Fuel Flow Too High When Boost Pump On??

[RV9Otter]

I have the new style Airflow Performance fuel pump in my RV9A with an IO320, and the GRT Engine Information System and the GRT IFIS. The aircraft has just over 60 hours on it and a new issues has popped up. On climb out, full throttle, full mixture, boost pump on, on the last two flights, the "Fuel Flow Too High" warning lit up on the EFIS. I think the high limit on the fuel flow is set at 20 GPH and the fuel flow was indicating about 21 GPH. When I shut the fuel boost pump off, the fuel flow returned to normal. In cruise, or at any power setting for that matter, whenever I turn on the fuel boost pump, I get a rise of 5 to 6 GPH on the fuel flow readout.

To make matters more confusing, I also just noticed that I'm getting a Zero readout on my fuel pressure reading from the EIS through the EFIS. I don't think the two issues are related but I'm always suspicious when two things appear to happen at the same time.

I know that I'm getting fuel pressure as the fuel injected engine is running normally, both with the boost pump off or on.

I checked all of the fittings today thoroughly for a leak and there are no leaks.

Right now, I'm leaning towards two separate problems that just happened at the same time. The first is the fuel pressure sensor is faulty. The second is that there is something wrong with the boost pump in that it is sending too much fuel through when turned on.

Any ideas would be appreciated.

Thanks,

Curt

[Mark Albery]

Depending on the type and positioning of your transducer relative to the pump and the amount of straight pipework in and out, the sensor can be vulnerable to the pulsing and turbulent nature of the fuel flow with the pump on.

I've heard that by tee-ing in a vertical length of pipe, closed at the end, and positioned downstream of the pump, you can dampen out the pressure fluctuations and get a more reliable measurement. Alternatively re-position the transducer where it is less subject to pressure fluctuation.

I think the Floscan transducer is particularly susceptible to this kind of error.

There have been several posts on this subject in the past.

[RV9Otter]

Thanks for the feedback. I've talked with a couple of other people more knowledgeable than me and they both think that the fuel boost pump is working normally, best indication being that the engine is operating normally when it is on or off. They both suggested that since I have verified that there is no leak in the fuel system, that I should simply set the maximum fuel flow limit higher in the EIS and EFIS. I'm going to try that and keep an eye on things unless someone has any thoughts that the high fuel flow indications with the fuel boost pump on are indicative of something being wrong.

[DanH]

Move the fuel flow transducer to a point in the -4 line between the fuel controller (aka the servo, or throttle body) and the flow divider. The jump in fuel flow when you turn on the boost pump should disappear.