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Fuel Flow Fluctuation

db1yg

db1yg

Well Known Member
Fuel Flow fluctuations

Need some help on where to start trouble shooting a fuel flow issue.

My fuel flow (red cube) was showing some unusual flow rates in my morning flight today. This is an IO340 Continental Titan engine (8:1 compression) with the fuel pressure pick up at the mechanical pump outlet and the fuel flow transducer just prior to the flow divider connected via a 90 degree fitting using the Aircraft Specialty mount. This set up has provided 130 trouble free hours to date.

I had leaned to my normal 25 to 40 degrees lop and after a few minutes the flow indication went from 7.2 gph (which is normal) to 5 gph and then fluctuated +-1-2 gph. Normally I see a steady reading with maybe a 0.2 gph fluctuation. Fuel pressure was steady as was egt and engine performance. The OAT was unusually hot at my cruising altitude at 75 degrees and the cht’s in climb had to be managed (#4 hit 390 prior to level off) so there was considerable heat in the cowl. Additionally I am running a 50% blend of 100LL with alcohol free 87 octane pump gas.

My thoughts on what could be the cause:

1. Fuel flow transducer is failing or I have a ground issue?

2. The unusually hot temperatures along with the 50% pump gas was releasing vapor at the 90 degree outlet from the fuel flow transducer.

Since I have never had this issue prior and the temperatures were unusually high today I lean toward cause #2. The problem with this conclusion is that the fuel pressure was not fluctuating, egt’s were steady, and performance was not affected.

My current plan is to fill with 100 LL and see if the issue goes away.

Comments—thoughts?

Cheers,
db
 
If that doesn’t fix it please post a picture of your FT-60 unit as it’s mounted, there could be answer there depending on what I see. I just replaced another FT (different aircraft to mine) as it ‘failed’’, due to a mounting issue.
 
Consider Hot fuel as a poosible issue. The hot fuel was starting to create vapor which effected the FF indication due to vapor in the red cube. You were managing your engine and it was hotter during the climb. The higher fuel flow during climb was moving faster thru the lines. When you reached top of climb fuel in the lines and thru the pump slowed down had time to pick engine/cowl heat. The pump transmits heat to fuel. The -6 line going to the servo from the pump is bigger and so fuel moves thru it slower.

I've been tracking mine. I found 3 meter long Themocouples on Amazon for about 3$ a piece and the box to read them for $30. I fixed the thermocouple tips on various fuel line fittings to see there temps. When it hit 130f at the servo inlet I saw the same thing you saw but I had 100% avgas. So yeah, the gas choice is good gouge. WHen it gets hotter outside it might be a problem with avgas too. Vetterman cross over exhaust heats up the cowl. Keeping mine cool is a fight. Its not an RV but a Bearhawk Patrol.
 
87 octane is below recommended. Understand you have 100 mixed with it. Up you mogas to 91 or greater, no alcohol might be a worthwhile experiment. I run straight 91/93 alcohol free unleaded never a issue. Good luck
 
My money is on #2. I see a lot of this every June, July, August, September, along with engine hiccups due to fuel vaporization. Turning the boost pump on should stabilize the reading. Also consider running straight 100LL during the warmer months, at least on one tank.
 
As others expressed, your plan to use 100LL will be a good test. Those insulated fireproof sleeves (red) help keep fuel temperature down
 
Thanks for the suggestions/info guys! What was confusing me about this being a vapor issue was that the fuel pressure was not fluctuating--which is usually the case with vapor release. However, since the fuel pressure is taken just out of the mechanical pump and the fuel flow is measured after the servo just prior to the divider could this line distance "buffer" the pressure sensor from vapor release downstream???? Wish I had thought to turn on the boost pump. Also, all fuel lines close to heat sources are double wrapped with fire sleeve. I will fly again with a much higher % 100 LL next week and see if the situation repeats. BTW, reference octane levels, the engine is an 8:1 compression and should handle a 50/50 mix of alcohol free 87 octane and 100 LL just fine--but not vapor release.

