Engine Driven Fuel Pump Failure Mode

[SMO]

After the last flight in my Rocket (IO-540) I fueled it up and then restarted it (engine was warm). It started fine but the fuel pressure did not come up properly - stayed around 6 - 8 psi. I switched on the boost pump and as the pressure came up over 24 psi I switched the boost pump back off. The fuel pressure stayed up. There were no issues with the fuel pressure during the flight immediately previous.

I have subsequently started it cold and the fuel pressure comes up fine.

There are 250 hours on the fuel pump, 100 in the current setup (and many flights on much warmer days), and I have never had this problem previously.

Is there a failure mode with the engine driven fuel pump that occurs when it is warm as opposed to cold?

Could this just be a vapor lock anomaly?

[airguy]

Sounds like classic vapor lock to me, the fuel in the pump was hot, the suction side of the pump generates vapor bubbles on the suction stroke, and you have difficulty getting pressure. As soon as you get the hot fuel moved out of the pump and replaced with cool fuel the problem goes away.

[pvalovich]

Agree. Vapor lock on restart is common here in the Mojave Desert with OAT's routinely approaching 110 degrees F.

First time it happened I was quite concerned about engine driven fuel pump failure, but now have procedures that work every time for my IO-360.

1. Throttle - full open
2. Mixture ICO
3. Prop - full increase
4. DO NOT TURN ON ELECTRIC BOOST PUMP
5. Start - when engine catches, retard throttle
6. If engine stumbles or fuel pressure low, electric boost pump on until fuel pressure climbs, then off

[SMO]

Do you find that this vapor lock issue can continue for a while after the engine is running?

The other day I refueled after a flight, and during the taxi back to the hangar the fuel pressure was low. I switched on the boost pump and it came back up but when I turned the boost pump off it went back down. Seems to me once the pressure comes back up, and the engine is running, it should stay up.........

The fuel pressure sender is between the engine driven fuel pump and the fuel injection servo. I have never had a problem starting, hot or cold.

[airguy]

Originally Posted by SMO

Do you find that this vapor lock issue can continue for a while after the engine is running?

The other day I refueled after a flight, and during the taxi back to the hangar the fuel pressure was low. I switched on the boost pump and it came back up but when I turned the boost pump off it went back down. Seems to me once the pressure comes back up, and the engine is running, it should stay up.........

The fuel pressure sender is between the engine driven fuel pump and the fuel injection servo. I have never had a problem starting, hot or cold.

The fuel pressure fell again when you turned the boost pump off because the fuel (and engine-driven pump) are still hot. They start to vaporize again, the pump cavitates again, and pressure drops as a result. The only way to fix the problem (other than keeping the boost pump on) is the get enough fuel through that line and pump so that the hot fuel is gone downstream, and the pump itself is cool enough not to boil the cool fuel coming into it. Sometimes that takes a while.

On my RV, when running 100LL I don't have any issues. But when I'm running 91-octane automotive pump-gas (not mogas, which means I'm running 10% ethanol) with its higher vapor pressure, if I do a hot-start after refueling and I taxi back to the runway for takeoff after heat-soaking everything, I usually need 3-4 minutes at taxi power settings for the cooler fuel to work its way through the system and the engine to stop "loping" from inconsistent mixture due to a mix of liquid/vapor moving through the injectors. On 100LL it does not do this, but the vapor pressure on the 91-octane pump gas is a good deal higher.