Fuel leaking ALOT out of overflow
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Skip Miller
Final Approach
No experience with a C-150. When I had the PA-28 topped off a year or so ago, the line guy came to me and asked if he did something wrong, fuel was "flowing" out. I had visions of gallons by the time we got to the plane....
Not so. I removed the fuel caps and he had indeed topped it off to the brim. I showed him where the quick drain was and told him that if the fuel was leaking from there, then we were in danger of putting 24 gallons on the tarmac. We then discussed that the fuel was likely cooler in the truck, being heated by the sun and expanding out the overflow, no where near as bad. But still bad. Don't fill it that full, leave the fuel level below the neck of the tank.
Sometimes it still expands and leaks. You can sometimes stop this by opening the tank and letting the heated, expanded air out, too.
This is probably your problem. Fill it full but not to the brim. That extra 1/2 gallon is just headed for the tarmac anyway.
Not so. I removed the fuel caps and he had indeed topped it off to the brim. I showed him where the quick drain was and told him that if the fuel was leaking from there, then we were in danger of putting 24 gallons on the tarmac. We then discussed that the fuel was likely cooler in the truck, being heated by the sun and expanding out the overflow, no where near as bad. But still bad. Don't fill it that full, leave the fuel level below the neck of the tank.
Sometimes it still expands and leaks. You can sometimes stop this by opening the tank and letting the heated, expanded air out, too.
This is probably your problem. Fill it full but not to the brim. That extra 1/2 gallon is just headed for the tarmac anyway.
That is a common problem and has a relatively simple fix. It happened to the 152 I learned in, my instructor told me about it and fixed it with some check valve or something like that.
I am sure someone on here will know exactly what is going on and what you need to do.
I am sure someone on here will know exactly what is going on and what you need to do.
Tom-D
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Cold fuel pumped out of the ground tanks, allowed to set in the sun to warm up. expands, where is it to go? --- but out the vent.
Or you fueled it level, and parked where it isn't. the high tank flows to the low tank and out the vent.
The Left tank has the positive vent, both tanks will have a vented cap. but the positive vent is about .75" below the top of the left tank. and it has a .125" hole in it to allow the over service to relieve its self.
Or you fueled it level, and parked where it isn't. the high tank flows to the low tank and out the vent.
The Left tank has the positive vent, both tanks will have a vented cap. but the positive vent is about .75" below the top of the left tank. and it has a .125" hole in it to allow the over service to relieve its self.
Challenged
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The 152 I owned did this, and yeah I stopped topping it off... could've been some other fix, but never did it to mine.
Dan Thomas
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Yup. If it's gushing out, the vent check valve is stuck open. It has a small spring that holds it closed so that fuel can't gush out but can drip through the small hole in the valve as Tom says. The idea is that air can get in in a hurry if it needs to but is restricted on the outflow. If the spring breaks, or if the hinge gets gummed up with old crud, it can stick open and fuel flows freely out of it.
The valve is on the end of a short section of tubing, inside the tank, that has a jog in it. That jog is supposed to hold the valve up high in the tank. Sometimes when mechanics are servicing the vent system, they could turn that tube when they turn the outer tube nut and that puts the check valve well down in the fuel and more of it gets away.
McFarlane sells a humped vent tube to help stop the loss, but if the tanks are right full and the fuel expands it will still force fuel out.
Dan
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Let'sgoflying!
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Dont tell the EPA or they'll try something like they did with our portable cans!
Yup. If it's gushing out, the vent check valve is stuck open. It has a small spring that holds it closed so that fuel can't gush out but can drip through the small hole in the valve as Tom says. The idea is that air can get in in a hurry if it needs to but is restricted on the outflow. If the spring breaks, or if the hinge gets gummed up with old crud, it can stick open and fuel flows freely out of it.
The valve is on the end of a short section of tubing, inside the tank, that has a jog in it. That jog is supposed to hold the valve up high in the tank. Sometimes when mechanics are servicing the vent system, they could turn that tube when they turn the outer tube nut and that puts the check valve well down in the fuel and more of it gets away.
McFarlane sells a humped vent tube to help stop the loss, but if the tanks are right full and the fuel expands it will still force fuel out.
Dan
Very nice post. I think this is my problem. Thanks all for the help!
Let'sgoflying!
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Sometimes that original valve is working fine, but someone accidentally rotates it so that the joggle is down instead of up.
I will try to rotate it and see if that fixes the problem i would imagine a 180 rotation should fix it if that were the problem?
If you aren't sure what you're doing, pay an A&P to do the job. It's not in the realm of owner maintenance anyway.
James331
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Yup. If it's gushing out, the vent check valve is stuck open. It has a small spring that holds it closed so that fuel can't gush out but can drip through the small hole in the valve as Tom says. The idea is that air can get in in a hurry if it needs to but is restricted on the outflow. If the spring breaks, or if the hinge gets gummed up with old crud, it can stick open and fuel flows freely out of it.
