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  #1  
Old 08-01-2017, 09:35 AM
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Bluelabel Bluelabel is offline
 
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Location: San Diego, Ca
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Unhappy Tank Switch Cough....

Wondering if anyone else has experienced this...
It's not consistent and I can't duplicate it.
Once in a while, when I switch tanks, it's almost as if the engine is without fuel for about a half a second. It comes right back, but at 160KTS, it will sure get your attention. Always scares the **** out of my wife and I. She won't watch when I switch tanks
On the 23 hours of flying to and from Oshkosh, it probably happened 3 times.
It does seem to always be when at altitude (=>10k)
I haven't documented the exact situations when it happens, because, to be honest, we are usually looking for clean underwear for a few minutes after it happens.
I thought I had the issue solved last year when I added an item to my runup checklist. "1800 rpm, toggle fuel tanks".
I always switch tanks very fast and smooth.
Andair duplex valve is installed per plans. I've checked for leaks. Nothing I can find.
Rich or lean has no effect.
I went over the issue with the Andair guys and went through inards of a valve at Oshkosh. Can't figure out what would cause it.
Only thing I can think is there is some situation causing an air bubble in the system. With our fuel system, we do have about 40 gallons/hour moving around the system at any time.

Anyone else had this issue?
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  #2  
Old 08-01-2017, 09:41 AM
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Auburntsts Auburntsts is offline
 
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No, but my SOP is to turn on my boost pump before I switch tanks and let it run for about a minute after.
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  #3  
Old 08-01-2017, 09:50 AM
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bkervaski bkervaski is offline
 
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Echoing Auburntsts .. in the Cherokee I'm flying now if I don't enable the boost pump before switching and leave on a few seconds after switching it will cough/complain.
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  #4  
Old 08-01-2017, 10:01 AM
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Bluelabel Bluelabel is offline
 
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Maybe I should have mentioned. With an electronic fuel injection system, there is no mechanical pump. I have pump 1 and pump 2. Pump 2 is back up and automatically kicks on if the fuel pressure drops below 22psi. My normal PSI in flight is around 28-30
I do have the ability to manually turn on both pumps at the same time. I may make that part of the procedure.
It's still puzzling as it's wildly inconsistent.
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RV-10 N265WD
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BPE IO-540, FULL EFII
KMYF, San Diego, Ca.
KEOS, Neosho, MO

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twin turbo, IO-540,
full EFII management system
!

"so I got that going for me... which is nice."
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  #5  
Old 08-01-2017, 10:05 AM
stancaruthers stancaruthers is offline
 
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Shut the fuel off for 1 minute each flight for 10 flights, after that you n the wife will never notice that cough again.
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  #6  
Old 08-01-2017, 10:43 AM
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Mine does this also, just switch tanks real fast, you are closing and then opening the ports during this movement and our fuel system requires constant fuel pressure to the injectors.
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  #7  
Old 08-01-2017, 10:50 AM
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Is there a possibility that the electric pump could suck air in the process of switching tanks that could cause the cough? I know it shouldn't be able to, but a loose line or fitting could cause something like this, especially when pulling so much fuel through. Also, something that comes to mind is that you have a small amount of fuel from the unused tank that is sitting in the tunnel for a long period of time before being used, which could be warming up slowly to the point that you switch and the suction drops the fuel pressure in that line below the vapor point and it gives you a very temporary vapor lock which the pump is able to overcome quickly but not immediately.
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  #8  
Old 08-01-2017, 10:59 AM
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I wonder if while flying, and the non used tank is low, and you encounter turbulence, could the pickup in the tank become momentarily un ported and allow some fuel to empty out of the pick up tube and allow a small air pocket to remain in there until that tank is selected and then you have your momentary air pocket getting circulated through the fuel system?
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  #9  
Old 08-01-2017, 11:04 AM
Tom Martin Tom Martin is offline
 
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Is it always the same tank?
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  #10  
Old 08-01-2017, 11:10 AM
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Bluelabel Bluelabel is offline
 
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Quote:
Originally Posted by Tom Martin View Post
Is it always the same tank?
That's what I need to determine. Like I said I'm always a little freaked out. I'm going to start documenting it.

Quote:
Originally Posted by bret View Post
I wonder if while flying, and the non used tank is low, and you encounter turbulence, could the pickup in the tank become momentarily un ported and allow some fuel to empty out of the pick up tube and allow a small air pocket to remain in there until that tank is selected and then you have your momentary air pocket getting circulated through the fuel system?
It happens even when both are full.
But glad to hear it not just me.
Quote:
Originally Posted by Jesse View Post
Is there a possibility that the electric pump could suck air in the process of switching tanks that could cause the cough? I know it shouldn't be able to, but a loose line or fitting could cause something like this, especially when pulling so much fuel through. Also, something that comes to mind is that you have a small amount of fuel from the unused tank that is sitting in the tunnel for a long period of time before being used, which could be warming up slowly to the point that you switch and the suction drops the fuel pressure in that line below the vapor point and it gives you a very temporary vapor lock which the pump is able to overcome quickly but not immediately.
Good info. Maybe on the ground I'll let each side run a little longer. Generally I climb out and if I'm going high (which usually do) i switch tanks after I level off.
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John Walker
RV-10 N265WD
"Miss Whiskey Delta"
BPE IO-540, FULL EFII
KMYF, San Diego, Ca.
KEOS, Neosho, MO

Crew Lead on the
Grove Aero Super Legacy Reno Racer,
twin turbo, IO-540,
full EFII management system
!

"so I got that going for me... which is nice."
2017 Dues Paid"
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