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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?  

 

Thanks

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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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Summarish...


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,

-a-

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  • 3 weeks later...

I may have discovered the problem and figured I would post some pictures and thoughts with the notion that it could possibly help someone else down the road. 

I'm still a bit puzzled. At first I didn't notice any change in EGT. But last flight I did during cruise the flow would go from 13 up to about 13.6 and back down. This would happen reasonably steadily and the EGTs would go down steadily and back up as the FF came back down. EGTs would lag slightly. Made sense. So the engine is getting more fuel right? Well... in take off it would jump up to and exceed 30GPH on occasion but would normally vary between 22.5 and 25 or so on the last couple flights. It would never run rough so I find it hard to believe the engine is actually getting 30 GPH. 

It looks like fuel was leaking around the wire connector coming out of the transducer. Not sure how or why because its brand new. The blue pasty stuff seems to be die from 100LL.  I tried dissolving it in fuel to be sure. I can only assume that while the unit is under pressure, fuel will seep out near the transducer showing a higher fuel flow.  I can only guess that under cruise conditions the engine gets FF steady and the when a leak developes around the wire then it gets less fuel and the ETGs change accordingly. Then the leak goes away and the engine then gets the extra fuel and EGTs change again. Rinse and repeat.

My hangar elf cleaned and sealed it with a tiny ring of epoxy. I will report back with the results.

20200816_092337.jpg

20200816_092028.jpg

20200816_092442.jpg

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The epoxy trick might be good for a test, but if fuel is getting out around the wire, there must be an internal leak in the transducer. A fuel leak in the engine compartment can be a serious risk especially in turbos where there are lots of hot parts.

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Your fuel flow transducer should be entirely wrapped in firesleeve - right? Otherwise it could can the symptoms you are experiencing.

Regardless as to whether or not you need a new one if its leaking.

Edited by kortopates
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The lines are wrapped. The unit itself is as pictured above. Thanks for the advice on the replacement. Its probably wise. Hopefully they take it back even though there is epoxy on it. Ill talk to EI if thats who made it and see if my mechanic can do the switch. Unfortunately the avionics guy is not in the picture. 

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