Fuel flow numbers are irregular.

[Paul K]

I noticed a really high fuel flow rate during a takeoff yesterday, it lasted for about a minute and then went back to normal. Some background:

RV7a
IO360
Cold air induction
WW RV200 prop
Red cube transducer
WOT
Full Mixture
Prop all the way in, 2650 rpm
Air temp 35f
Winds calm
Full tanks, 100LL
With passenger
GRT Horizon and EIS but didn't have the record function running.

These conditions usually show a max fuel flow rate of about 17gph. For this launch, the GRT warning went off and I saw 27gph! As soon as we were at pattern altitude and on downwind, I retarded the throttle, prop, and mixture and all went normal. Boost pump on/off, no difference. Obviously thinking leak downstream of the transducer, I landed and checked everything. No fuel anyplace and nothing out of order! Wires good, plumbing good, everything!

So, I climbed back in, sans passenger, and took it around the pattern a few times including a couple touch-N-go's. Everything normal!

After thinking about it, I continued to test it in the air (after landing again and looking for anything our of place) doing about everything I could think of, pump on, pump off, full lean, full mixture, dial prop in, dial prop out, WOT, or not, pulled several 3G turns, steep climbs as if taking off, and every combination of everything I could think of, and couldn't get it to happen again.

The question is, what caused the very high flow indications and is this a messenger that something is failing?

[Russ McCutcheon]

Originally Posted by Paul K

I noticed a really high fuel flow rate during a takeoff yesterday, it lasted for about a minute and then went back to normal. Some background:

RV7a
IO360
Cold air induction
WW RV200 prop
Red cube transducer
WOT
Full Mixture
Prop all the way in, 2650 rpm
Air temp 35f
Winds calm
Full tanks, 100LL
With passenger
GRT Horizon and EIS but didn't have the record function running.

These conditions usually show a max fuel flow rate of about 17gph. For this launch, the GRT warning went off and I saw 27gph! As soon as we were at pattern altitude and on downwind, I retarded the throttle, prop, and mixture and all went normal. Boost pump on/off, no difference. Obviously thinking leak downstream of the transducer, I landed and checked everything. No fuel anyplace and nothing out of order! Wires good, plumbing good, everything!

So, I climbed back in, sans passenger, and took it around the pattern a few times including a couple touch-N-go's. Everything normal!

After thinking about it, I continued to test it in the air (after landing again and looking for anything our of place) doing about everything I could think of, pump on, pump off, full lean, full mixture, dial prop in, dial prop out, WOT, or not, pulled several 3G turns, steep climbs as if taking off, and every combination of everything I could think of, and couldn't get it to happen again.

The question is, what caused the very high flow indications and is this a messenger that something is failing?

I have seen this after switching from an empty tank,the air in the line buzzes the impeller in the transducer faster as it's being forced through the fuel system. Is there anything you might have done during your taxi/ run up that might have introduced air into the system, switching tanks to one that had been dry and refueled spence your last flight? Maybe a suction leak between your pump and tank? However it might happen, air will cause this.

[Paul K]

Russ,

Thanks for the suggestions.

I had filled both tanks the day before but they were not empty before filling, just topped them off. I normally switch to the fullest tank before tax/runup, but not this time. I never switch tanks in the pattern, always being on the fullest before I get to the airport environment.

My high reading lasted close to a minute (guess as time in the pattern is easy to loose track of). If I had a bubble or slug of air in the line, wouldn't it have passed through the transponder relatively quick? If I have a leek that is sucking air, wouldn't it have persisted? Also, I couldn't replicate it both boost pump on or off.

I do think this calls for a deeper inspection of the fuel system from the tanks all the way to the heads.

[Russ McCutcheon]

Originally Posted by Paul K

Russ,

Thanks for the suggestions.

I had filled both tanks the day before but they were not empty before filling, just topped them off. I normally switch to the fullest tank before tax/runup, but not this time. I never switch tanks in the pattern, always being on the fullest before I get to the airport environment.

My high reading lasted close to a minute (guess as time in the pattern is easy to loose track of). If I had a bubble or slug of air in the line, wouldn't it have passed through the transponder relatively quick? If I have a leek that is sucking air, wouldn't it have persisted? Also, I couldn't replicate it both boost pump on or off.

I do think this calls for a deeper inspection of the fuel system from the tanks all the way to the heads.

Running a tank empty and switching tanks in flight has caused high readings for several minutes after the engine regains normal running, so I would not be surprised if a slug of air could cause hi readings for a minute or more. Don?t know what your cause is, just spit balling so to speak.

[schristo@mac.com]

The red cube signal is driven by a small impeller calibrated near 68,000 pulses per gallon... any vibration at the signal wire connection that is not perfectly mated gets picked up as a pulse. Newer red cubes from EI come with speciality high vibration connectors that work great. I spent a bit of time chasing down a similar problem and resolved it using D-sub pins and heat shrink. My red cube failed after 600+ hours and my new one came with the new connectors.

[Paul K]

All good to know!

Air bubble theory bothers me in that an air sucking leak could also be a gas spraying leak

My red cube is about three years old and I can't remember what the connections look like. I'll check them also.

Thanks for the excellent suggestions. Thanks Doug for providing such a great resource for just pennies a day! Happy to be a contributing member!

[RV10inOz]

I would go spattering all your fuel line connections. And being colder things shrink, and it takes very little to suck in air on a fuel line, fuel tap? Is it an old Vans type valve?

Lots of places to look.

[Paul K]

RV10inOz,

Haven't thought of the temp/cold shrinkage! It was fairly mild, 35f when this happened and I had been flying just a few days earlier when it was 12f and no problems then. However I can understand adding vibration into the equation along with shrinkage and stranger things could happen!

Thanks