Where is the fuel going?

Maybe you can help me with this riddle. I just completed the annual on my 300 hour 7-A with ECI IO-375 and Precision Silverhawk injection. I installed a fuel pump shroud with blast tube to see if I could improve persistent vapour locking with mogas. (I keep avgas in left tank for TO, landing, and backup to electric boost pump). I damaged the original fuel pump by installing the push rod beside the pump rocker arm instead of on top, and replaced the pump with a new Tempest unit installed correctly. I had an experienced mechanic check firewall forward before flight

The new mechanical pump puts out 35 psi, about 3 psi more than the old one, but well within the specified 20-50 psi required for the servo. My Airflow Performance electric pump continues to work well.(stock Van's setup, except filter is replaced with firewall gascolator to meet Canadian reg's). Since the AFP pump is regulated to 35 psi, there is very little change to indicated fuel pressure when boost pump is switched on or off with engine running. This makes sense to me, since the internal spring in the new fuel pump is holding pressure at 35 psi.

Engine performs normally, but on test flight I noticed that fuel flow gauge indicated 3-4 gph higher with boost pump on than with mechanical pump alone. Fearing a fuel leak, I returned to airport for trouble shooting. A helper looked for leaks with engine running and fuel pump on, no fuel coming from pump overflow line or cowl area, despite the fact that gauge indicated 2gph higher flow with boost pump, although engine did not change its idle tempo or EGT.

By the way, my Dynon fuel flow transducer is located in the cockpit, downstream of the Fuel tank selector, boost pump, and pressure regulator. No way for fuel to loop around it. Fuel pressure transducer is on firewall, "T'd" with flexible hose from the output fitting on mechanical fuel pump.

Removed cowls and found nothing wrong:
-No sign or smell of fuel leaked anywhere
-No fuel in the crankcase
-No fuel in the cockpit
-No leaks visible with boost pump on and mixture in idle cut-off position
-It took system 30 minutes to leak down from 35 to 30 psi after boosting in the idle cut-off position

Buttoned up cowls and test flew again, taking careful note of performance and readings. Take-off and climb performance is normal (great!), with CHT's and EGT's where I expect them. Application of boost pump increases indicated fuel flow 4-5 gph at cruise and climb, 2-3 gph at idle. No change to engine output or EGT in response to boost pump, however the engine does not run as well LOP as it used to. (Engine roughens up just LOP, resulting in having to run too close to peak, and correspondingly higher CHT's. LOP cruise yeilded 340 to 360, when I am used to seeing 320 to 330. Number 4 jug increased the most, which is where I pulled fuel pump blast air from. I don't think the blast air affected CHT, since I also installed my winter oil cooler shutter, which blocks off a big chunk of oil cooler air also coming from number 4 cylinder.

Writing this, I think I will look at the servo finger screen for debris, then run a GAMI test. Maybe I introduced debris downstream of fuel pump while fixing stuff which is contributing to LOP changes.

Any ideas what is causing indicated fuel flow to increase with boost pump application? I considered some electrical interference with the fuel flow transducer, but it always worked well before, and I don't like believing in coincidence. Prior to this work, fuel pressure would increase 3-5 psi, and fuel flow would temporarily increase with application of fuel pressure, but flow would settle down to previous value in a couple seconds
Thanks for any ideas
Jay

Jay,

Since the fuel flow sender is located between the two pumps, what MAY be happening is the fuel flow has an oscillating component when the boost pump is on due to an interaction between the boost pump and the mechanical pump's check valves and diaphram. Meaning the fuel is moving back and forth in addition to the forward movement. This causes the little wheel in the sender to also oscillate causing the LED counter to "see" and therefore count more flashes which are not always representative of full rotations.

My opinion only and worth less than what you've paid for it.

Whatever the cause, this is common. Since the boost pump is not used the majority of the time, the total fuel count is still pretty accurate.

Bevan

Flow Meter Transducer Location

Perhaps if you re-locate the flow meter to the correct location between the fuel servo and the flow divider your incorrect flow meter readings will be correct boost pump on or off.

Don

Don,

I'm currently working on relocating mine. Where exactly would be the best place to locate a Floscan on a lycoming? Orientation, mounting harware, vibration, heat and available space conspire to make this a compromise at best. Have you got some pictures of successful installations?

