Fuel flow diagnostics

[Let'sgoflying!]

My fuel flow indication has become intermittent.
Before I look into it, I wanted to make some kind of diagnostic plan.
It reads zero on start up and for the first 10 minutes of flight, then comes online and reads accurately for the rest of the flight.

It’s a JPI transducer:

JPI EDM Pressure Fed Fuel Flow Transducer Floscan 201 For Fuel Pump Aircraft | Aircraft Spruce ®

JPI EDM Pressure Fed Fuel Flow Transducer Floscan 201 For Fuel Pump Aircraft JP Instruments EDM 700 Transducer (Fuel Pump).

www.aircraftspruce.com

Which connects to an “EAU” (Engine and Airdata Unit) which feeds all data to the Chelton (GRT) displays.
There are probably a few connections bettween the transducer & the EAU
I soldered the (new transducer) connections between it and the engine compartment wires when it was installed new at overhaul 150hrs ago and they look solid.
I inspected and cleaned (electronics spray) the multipin cpc connector at the firewall:

CPC Connector Kits | Aircraft Spruce ®

CPC Connector Kits The new Sealed CPC using UL F1-rated resin is ideal for UV resistance in outdoor applications.

www.aircraftspruce.com

 

[AV8R_87]

Is the non-operation time (10 minutes) pretty consistent? The way they work is that a little turbine gets turned by the fuel flow, and a magnet on the shaft triggers a Hall effect sensor that generates pulses. Sensor gets fed power (I recall 5 volts) and ground and outputs a square wave proportional to the fuel flow.

My first thought is that the turbine is sticking and as it warms up it unsticks and starts operating. What's not convincing me on that is that you're stating the fuel flow is correct once it starts working. I'd expect some level of under-indication.

There's also a chance that you have a bad connection somewhere that fixes itself as things warm up.

Troubleshooting-wise, what kind of tools do you have available?
It might be best to disconnect the fuel line and use the electric pump to flow fuel as you're troubleshooting.
I recall the JPI sensor puts out 29 thousand pulses per gallon of fuel, so at your typical fuel flow you're looking at 60-ish pulses per second, and about 5 times less at idle. You might be able to use a good digital multimeter to see the pulses at idle. You'd need one that has a bar graph below the digits. An oscilloscope would be the best troubleshooting tool.
A bit of light percussive maintenance should help you troubleshoot any loose connections. The JPI sensors can be a bit unreliable as well.

 

[Stewartb]

My JPI FS450 registers instantantly.

My bet is a bad pin on one of your connectors.

 

[Albany Tom]

I kinda hate intermittent electronic problems. But at the same time sometimes the challenge is good.

If what you say is repeatable, I'd try to isolate the problem a bit. I'd power up the electronics and the fuel pump, if you safely can, for 15 minutes before starting. If it works right away, then you might have a thermal intermittent in either the transducer or receiver...unlikely to be wiring related. If it still takes 10 minutes, then try those 10 minutes on the ground. If it fails on the ground, it might be something mechanical, either the sensor or the wiring responding to vibration. If you fly it and it fails faster, it might mean fuel flow related, or mechanical. And I realize as I write this, it's not going to give you an absolute answer, just places to look.

If I had to guess it, like Stewart, I'd look at the connector first. But slightly different, I'd guess an intermittent short between the signal pin and ground. I like those little circular connectors, but they're a PITA to solder, and it's easy to have one strand floating around wanting to touch something, sometimes. A short between signal and ground wouldn't damage anything with that setup, but it would disable signaling. A short between power and ground is more likely to cause damage, and a short between ground and well, ground, is probably mostly or completely harmless.