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Fuel Flow Jumping up and down

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And the problems with the new installation continue....

Mooney M20K 231.  Merlyn Wastegate and intercooler.  I have a G500 TXi with the Garmin engine monitor and Garmin Fuel flow transducer.  On take off yesterday I was showing 24.5 GPH on the fuel flow and it was flashing red at me indicating too much fuel flow.  I believe its set to 24 as red line.  I realized i had mistakenly set 38" MP and with the intercooler I usually use 36-37".  Even after bringing it back to 35.5" I was still getting the red line fuel flow in the climb out.  It dropped down to 22.5 GPH, then back up to redline.  It did this up and down thing quickly many times in the climb out.  Everything seemed to be running smoothly with no other issues.  CHT and EGT were steady.  Leveled off in cruise, same thing.  13 GPH steady for a bit.  Then up to 15.5 GPH and back down.  These are quick jumps up and down.  It would settle down and stay at 13 GPH for a while without anything so it seems intermittent.

I had a Shadin fluel flow totalizer and transducer that was removed and in the 3 years I used that I had no issues.  This new one was mounted in a different position as the avionics guy said the last one wasn't done right and it had to be horizontal and new fuel lines in and out were run to it.  

I figured I would try here before flying her to the shop.  Anyone have any ideas of things I can check before I do that.  Loose wires?  Leaks?  



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Avionics guy is correct, it should be mounted horizontally.

I would fill tanks, fly for a couple of hours, refill tanks and compare actual usage with computed usage. JPI allows you to calibrate the FF, don’t know Garmin.

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Let your tanks set for an hour after filling to make sure all absorbed air is purged just for a baseline. It sounds as if there is some turbulent flow, some air aspirating, or a poor connection.

I had a boat a while back that was erratic on fuel flow and I found a small pinhole in the suction side of a fuel line leading to FF transducer that was sucking air.



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0) Could be instrument related... could be a real problem...

1) Pics are required... for proper discussion of FF sensor installation challenges...

  • horizontal... Or slight incline for better bubble handling....
  • no nearby elbows... in or out...
  • not next to hot or vibrating things...

2) Bubbles... either air leaking in or vapor related... if you are getting higher numbers than expected... bubbles are often the challenge... fuel injectors systems have a way to handle bubbles once started...

3) vapor is usually a challenge related to hot starts... if you suspect vapor to be a challenge, sometimes running the fuel pump adds enough pressure to help move the vapor through the system...

4) New plumbing... nothing makes you feel like a leak is occurring like High FF with a new sensor...

The K has one of the most complex fuel systems of any Mooney.... lots of places available to have a leak...

Start with the new plumbing...

Usually when a fuel line has bubbles drawn in during operation.... blue stains occur while it is not operating...

PP thoughts only, not a mechanic...

Best regards,


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