High Fuel Pressure after shut down

[Skates97]

I have seen this a few times but happened twice today. I have a O-360-A1D in my plane. My shut down procedure is out of the POH. Increase RPM to 1,100, pull mixture to idle/cutoff, as engine dies pull throttle out. On today's flight I stopped at a fuel stop, shut down, fueled, (took a restroom break), and got back in to start up to see that the fuel pressure gauge was showing about 7 PSI. I finished the flight to my destination and refueled before parking so I wouldn't have to when I got ready to fly home. Upon getting in after refueling (about 10 minutes) the fuel pressure gauge was showing a little over 9 PSI.

When it does this if I leave the mixture at idle cut off and crank the engine while advancing the throttle it will start up, the fuel pressure drops into the green, and I can enrichen the mixture to keep it running as the fuel pressure drops down to normal levels. (Similar to the flooded engine start procedure)

This only seems to happen when it is hot outside and it is not consistent. Any ideas?

[Andy95W]

I haven't really noticed mine, but I will say it's probably normal.  The cold fuel stops flowing in the lines inside the hot cowling.  The fuel heats and expands which increases the pressure at the gauge.

The good news is that your fuel system, up to and including your carburetor must be in very good shape to not let the pressure bleed down.

[carusoam]

Chemical engineering thoughts ahead, not PP ideas...

1) there is a small amount of fuel blocked in the line.

2) It is a pretty hot environment under the cowl, the engine has been run, it is hot outside...

3) all the heat under the cowl is trying pretty hard to expand the volume and evaporate the fuel.

4) as long as the fuel stays locked in the same volume, the pressure will build.

5) opening the mixture knob will allow the pressure out.

6) releasing the pressure of hot fuel will allow the evaporation to occur.  Bubbles forming in the line will press even more fuel towards the fuel injectors...

7) This is the basic challenge of hot starts.  More fuel gets delivered than wanted, the heated fuel is easily evaporated.  Air gets overwhelmed by fuel and the mixture percentage is not flammable... until better controlled, cool fuel is flowing....

How does that sound?

Best regards,

-a-

[N201MKTurbo]

The true vapor pressure of avgas at 120 F is 10.3 PSI.

So the pressure you are seeing look correct for a sealed volume of avgas on a hot summer day.

I don't think it is a problem. It is illustrating the health of your fuel system.

http://www.dtic.mil/dtic/tr/fulltext/u2/a132106.pdf

 

[Hank]

Sounds good, Anthony, except Cs & Ds are carbureted. It's simply engine heat making the fuel in the line expand, and a good shutoff in the carb not letting it expand.

While I was typing this, someone else chimed in with a detailed answer that looks pretty correct to this (non-chemical) engineer.

[Skates97]

Thanks guys. The first time it happened months ago it took me a while to get it started. Once I figured out that I needed to leave the throttle out and mixture at idle cut initially to get it started it hasn't been an issue. I figured it was the heat causing the fuel to expand but I wanted to make sure it wasn't indicative of a problem. 

[Jim Peace]

mine does it,,,,