Fuel flow reading

[Towner]

My fuel flow on the Dynon d100 usually shows about 5.6 gph between 5100 and 5200 rpm. Over the last 6 months, this has been pretty accurate. 

I just did an oil and plug change, along with a wash of the plane. The following flight, fuel flow was showing about 4.5 gph for the same rpm range. This is not going to be correct for the plane. Any idea where to find the problem?

[Tom Baker]

Maybe cleaning the plane reduced the drag enough to reduce the fuel flow. The fuel flow transducer is a fairly simple device. There is an impeller that rotates in the flow of the fuel. It spins at a certain rate to show on the instrument. So either the actual amount of fuel flowing is lower, or the impeller is not spinning freely. You can test this fairly easy. Shut your fuel valve off, get a quart or larger container that has a measure on the side, turn on your master switch, open the quick drain on the gascolator, open the fuel valve and start a timer at the same time, read the fuel flow on the Dynon, shut off the valve and stop the timer when the fuel gets to a measurable level, calculate the fuel flow based on time and volume, and compare the the Dynon. If they don't match somewhat close you need to probably replace the fuel flow transducer. If the number match, then you need to figure out why the fuel flow is reduced.

[Ed Cesnalis]

RPM is a poor proxy for power or fuel burn.  Both prop pitch and manifold pressure and change resulting power/fuel burn at the same RPM

[Tom Baker]

BTW, I normally expect to see between 18 and 20 gallons per hour on the fuel flow test I described above.

[Towner]

When comparing the fuel flow before and after, it was a similar cruise altitudes, speeds and rpm, so there was definitely something amiss. I've flown the airplane 90 hours in the last six months and I'm familiar enough with the settings while flying the airplane to see it was obviously different from prior flights.

Prop pitch, dynon setting, floats and that kind of stuff shouldn't have changed between 2 flights in one week.  I had also checked the air filter and it was good.

The impeller or a clogged fuel filter would make more sense to me. I certainly appreciate this information. 

Today, fuel burn was back to normal, so not sure what the problem was.

I wish a good wash and wax would have helped my fuel burn. At least it looked pretty! 

[Anticept]

Your fuel flow sensor may be going bad. I replaced several, and increasingly inconsistent readings was the first sign.

[Towner]

Thanks for all the help guys. Was normal for the last 15 hours of flight time, but it was probably giving me a warning. At least now I know where to begin my troubleshooting.