Bevan

PS Thank you for monitoring this forum.

Email me a [email protected] and I can send you pictures. You running a front sump or up draft sump?

I agree with the oscillation phenomena. I see the same situation in a friends RV6 which I fly often. It has a 2-3 gal per hr increase with the boost pump on. All EGT and CHT remain constant.

jay bell said:

Engine performs normally, but on test flight I noticed that fuel flow gauge indicated 3-4 gph higher with boost pump on than with mechanical pump alone.

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Jay, it's normal for your installation arrangement. Inquiries to the manufacturers (Why does a fuel flow transducer mounted between the electric pump and engine pump register extra flow when the electric pump is running?) have yielded nothing; if they know, they ain't tellin'.

As Don said, if you don't like it, mount the transducer between the servo and the divider. All design is compromise; such an installation may entail extra hose length, or a vertical alignment, or some other issue which may be undesirable. However, it is per the manufacturers instructions.

The counter is, of course, that a transducer between the pumps typically allows a cleaner install, with the unit hard-mounted to the firewall, or living cool inside the cabin with the electric pump.

Frankly, either works. Things like hose routing, clamping, security, and fitting tightness should probably be the serious concerns.

I mounted the FF transducer inside the firewall on the -8 and it shows higher fuel flow with the boost pump on. I mounted the FF transducer directly to the flow divider on the Rocket and the flow is stable in all conditions. Both airplanes register accurate fuel qty at fill up.

Thanks everyone for your comments. I feel confident flying the plane again now that I think I understand what is happening. The new fuel pump's internal spring pressure is 3psi higher than old, changing the interaction it has with the boost pump, and creating oscillations that affect the transducer.

I originally located the flow transducer in the cockpit to reduce fuel heating, and I think that is still valid given I run Mogas in one tank. I will accept the weird flow readings because I would rather have a happy engine than happy gauge, and have been making progress on reducing vapour lock frequency while using Mogas.

By the way, I checked the servo finger filter yesterday and ran three GAMI spread tests at different cruise MP's. Screen was clean, and maximum spread was 0.3 gph.
Jay

jay bell said:

...I originally located the flow transducer in the cockpit to reduce fuel heating, and I think that is still valid given I run Mogas in one tank. I will accept the weird flow readings because I would rather have a happy engine than happy gauge, and have been making progress on reducing vapour lock frequency while using Mogas...

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If your concern is vapor lock with mogas, then you are actually hurting yourself with the FF transducer in the cockpit. This places it on the "suction" side of the engine driven pump and these FF tranducers are just about the most restrictive thing in your whole system. The increased resistance lowers the atmospheric pressure of the fuel ahead of the pump and will actually promote vapor lock.

Indication anomalies are one thing, but if you have vapor lock issues, then you need to get that thing downstream of all pumps.

Toobuilder said:

If your concern is vapor lock with mogas, then you are actually hurting yourself with the FF transducer in the cockpit....
Indication anomalies are one thing, but if you have vapor lock issues, then you need to get that thing downstream of all pumps.

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Darn good point. Jay, if vapor lock has been an issue, it's time to re-plumb.

Michael, that is a good point about the flow transducer restricting the suction side, thanks. If the fuel pump shroud doesn't correct my vapour lock issues I will try re-locating the transducer per Don's recommendation.
Happy flying today!
Jay

DanH said:

Jay, it's normal for your installation arrangement. Inquiries to the manufacturers (Why does a fuel flow transducer mounted between the electric pump and engine pump register extra flow when the electric pump is running?) have yielded nothing; if they know, they ain't tellin'.

Click to expand...

If the flow meter consists of a rotor and a Hall Effect sensor that counts the vane passage, brief pulsations in the flow can cause double counts by the sensor, effectively showing an incorrect higher flow. Vortex type flowmeters can be susceptible to pulsations as well.

In general it is considered important to damp pulsations in the flow before attempting to measure it. Pulsation dampers can be installed on water lines to minimize these effects, but in an aircraft application measuring fuel flow, I would stick to installing the meter in a section of the system where pulsations don't exist in the first place. This would save the weight and reduce system failure modes associated with the pulsation damper.