.

48RagwingPilot wrote:Color me embarrassed. As sure as I was that mine fell to zero after shutdown, here I am at a neighboring field for fuel looking at a 3.25 psi on my gauge approximately 20 minutes after shutdown. Sorry for the error.

Not a problem, I think the pressure we see is the pressure developed by the spring in the pump. and will vary from pump to pump. The pressure we see is the pressure in the fuel trapped between the pump diaphragm/pump check valve and the needle valve in the carb.

Thanks for the explanation. I do know that it does leak down over time.

I'm curious ... I'll check mine and report back. I've never paid any attention to what it reads after shut down.

Thomas

My pump was removed sometime in the 70's.

Poncho73 wrote:My pump was removed sometime in the 70's.

your serial number better be 18250 or higher.

Poncho73 wrote:
My pump was removed sometime in the 70's.

your serial number better be 18250 or higher.

Tom,
Why sn 18250? Per TCDS all 48 170's have a fuel pump.
Just curious
Thx,
Dan