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threein60
Joined: 04 Feb 2016
Posts: 51
Location: Everett Wa
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 Posted: Wed Mar 22, 2017 1:53 pm Post subject: Fuel oddity and the little check valve no one touches. |
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Just wanted to give everyone a heads up as to an issue that I found on my CJ while doing the annual. After my annual was complete, I rolled the aircraft outside to do a run and leak check. I realize this was the wrong time to do this since snow was moving in, but I wanted it to be complete. I had the cowls off and with the cold temps, I needed to pay special attention to get the temps up before I performed my checks. After about 10 minutes at warm up RPMs, I ran to 85% for a few minutes than shut down to check for leaks. No leaks noted! So I wanted to perform a second run and continue my systems checks for the final inspection. I performed the same engine warm up procedure as before and when the temps were sufficient I added power. The engine accelerated to 50% and died! Let me tell you, the engine dying on the ground is somehow just as alarming as if I were in the air. I performed a restart again. My start procedure is to pressure with my electrical pump and then prime with the electrical primer first. I found out each time I hit the start button with the electric pump on, the engine would start immediately but when I turned the electrical fuel pump off, the engine would die after a few seconds late. I also noted that I could not accelerate past 50% with the electrical pump on. This is odd because it is designed to supply more fuel while on the electric pump than the engine can consume on takeoff. Sent the mechanical fuel pump to M14P for overhaul (They do a great job) and no real issues found. The odd part was with the fuel strainer opened, I could not get any fuel to flow when I blew air into the wing vents.
Long story short, the fuel check valve under the floor that supplies fuel to the wobble pump, or in this case an electric pump was somehow failing closed. When I removed it, it looked fine but randomly I could force it to fail open and closed by shaking it. I removed the flapper and filed away any edges on the hinge line as well as lapped the valve flapper surface. I also, noticed that the brass hinge pin had VERY slight bend in it. I replaced this brass hinge pin with a new, slightly larger diameter brass pin and reassembled. This seems to have fixed the problem. Just wanted to pass this oddity along if you ever have a similar issue.
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wlannon(at)shaw.ca
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| Posted: Wed Mar 22, 2017 8:51 pm Post subject: Fuel oddity and the little check valve no one touches. |
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I agree that really is an oddity!
However I don't believe it has anything to do with the fuel inlet check
valve. That valve does not supply fuel to the auxiliary pump (wobble or
otherwise). The fuel comes directly from the header tank via the 90 deg.
fitting on the input side of the check valve.
The valve is a simple flapper type identical to the two in the header tank.
It opens or closes in direct response to fuel flow. It almost literally
cannot fail if the hinge
is correctly positioned at the top (critical), the hinge pin is not too
tight (or falls out) and the flapper is not glued closed from storage with
serious fuel contamination. Held in your hand the flapper must fall freely
in either direction.
The only purpose of the valve is to close under aux. pump pressure and
direct fuel to the carburettor rather than back to the aux. pump. In normal
operation it opens instantly on sensing the negative pressure differential
from engine fuel pump operation.
I wish I could give you some idea what the real problem is but at this point
I really don't know.
Am I correct in assuming this is an M14P conversion and you have a non
standard vent system? Do you know the output pressure and volume (GPH) of
your aux. fuel pump?
The pressure ideally should be the same or slightly less than your engine
driven pump (usually 0.3 - 0.4 Kg/ sq. cm. or 4-6 psi ). Excessive over
pressure or GPH could
cause some problems but does not seem to fit your description.
Walt
From: threein60
Sent: Wednesday, March 22, 2017 2:53 PM
To: yak-list(at)matronics.com
Subject: Fuel oddity and the little check valve no one touches.
Just wanted to give everyone a heads up as to an issue that I found on my CJ
while doing the annual. After my annual was complete, I rolled the aircraft
outside to do a run and leak check. I realize this was the wrong time to do
this since snow was moving in, but I wanted it to be complete. I had the
cowls off and with the cold temps, I needed to pay special attention to get
the temps up before I performed my checks. After about 10 minutes at warm
up RPMs, I ran to 85% for a few minutes than shut down to check for leaks.
No leaks noted! So I wanted to perform a second run and continue my systems
checks for the final inspection. I performed the same engine warm up
procedure as before and when the temps were sufficient I added power. The
engine accelerated to 50% and died! Let me tell you, the engine dying on
the ground is somehow just as alarming as if I were in the air. I performed
a restart again. My start procedure is to pressure with my electrical pump
and then !
prim with the electrical primer first. I found out each time I hit the
start button with the electric pump on, the engine would start immediately
but when I turned the electrical fuel pump off, the engine out die a few
seconds late. I also noted that I could not accelerate past 50% with the
electrical pump on. This is odd because it is designed to supply more fuel
while on the electric pump than the engine can consume on takeoff. Sent the
mechanical fuel pump to M14P for overhaul (They do a great job) and no real
issues found. The odd part was with the fuel strainer opened, I could not
get any fuel to flow when I blew air into the wing vents.
Long story short, the fuel check valve under the floor that supplies fuel to
the wobble pump, or in this case an electric pump was somehow failing
closed. When I removed it, it looked fine but randomly I could force it to
fail open and closed by shaking it. I removed the flapper and filed away
any edges on the hinge line as well as lapped the valve flapper surface. I
also, noticed that the brass hinge pin had VERY slight bend in it. I
replaced this brass hinge pin with a new, slightly larger diameter brass pin
and reassembled. This seems to have fixed the problem. Just wanted to
pass this oddity along if you ever have a similar issue.
[Shocked]
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=467514#467514
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threein60
Joined: 04 Feb 2016
Posts: 51
Location: Everett Wa
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| Posted: Thu Mar 23, 2017 8:07 am Post subject: Re: Fuel oddity and the little check valve no one touches. |
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Walt, I guess there are many different variants to electrical fuel pump installations. Mine is reasonably simple. But let's look at a standard CJ's fuel system. During normal operation (no wobble pump), 100% of the fuel from the tanks feed through this check valve on its way to the engine driven pump. I know of many people who have had incidents of center tank (hopper) check valves sticking, I have been one. So it is entirely possible that the same type of flapper check valve located in the floor can cause issues. In my case, the hinge pin was slightly bent, allowing the flapper to move side to side in such a way that it makes contact with the hinge pivot points. When I removed this valve, and after I cleaned it, I was able to get it to stick closed and stick open just by shaking it. So it stands to reason that is the engine receives 100% of its fuel thru this check valve and this check valve fails closed, no fuel will reach the engine.
Now the reason it worked while on my electric fuel pump is because my pump sucks fuel aft of this valve and dumps it forward of the valve, thereby bypassing the check valve. The reason I couldn't get more than 50% power while on the elect pump was because this valve was closed. The pump which supplies 5psi at 35 gal/hr still needs the original normal -8 (equivalent) fuel line to supply volume while the elect pump supplies volume and pressure. Once corrected, a flow test was performed, with engine off and electric fuel pump on, to provide 35 gal/hr.
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