Fuel Pressure with engine off

[Stanley]

Hooked up my new FP gauge today, like the gauge it's a 0-10 psi McDaniels, easy to see under glove box from driver seat. Line is steel braid over Buna-N with black braided Dacron cover good for 3025 psi, all OK.

Got steady 3.8 psi at idle which is ok I guess. Bled out the air, think I got most of it. One question though, when I shut down the engine, FP drops fast to 3 psi, then very slowly to 1.5 or 2. After a minute or two, looked at it again - it went back up to about 4 psi again. Whaaat? Then it dropped off again. After 15 minutes it was down to 1 psi.

Doesn't seem like much of a problem, but I don't get that strange behavior at all. Any ideas? I got zip.

Edited July 27, 2013 by Stanley

[beermanpete]

The fuel pressure will drop after shut-down due to bleed off thorugh the return line. The increase after a few minutes could be due to heat soak heating the fuel to the boiling point and causing it to turn to gas. You seem to have made a vapor lock indicator.

[Stanley]

Sounds reasonable , but in this case the engine wasn't even warmed up. No return line, but the gas (pressure) has to go somewhere. I guess it goes backward through the fuel pump, filter, and back down the fuel line to the tank to level with the fuel in the tank. Also, I removed the electric fuel pump by the tank a while back.

Wish I had clear fuel lines to see where it was going but the tech inspectors at the strip wouldn't be thrilled with that. That's why ultralight aircraft have clear fuel lines and change fuel pumps every year- so they see that the gas is running. I do have a clear filter before the pump that usually runs about half full - I've seen it fill up after shutting down the engine.

Maybe it's a hiccup or burp due to siphoning or something? Pressure from air in the tank, or maybe because I didn't get all the air out of the line to the gauge? At least it's doing something interesting besides just sitting at 3.8 psi. Maybe it will save me from buying unnecessary fuel pumps.

Guess there are some gaps in my understanding of the fuel system.

[Stanley]

Tried it again today, only went to about 3.0 psi, shut it off and watched the gauge FP drop and rise like yesterday. Noticed the hose from the tee on the pump went up few inches vertically before going down to the grommet on the firewall. Figure gas was draining out and back to the tank went it's shut down, so I moved it so the whole line descends to the gauge. Took another drive, max FP was about 3.3 psi. When I floor it for 5 sec or so the FP drops to about 2.8, tried it 3 or 4 times with the same result. The rise in FP after turning it off seems to be gone. FSM says 3.4 to 4.4 psi is correct - maybe I'll should try a new fuel pump when I get paid.

[Stanley]

OK, learned something. Liquid filled gauge is supposed to have a pinhole in the rubber vent nipple, otherwise the pressure goes up when it's cold, and down when it warms up. The vent needs to be up so the glycerine doesn't leak out. Maybe I'll install a union at the gauge before venting it, so if I want to remove the gauge from the line I can do it without rotating it and losing some liquid. Guess the pump is OK. I Like being able to see the FP while driving. Thanks to McMaster-Carr for getting my BSPT adapter and chrome brass tee to fit on the mechanical pump to me within two days. And thanks Mr. Hose for putting the 3050 psi pro-looking hose together with all the necessary dash-4 and NPT fittings while I waited, made it all easy. McDaniel gauge is 0-10 psi, case and glass safe to 300 psi, about $55 plus shipping. Under $100 for everything and nothing Mickey Mouse.