Just tried MOGAS, didn't go well

[Elwell81]

After getting the first 90 hours on my -9A, I decided to tried the 91 Octane autogas that is sold at my home airport. Being overly cautious, I completely drained the left tank of 100LL and filled it with autogas. On the next flight, I took-off on the right tank (100LL) and once leveled off and with everything stabilized, I switched over to the left tank. About 30-45 seconds later, I felt a noticeable loss of power and the engine began surging from cruise power to lower power. I had the engine page up on the PFD during the tank switch just in case and the fuel pressure was dropping, accompanying the power surge. My IO-320's fuel pressure usually runs at a steady 18 psi, but it was dropping to 8-9 psi for a second, then back up to normal, then down again (again with a power loss).

Flipping on the boost pump (Andair) instantly brought the fuel pressure up to 29-30 psi and the engine ran normally. Turning it off, same symptoms returned. Going back to the right tank, after the 91 octane got run out of the lines, fuel pressure stayed at 18 psi like normal so it would seem the 91 octane was the cause? To make sure it wasn't some problem I induced while draining the tank, I ran on the left (91 octane) tank with the boost pump on, way ROP to burn as much as I could on this leg so I could refill with 100LL.

Upon refilling tank with 100LL, I had approx. 1/4 tank of 91 left so it was much more 100LL than 91 octane. On the next leg, I performed the same test. The fuel pressure still fluctuated, but nowhere near as much. I've since refilled 2 more time so it should be back to nothing but 100LL. Now the fuel pressure is running 18 psi on both tanks, just like before this experiment.

I'm just wondering what is going on with this? I've read countless posts about people running auto gas with no problems at all. What could be causing a problem specific to this plane?

[mburch]

Originally Posted by Elwell81

Being overly cautious, I completely drained the left tank of 100LL and filled it with autogas. On the next flight, I took-off on the right tank (100LL) and once leveled off and with everything stabilized, I switched over to the left tank.

^ Excellent risk mitigation in evidence here.

[Fred.Stucklen]

Originally Posted by Elwell81

After getting the first 90 hours on my -9A, I decided to tried the 91 Octane autogas that is sold at my home airport. Being overly cautious, I completely drained the left tank of 100LL and filled it with autogas. On the next flight, I took-off on the right tank (100LL) and once leveled off and with everything stabilized, I switched over to the left tank. About 30-45 seconds later, I felt a noticeable loss of power and the engine began surging from cruise power to lower power. I had the engine page up on the PFD during the tank switch just in case and the fuel pressure was dropping, accompanying the power surge. My IO-320's fuel pressure usually runs at a steady 18 psi, but it was dropping to 8-9 psi for a second, then back up to normal, then down again (again with a power loss).

Flipping on the boost pump (Andair) instantly brought the fuel pressure up to 29-30 psi and the engine ran normally. Turning it off, same symptoms returned. Going back to the right tank, after the 91 octane got run out of the lines, fuel pressure stayed at 18 psi like normal so it would seem the 91 octane was the cause? To make sure it wasn't some problem I induced while draining the tank, I ran on the left (91 octane) tank with the boost pump on, way ROP to burn as much as I could on this leg so I could refill with 100LL.

Upon refilling tank with 100LL, I had approx. 1/4 tank of 91 left so it was much more 100LL than 91 octane. On the next leg, I performed the same test. The fuel pressure still fluctuated, but nowhere near as much. I've since refilled 2 more time so it should be back to nothing but 100LL. Now the fuel pressure is running 18 psi on both tanks, just like before this experiment.

I'm just wondering what is going on with this? I've read countless posts about people running auto gas with no problems at all. What could be causing a problem specific to this plane?

Some things to look for:

1. Do not use 90* fittings on the INPUT side to the engine mounted fuel pump. If the fuel flow turns suddenly, you will get cavitation (air bubbles) which cause loss of mechanical fuel pump pressure. Same issue with the output of a firewall mounted gascolator, no 90* fittings. All fuel line runs inside the engine compartment hot area should only have gradual turns (i.e., radiused runs). This is not an issue of fuel line runs between the mechanical fuel pump and the fuel body.

2. Consider a cooling showd over the mechanical fuel pump with cooling air.

3. Be sure that you DO NOT have anything in the fuel lines between both tanks and the mechanical fuel pump that causes any excessive fuel pressure drop. Sources of these pressure drops include paper fuel filters, too small or restrictive fuel lines (for the required fuel flow), and in some cases, the AUX fuel pump itself. The more suction that the mechanical pump has to pull to get the required fuel flow, the more the possibility of fuel vaporization in that line. You stated that turning ON the AUX pump appeared to solve the issue. This leads me to believe you have something in the fuel line between that pump and the mechanical engine mounted pump that represents a pressure drop.

I've been running 93 OCT E10 autogas with no issues for over three years....