Cheers,
db
 
Last edited:
db1yg said:
Thanks for the suggestions/info guys! What was confusing me about this being a vapor issue was that the fuel pressure was not fluctuating--which is usually the case with vapor release. However, since the fuel pressure is taken just out of the mechanical pump and the fuel flow is measured after the servo just prior to the divider could this line distance "buffer" the pressure sensor from vapor release downstream???? Wish I had thought to turn on the boost pump. Also, all fuel lines close to heat sources are double wrapped with fire sleeve. I will fly again with a much higher % 100 LL next week and see if the situation repeats. BTW, reference octane levels, the engine is an 8:1 compression and should handle a 50/50 mix of alcohol free 87 octane and 100 LL just fine--but not vapor release.

Cheers,
db
Click to expand...
in my experience, vapor does not impact fuel flow with a red cube. fluctuating pressure is pretty common. i have experienced FP fluctuations from what i speculated was from vapor, but never FF. The red cubes don't seem to last all that long, so my first guess would be the cube starting to go. Just one mans experience.

The paddle wheel style of the cube lets it pass air in the same way as liquid, therefore very tolerant of vapor.
 
I suspected vapor does not turn the Red Cube's paddle wheel like a liquid and caused the irratic indication. But that was just my hunch. I could be wrong, that is for sure. I also have no pressure data in that line from the servo to the flow devider and suspect that is standard installation. The fuel pressure is read in the line before the servo while the red cube is after the servo before the flow divider. The fuel fitting at the serve exit was 130F when my red cube flow variance was unsatisfactory.
 
Bcone-my thinking was similar to yours--ie, vapor release at the 90 degree fitting out of the red cube was slowing the red cube sensor wheel and therefore indicating a lower ff. We know the pressure in my line from the mechanical pump to servo is 27-28 psi because that is where the pressure sensor is, and the pressure from the divider to the injectors is 2-3 psi limited by a spring in the divider, but what is the pressure from the servo to the divider??? If it is also 27-28 psi this reduces the chance of vapor release--however, if it is low like the divider to injector then the chance of vapor release at a hot 90 degree fitting is probably much higher??!!

So much to learn--so little time.

Cheers,
db
 
db1yg said:
Bcone-my thinking was similar to yours--ie, vapor release at the 90 degree fitting out of the red cube was slowing the red cube sensor wheel and therefore indicating a lower ff. We know the pressure in my line from the mechanical pump to servo is 27-28 psi because that is where the pressure sensor is, and the pressure from the divider to the injectors is 2-3 psi limited by a spring in the divider, but what is the pressure from the servo to the divider??? If it is also 27-28 psi this reduces the chance of vapor release--however, if it is low like the divider to injector then the chance of vapor release at a hot 90 degree fitting is probably much higher??!!

So much to learn--so little time.

Cheers,
db
Click to expand...
I understand that the Servo's input is high pump pressure, and the output to the servo is a low regulated pressure. The variation in airflow across the servo's venturi (which the operator controls with throttle) PLUS a mechanical input from the mixture control effects in changes to an orfice to regulate lowish fuel pressure to the servo. When I flew these type of aircraft In the 1980's the fuel flow indication was displayed from the pressure signal at the flow devider.
 
Your idea of mixing avgas 50/50 with ethanol free sounds good to me. I need 100 octane and tried mixing avgas with premium auto gas, engine ran great but even that 5% ethanol content made me uncomfortable. Using 100% avgas now. Maybe collecting butterflies would have been a cheaper hobby 😂
 
Bcone1381 said:
I suspected vapor does not turn the Red Cube's paddle wheel like a liquid and caused the irratic indication. But that was just my hunch. I could be wrong, that is for sure. I also have no pressure data in that line from the servo to the flow devider and suspect that is standard installation. The fuel pressure is read in the line before the servo while the red cube is after the servo before the flow divider. The fuel fitting at the serve exit was 130F when my red cube flow variance was unsatisfactory.
Click to expand...
The most common place for vapor to form is in the lines upstream from the mechanical pump, due to the lower boiling point created by the lower pressure. These air bubbles get sucked into the diaphragm pump and the air compresses more easily than the liquid. This then reduces the pressure on the output side, where your sensor is, and creates the observed fp reduction. The bubbles come and go, so these fluctuations are often intermittent. It would be rare to have no vapor impact in the fp, assuming boost pump is off, yet have it on the downstream side with its positive pressure.
 
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