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  #21  
Old 12-20-2018, 08:08 AM
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BruceMe BruceMe is offline
 
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Location: Shawnee, Kansas
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Default I think I know what it is...

I ran the return lines back to the fuel tank to an open port...

What if... when I'm running mechanical it's sucking air through the return line. When I turn on the electric, it pressurizes that line and there's no more air?


Now, how can I prove this?
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  #22  
Old 12-20-2018, 08:25 AM
rv7charlie rv7charlie is offline
 
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Location: Pocahontas MS
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Default

Quote:
Originally Posted by BruceMe View Post
I ran the return lines back to the fuel tank to an open port...

What if... when I'm running mechanical it's sucking air through the return line. When I turn on the electric, it pressurizes that line and there's no more air?


Now, how can I prove this?
See post #17 (and my earlier posts).

Charlie
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  #23  
Old 12-20-2018, 08:34 AM
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Quote:
Originally Posted by rv7charlie View Post
See post #17 (and my earlier posts).

Charlie
Yup... just did. Ok, I'll pull the pump
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  #24  
Old 12-20-2018, 09:13 AM
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airguy airguy is offline
 
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Quote:
Originally Posted by rv7charlie View Post
See post #17 (and my earlier posts).

Charlie
Charlies right - any minor air leak into the suction side will cause issues. I had an O-ring go bad on a fuel filter on the suction side that started sucking air, and it ended up trashing an AFP fuel pump before I recognized it and corrected it. In my case I never saw any engine hiccups, but the pump failure got my attention as fuel pressure was declining on that pump.
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  #25  
Old 12-20-2018, 10:50 PM
amerkarim amerkarim is offline
 
Join Date: Nov 2017
Location: Houston Tx
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Default Non return valve

Just a few ideas:

What about fitting a non return valve in the return line? This should deal with the suction theory and take that out of the equation.

Could your tanks not be venting properly. As you use up fuel in the tank is there a possibility that the generated back pressure puts extra strain on the pump?

Best

Amer
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  #26  
Old 12-21-2018, 12:34 PM
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BruceMe BruceMe is offline
 
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Thumbs up ! SOLVED !

Ok, so this is pretty much what a couple folks had thought, but not why.


The small blue fitting on the boost pump that says...

"FLOW ==>" is NOT a check-valve; it's a flow restrictor

So...

Boost Pump On - All works great, there's positive pressure on the return lines, no air

Bust Pump Off / Lots of gas - The return line fitting in the tank is under 100LL and the up-stream mechanical pump is slowly pulling some unfiltered 100LL (not great) through.

Bust Pump Off / Low gas - The return line fitting is sucking air from the tank and bleeding it into the fuel system. Mechanical pump sucks air, stops working right.

The change to add the return lines is new, but I used to have that return line plumbed ahead of the filter and I suspect that was causing unfiltered debris into the throttle body and causing odd things to happen periodically.

This explains a lot of my fuel problems over the years. I'm going to re-plumb the return line ahead of the filter (so all fuel is always filtered) and I have every reason to believe this problems is gone for good.

Thanks everyone!
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  #27  
Old 12-21-2018, 01:03 PM
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Quote:
Originally Posted by BruceMe View Post
The change to add the return lines is new, but I used to have that return line plumbed ahead of the filter and I suspect that was causing unfiltered debris into the throttle body and causing odd things to happen periodically.
Dont you mean after the filter-------otherwise how could you get crud in the throttle body?


No matter which side of the filter the return line is tied in, as long as it is seeing air in the tank, it could introduce air into the fuel flowing into the suction side of the mechanical pump (unless there is a check valve)-------or am I misreading how you have things plumbed?
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  #28  
Old 12-26-2018, 08:27 AM
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BruceMe BruceMe is offline
 
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Default tested/closed

After test flights... this is solved.

I routed the return line after the filter and before the boost pump as designed.

One regressed issue... with the boost pump on, the fuel flow is reading 2-3 gph high as it used to. That had been solved by returning to the tank. I need to move the red brick further up the fuel lines. Some install it up by the spider, I could do that... some other time.

Thanks everyone,
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  #29  
Old 12-26-2018, 10:02 AM
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Hartstoc Hartstoc is offline
 
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Quote:
Originally Posted by BruceMe View Post

Tank -> Electric Boost Pump -> Reguator -> Mechanical Pump -> TB -> Spider -> Injector

The regulator has a regulator bi-pass fuel outlet (not like the current AFPs) that goes through a duplex valve back to the thank it came from.

This was the recommended setup back them.
Bruce- No insight on your loss of pressure problem, but you are very lucky to have return lines to your tank(and the dual selector valve). In fact, ALL aircraft with constant volume electric boost pumps really should have return lines. I believe the now-common expedient of returning bypass fuel to the pump inlet, either internally or externally, is a dangerous practice, and the only thing that prevents serious problems with it is how rarely boost pumps get used for extended periods. All of these pumps are fuel-cooled, and with current draws in the 4-6Amps they generate quite a lot of heat.

With fuel re-circulating constantly back through these pumps at a rate of 35-50 GPH (less actual fuel burned), this heat can easily elevate fuel to the vapor point. These pumps do not pump vapor very well at all, so at some point sufficient vaporized fuel will impede flow through the pump andheat will rise even more- a viscious cycle of the worst sort! Sending the fuel back to the tank is the only way to properly cool it.

A side note- If an engine driven pump problem ever necessitates running on the boost pump for an extented period on an aircraft lacking fuel return lines, do not “baby” it to an airport. Instead, go full throttle-full rich to maximize throughput of fuel through the electric pump to minimize heat buildup and head for the nearest safe airport.- Otis
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Last edited by Mike S : 12-26-2018 at 10:07 AM.
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  #30  
Old 12-26-2018, 01:18 PM
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Hartstoc Hartstoc is offline
 
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Location: Sebastopol,CA
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Quote:
Originally Posted by BruceMe View Post
I ran the return lines back to the fuel tank to an open port...

What if... when I'm running mechanical it's sucking air through the return line. When I turn on the electric, it pressurizes that line and there's no more air?


Now, how can I prove this?
The bypass valve aspect of the regulator should serve as a check valve against this. You might consider replacing the regulator with a simpler fixed bypass valve nominally set to 25-30psi. The metering valve should tolerate a range of pressures. Airflow performance systems rely entirely on a high-quality bypass valves and don’t require regulators. The hexagonal brass colored bodies shown in this photo of the dual pump system I’m installing are bypass valves that feed back into the return lines.(I’m eliminating the engine drive pump, thus redundant electric pumps).

The bypass valveswill feed into the retur portion of the dual Andair valve via the wye-fitting shown here
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Last edited by Hartstoc : 12-26-2018 at 01:52 PM.
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