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  #1  
Old 07-27-2017, 10:04 PM
DaleB's Avatar
DaleB DaleB is offline
 
Join Date: Sep 2012
Location: Omaha, NE (KMLE)
Posts: 1,729
Default Aggravating fuel flow alarm

It seems that any time I get over 7500 MSL, I get a "FUEL FLOW HIGH" alarm. It's particularly annoying because it comes and goes, meaning I silence it and a few seconds later it's back again. Over and over, during climb and level cruise, with the D180 erratically indicating anywhere from 8 or 9 to well over 10 GPH fuel flow. Obviously there is not really 10-plus GPH flow. No gas smell in the cockpit, no evidence of overflow, no flames or explosions.

It happens with the unleaded, ethanol-free MOGAS that I normally run. It happens with Swift 94UL. Today it happened after I'd just added 15 gallons of 100LL to the last remaining 5 gallons of 94UL. It's happened in winter with OAT well below freezing, and today with OAT at 8500 MSL of around 10 C. The only constant seems to be, 7500 MSL or higher. I haven't tried drastically reducing throttle, but I'm typically running 5200 - 5400 RPM at most.

If I pull the fuel pump fuse, the alarm immediately stops, fuel flow indications return to a normal value of 5.5-6.5 GPH, depending on throttle, and all's well. When I put the fuse back in, it may go back to the constant bouncing alarm -- or on rare occasions not, although the indicated fuel flow always goes up a little (no change in EGT or CHT, just higher fuel flow indication). If I descend to 6500, all is well with no alarms. Fuel pressure stays within the green range at all times... on the low end with no boost pump, on the higher end with it on but always in the green. CHT and EGT don't vary with the fuse in or out.

My indicated fuel use from the D180 is reasonably close to actual usage after most flights. The "k" factor could use a little tweaking for better accuracy, but it's not THAT far off. And if I stay below 7500 MSL, I never see the fuel flow alarm.

Ideas, folks? I'm just about out of them. Have not run this past Van's yet, but will in a week or so if a root cause and/or solution does not become apparent.
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Omaha, NE
RV-12 # 222 N980KM "Screamin' Canary" (bought flying)
Fisher Celebrity (under construction)
Previous RV-7 project (sold)
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  #2  
Old 07-28-2017, 04:24 AM
BigJohn BigJohn is offline
 
Join Date: Nov 2008
Location: Gloversville, NY
Posts: 1,470
Default

Dale,

This is a very old issue. It first surfaced about 2012 I believe. First time it happened to me nearly scared my wife to death! I worked the problem with both Vans and Dynon. Various theories were developed for the false high flow indications at higher altitudes but nothing was ever pinned down. There is a simple solution. Install a switch in the electric fuel pump circuit and turn it off after takeoff, just as if you were flying a Piper. Add an item to pre-landing checklist to turn it on.

John

Here is just one of the earlier threads on topic: http://www.vansairforce.com/communit...ight=Fuel+flow

There are several others if you search.
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Last edited by BigJohn : 07-28-2017 at 04:38 AM. Reason: Added links
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  #3  
Old 07-28-2017, 09:00 AM
Sunriver Ken's Avatar
Sunriver Ken Sunriver Ken is offline
 
Join Date: Jul 2010
Location: Sunriver, Oregon/Surprise, Arizona
Posts: 125
Default Another Option

I had the same problem only with a low fuel pressure warning. Several suggestions were received (including from Van's) to simply reset the value that the warning comes on. That means change the limits of your yellow areas.

As long as you are not seeing actual values that indicate high fuel flow then why not? The switch idea is good but I don't like messing with non-standard wiring if it is not necessary. That said, I also do not like pulling the electric fuel CB on the ground when working on the Dynon.
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  #4  
Old 07-28-2017, 10:39 AM
DaleB's Avatar
DaleB DaleB is offline
 
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Location: Omaha, NE (KMLE)
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Default

I already did adjust the fuel flow value a little... as I think about this, I seem to recall it happening once before, but not nearly as bad as it does now. I think I bumped it up to 10 GPH from whatever it was before, I'd have to go back and see if/what was logged at the time.

The builder had put a pull type CB right next to the fuel valve lever.. close enough, I fact, that you needed to pull the knob up about an inch to turn it off. I removed that during the most recent condition inspection. The electrical diagram shows a fuel pump switch on the switch panel; I may see if I can add that. I don't like pulling fuses in flight. I like screaming alarms scaring the bejezus our of passengers even less. I'll check the high RPM carb balance again though. I just did it last weekend at about 3500 RPM, maybe I'll run it up to 4K or higher and see how that looks as well.

I also wonder if mounting the flow transducer with some vibration isolating hardware would help. Has anyone tried that?
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Omaha, NE
RV-12 # 222 N980KM "Screamin' Canary" (bought flying)
Fisher Celebrity (under construction)
Previous RV-7 project (sold)
-=VAF=- 2017 dues paid
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  #5  
Old 07-28-2017, 11:15 AM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
Posts: 2,132
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I had this happen to me recently along with low fuel pressure. My electric fuel pump has a switch. I could reproduce clearing both alarms several times by turning on the pump. I concluded it is vapor bubbles forming in the line from the tank to the mechanical pump (vapor lock) that collapse when the line is pressurized by the electric pump. The indications stabilized in the green every time I turned it on. I was using auto gas that was about 7 weeks old.
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  #6  
Old 07-28-2017, 02:50 PM
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DonFromTX DonFromTX is offline
 
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Location: La Feria Texas
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In reading this all over to become familiar with it, I noticed that it was stated earlier that whenever you updated the Dynon software to a new one, you lost your K factor setting. Is this still the case?
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  #7  
Old 07-28-2017, 04:49 PM
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tomkk tomkk is offline
 
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Location: Port Orange, Fl
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Default

Quote:
Originally Posted by DonFromTX View Post
In reading this all over to become familiar with it, I noticed that it was stated earlier that whenever you updated the Dynon software to a new one, you lost your K factor setting. Is this still the case?
Can't answer the question but I'd advise Exporting User Settings and taking screen shots of setup screens when settings are changed to guard against the eventuality of something getting lost.
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