[dweyant]

Originally Posted by Fred.Stucklen

Some things to look for:

1. Do not use 90* fittings on the INPUT side to the engine mounted fuel pump. If the fuel flow turns suddenly, you will get cavitation (air bubbles) which cause loss of mechanical fuel pump pressure. Same issue with the output of a firewall mounted gascolator, no 90* fittings. All fuel line runs inside the engine compartment hot area should only have gradual turns (i.e., radiused runs). This is not an issue of fuel line runs between the mechanical fuel pump and the fuel body.

How can a 90 turn cause air bubbles? Are they somehow coming out of solution?

-Dan

[rolivi]

I'm ready to learn on this one...

The specific gravity of AVGAS is right in the middle of the 100LL specific gravity range.

Why would any pump behave differently with a fluid of same density?

[aerhed]

Isn't 18psi low to begin with? Isn't 23-27 most normal?

[WA85]

I use 93 E10 in my IO-320 (AFM FM 100 system) about 70% of the time and there are several known issues with using auto fuel in fuel injected engines on acft that have to be taken into account before using it. Auto fuel typically will "boil" at lower temperature and pressures that 100LL, meaning that instead of running through the fuel lines as a liquid, it may actually begin to boil and produce bubbles fuel, which causes fuel pressure fluctuations, extreme cases vapor lock. At ambient temps about 90 F, I have to run my aux pump continuously on the ground to avoid pressure fluctuations and a very rough idle. Once in the air and things have cooled off, I typically shut off the aux pump with no pressure fluctuations. I see 23-25 psi from my mech fuel pump and 28 to 35 psi with the aux pump on, so 18 psi seems a bit low, which may be part of the issue. In order to avoid boiling the fuel, it needs to be pressurized and kept cool. Suggest looking at the fuel lines from the tank to the pump to ensure that there are no leaks, which in a suction type system will draw air in, causing aeration. Also, for a fuel injected engine, I would definitely look at why you have 18 psi instead of something closer to 23 to 25 psi or whatever the Mfg requires.

[airguy]

The pump is not behaving differently - the vapor pressure of the fuel is changing. 100LL has a lower vapor pressure than mogas, meaning that mogas will boil easier. Boiling occurs not only at elevated temperature, but also at reduced pressure. If you have restrictions of ANY kind in your fuel line, that will create a region of lowered pressure as the fuel flows through that restriction. If the fuel is warm enough, it can create its own vapor bubbles in the fuel, and the fuel pump does not do well at all with vapor. The fuel pump itself is bolted to that great big heating element up front, and the fuel will pick up heat in the pump itself as well, making the problem worse.

Keeping your fuel line routing as clean as possible with cooling air on your fuel pump will help, but it's going to be trial and error to eliminate the cause. You'll notice a difference between summer and winter fuel blends also, the winter blend will be more likely to cause the problem you are seeing.

[Elwell81]

Originally Posted by Fred.Stucklen

Some things to look for:

1. Do not use 90* fittings on the INPUT side to the engine mounted fuel pump. If the fuel flow turns suddenly, you will get cavitation (air bubbles) which cause loss of mechanical fuel pump pressure. Same issue with the output of a firewall mounted gascolator, no 90* fittings. All fuel line runs inside the engine compartment hot area should only have gradual turns (i.e., radiused runs). This is not an issue of fuel line runs between the mechanical fuel pump and the fuel body.

2. Consider a cooling showd over the mechanical fuel pump with cooling air.

3. Be sure that you DO NOT have anything in the fuel lines between both tanks and the mechanical fuel pump that causes any excessive fuel pressure drop. Sources of these pressure drops include paper fuel filters, too small or restrictive fuel lines (for the required fuel flow), and in some cases, the AUX fuel pump itself. The more suction that the mechanical pump has to pull to get the required fuel flow, the more the possibility of fuel vaporization in that line. You stated that turning ON the AUX pump appeared to solve the issue. This leads me to believe you have something in the fuel line between that pump and the mechanical engine mounted pump that represents a pressure drop.

I've been running 93 OCT E10 autogas with no issues for over three years....

There are no 90 degree fittings in the engine compartment. There is only a about a 2' run from the AUX pump to the mechanical pump with nothing in between.

I understand the fact that excess restriction can cause issues and ran my fuel lines accordingly. The only thing different in my fuel system is an Andair gascolator (for easily servicable filter) between the tank selector valve and Aux. pump.

Temperature also seems like it wouldn't be an issue because OAT was 25 degrees. Running on 100LL this summer with 90 plus degrees OAT, I never had any hint of an issue.

Is the difference between auto gas and 100LL that substantial, that I have no issues 100LL but can hardly pump 91 octane?

[BobTurner]

If your auto gas was bought locally in Green Bay, you probably got winter gas. Auto gas is seasonally adjusted to make it easier to vaporize in cold winter conditions - just as you experienced.