Fuel pressure on a hot day...?

Yesterday about noon I was returning from a fly-in in another part of the state. It was a record hot day here, about 96 degrees. I had no trouble starting the engine and the plane performed well (hot in the cockpit, though). All the engine gauges read normal on the flight back. When I landed, as I started to taxi back, I turned off the boost pump. I got no aural warning, but I noticed that the fuel pressure was down in the red, about 8-10 psi. The engine was running fine, engine temps all normal. Flipping the boost pump back on, there was no engine change but the fuel pressure did come back up. Flipped it off and the pressure dropped back down into the red. I ran the engine up to about 2000 rpm and the fuel pressure gauge continued to read low.

I’m sensitive to this because about 30 tach hours ago I had a failure of the engine -driven pump. I saw those same pressure gauge changes, although that was associated with definitely inadequate fuel flow..the engine ran poorly when the boost pump was off.

I thought I read awhile ago that occasionally, when a fuel-injected engine is hot, there can be fuel pressure wonkiness when running on the engine-driven pump only. Is that correct? Am I remembering that right?

Engine is a factory-new Lycoming IO-320 D1A with about 360 hours TT.

MacCool said:

I thought I read awhile ago that occasionally, when a fuel-injected engine is hot, there can be fuel pressure wonkiness when running on the engine-driven pump only. Is that correct? Am I remembering that right?

Click to expand...

Not just fuel injected. The inlet line to the engine driven pump is subject to heating from engine compartment air, and often exhaust pipe radiant energy. The pump itself is warm. The feed has some level of restriction, thus there is a pressure drop. So, elevated vapor pressure (due to heating) and lowered line pressure (pumping against a restriction) means the more volatile components of the fuel can flash into vapor, i.e. form bubbles. Bubbles are squishy (for lack of a better term), and don't pump well.

The boost pump raises line pressure to 25 psi or so between the boost and the engine driven pump. Even if the fuel gets heated, it doesn't flash into vapor if the line pressure is kept above vapor pressure.

As for affecting the fuel injection system itself, it depends on how low the pump outlet pressure becomes. The Bendix style system will meter ok with feed pressure well below normal, assuming it doesn't include a bunch of bubbles.

(Opinion warning) I'm fine with an installation which idles a bit rough when hot, with the boost pump off. I do not like to see a system with pressure loss at high power. If it needs boost pump to maintain pressure at high power, the airframe needs modification to reduce fuel supply heating and flow restriction.

Great explanation, Dan. Thanks for taking the time. These were pretty extraordinary conditions for Minnesota. Very rare to see it this hot.