Fuel Pressure Low

[designrs]

Unusually warm Spring days can cause vapor lock and low fuel pressure because the MOGAS stations have not switched to Summer blends yet. Mixing in 30% 100LL usually solves the problem. Question for the mechanic... if fuel pressure drops from the normal 4.5 psi to 2.2 or 1.8 psi intermittently. How serious or threatening is this condition?

[drseti]

If your plane is equipped with an electric fuel boost pump, when the pressure drops you should turn that on and see what happens. If it comes back up to middle of the green, fly to either your destination airport, a fueling alternate, or your home base, whichever is closer, and top off with 100LL. If pressure doesn't come up with the boost pump on, change tanks. If that doesn't help, get the airplane on the ground ASAP!

(Disclaimer: this advice applies to Rotax 912ULS engines only.)

[roger lee]

Hi designrs,

It's actually not vapor lock just vapor pressure within the hose. It hasn't stopped any fuel flow.

This can happen fairly frequently in tight cowled engines and after the engine has been shut down for 30-45 minutes and re-started. It is especially prevalent with fuel lines not in fire sleeve. Fire sleeve helps insulate as well as protect against fire.
Here is what I have noticed over the last 10 years and it happens to me still.
After the engine is re-started and you will notice the low fuel pressure. This is where the cooler fuel will start to replace the hotter fuel, but flow at idle is minimal. Even though the engine is warm, idle it up at 2400+ rpm during the pre-flight check period and or taxi. Usually upon full rpm at take off or shortly there after the fuel pressure returns to normal any way. I have taken off with the lower pressure many times in my CT over 6 years and have never had a hiccup. I have done fuel flow and pressure testing and the engine can easily run on lower than 2 psi pump pressure. The lower fuel pressure is not a direct indication of the fuel flowing through the carb and only what the pressure is. Once you have high high fuel flow and additional air flow for cooling the pressures should come back to normal. Many times just running it at higher rpms on the ground for a few minutes will take care of this all together. You just need to replace some of the warmer fuel with some cooler fuel.

If it makes you feel any better I have flown with 1.2 psi fuel pressure at full throttle without any issues during other testing.

[drseti]

Adding to Roger's informative response: I thought Richard was asking about intermittent fluctuations in fuel pressure, which would of course be a concern. If the pressure is low, but steady, and the engine is warm, do as Roger says.

[dstclair]

Roger knows more than I ever will on Rotax so he may correct me but I was told by the Rotax mechanic at SportAir (Sting, Sirius, iCub, SeaRay distributor) that the Bings will operate normally down to 0.5 psi. Not advocating this as normal operation and that lower-than-normal pressure should be ignored. Personally, I'd become somewhat nervous if the pressure jumped below 3.0.

[designrs]

It's actually not vapor lock just vapor pressure within the hose.... You just need to replace some of the warmer fuel with some cooler fuel.

This makes sense, usually only after being on the ground with a hot motor on a hot day.
Also good to know that the engine will still run at low psi.
Rotax manual acknowledges "vapor lock" on hot Spring days using Winter formulation MOGAS.
Roger's explanation of "vapor pressure" is much more comforting.

Senario #1) land at airport on hot Spring day, sit for a few minutes with the engine idling to flight plan next destination, set radios etc. Taxi to runway. Pump on. Power full, T/O roll, warning "Fuel pressure low" (about 2 psi.) Now what? Abort (most prudent and safest decision)? Continue, because "it always does this", hot fuel in lines, motor can run at low fuel psi, will clear in the air momentarily, etc. (probably an accurate assessment, but jeez, it sure doesn't feel good to do this!)?

Senario #2) Hot Spring day, pattern work, "Fuel pressure low" on takeoff roll. Abort. Taxi. Runup OK (cool fuel in lines now). Fuel pressure fine. Pump on. Fuel pressure fine. Full throttle. Takeoff roll. "Fuel Pressure Low". Now what?

Anybody else finding the need to add about 30% 100LL to their MOGAS in Spring?
Maybe another solution would be to abort takeoff and do a looong runup to get cool fuel in the lines? Or would the fuel heat up so fast just while the plane is on taxi?

[roger lee]

The Bing 64's on a high wing can absolutely run an very low pressure. During may testing I removed the pump and just joined the intake and pressure side of the hoses on my CT and it never stumbled at full throttle. So that was only very little head pressure from the wings. I joined the hoses back to the pump and pulled the pump off the engine and ran it. It did stumble at full throttle, but I could have flown all day at 5000 rpm. Low wings may be a whole new issue for those test, but most low wings have an electric back up pump.

I live in Arizona as do many other LSA friends of mine. 7 at my airport. We all experience the low fuel pressure after the hot engine and re-started after it sat a while. Not a single person here worries and no one that I know of with my AZ. buddies have ever experienced a hiccup because of it. I use only 91 oct. with ethanol since that is what we have and so do all the other guys.

[designrs]

Thanks Roger. Most informative!