The valve is on the end of a short section of tubing, inside the tank, that has a jog in it. That jog is supposed to hold the valve up high in the tank. Sometimes when mechanics are servicing the vent system, they could turn that tube when they turn the outer tube nut and that puts the check valve well down in the fuel and more of it gets away.
McFarlane sells a humped vent tube to help stop the loss, but if the tanks are right full and the fuel expands it will still force fuel out.
Dan
Good diagram.
I'd just not fill it to the brim
Dan Thomas
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The fitting that holds the vent tube in the tank has a gasket on it that is easily disturbed and can start leaking if it's old and deteriorated. Further, the tank cover has to come off to get at it properly, and the fuel has to be low enough to use a mirror to see where the valve is inside the tank. It's not an easy or quick job even for a mechanic. If that tank cover hasn't been off in years the screws could be corroded and stuck and take some time to extract without damaging the spars or the anchor nuts attached to them. And while the covers are off the tanks should come out and the rubber bumbers that support the tank need to be checked to see that they aren't crumbling and falling off their hat sections; if they do that, the tank gets chafed and starts leaking. VERY expensive to repair tanks damaged like that.
AIrplane ownership can get expensive. Old airplanes bought cheap are not usually cheap unless they've had topnotch care throughout their lives. When someone asks me about buying an older airplane, I tell them to use half the money they have to buy it and the other half will pay to fix up all the neglected stuff we'll find. Too many owners have had cheap annuals for too long and the diseases accumulate.
Dan
azure
Final Approach
This.
And, if they've had topnotch care throughout their lives, it's rare the owner will want to part with them cheaply. Expect to get what you pay for, and to either pay now, or pay later.
(To be completely accurate, that should really say expect to get *no more than* what you pay for, and to pay now, or later, or both. Often both.)
Let'sgoflying!
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No, just come back <half tanks one day and use a t̶o̶r̶c̶h̶ .... er sealed flashlight and a mirror. Take pics or report back what you see. Don't mess with it. Just look for now, see what you got.
Silvaire
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This is true and the fact that so many 150/152's exhibit this problem should lead to the conclusion it is not because the vent tube or valve was at sometime installed upside down by a hapless A&P mechanic. The vast majority of these things have never been touched since they left the factory in Wichita.
The problem is the check valve. Because it is exposed to the atmosphere on one side and allows air to flow into the tank as fuel is used to equalize the pressure the fuel that is on it evaporates but the dye in the fuel doesn't. Over a period of decades it builds up and the valve no longer makes a perfect seal in it's "check" direction.
The problem is the check valve. Because it is exposed to the atmosphere on one side and allows air to flow into the tank as fuel is used to equalize the pressure the fuel that is on it evaporates but the dye in the fuel doesn't. Over a period of decades it builds up and the valve no longer makes a perfect seal in it's "check" direction.
Tom-D
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Problem is, it's not a true check valve. it has a 1/8th inch hole in the center that will allow fuel to escape when over filled.
the eccentric mounting hole is to ensure that the hinge for the flapper is at the top. so gravity can hold the flapper closed.
Silvaire
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I know but these guys aren't talking about the normal drips, they're talking about fuel pouring out which isn't normal even when topped off.
Tom-D
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Describe "fuel pouring out" Is that 10 drops per minute? or 10 gallons?
A 1/8th inch hole will produce a pretty good stream. Is that pouring like from a bucket? or a ketchup bottle?
The vent line is only a #4 line, just twice the size of the hole, so half of a #4 line is to some pilots " Pouring" to others, Oh Well.
txflyer
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Fly it like you STOL it ♦
I try to fill mine and keep it an inch below the cap. Leave some air.
My wings are polished so it will boil the gas out in no time sitting in the sun if you fill it up too much.
My wings are polished so it will boil the gas out in no time sitting in the sun if you fill it up too much.
Dan Thomas
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Ten gallons. We had a 185 in a few weeks ago that was streaming fuel out the vent whenever the airplane turned right while taxiing and it was running out pretty good when the tanks were full. Way more than the usual Cessna drip or small stream. The check valve flapper was stuck wide open: broken spring, gummed up.
Dan
Let'sgoflying!
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maybe in some models but the one I took out was not a "D" but more like an "O" with flattened sides. So it could be installed upside down; 180degrees from the normal way:
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Tom-D
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If you do that, it will syphon out half the tank. If the right wing is higher it will do more than that.
buzzard86
Pre-takeoff checklist
I did the McFarlane vent tube mod to my C150 last year. It definitely helped but I still do get a decent drip if I top to the collar. My mechanic also discovered a broken tank strap during the install which added to the costs.
It definitely made things better so I have no regrets. I now top off before each flight (I used to top at the end of the flight). Also, I try not to fill to more than about 1" below the collar. This combo has effectively stopped my leak.
Jim
It definitely made things better so I have no regrets. I now top off before each flight (I used to top at the end of the flight). Also, I try not to fill to more than about 1" below the collar. This combo has effectively stopped my leak.
Jim
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Expansion is most likely what you are seeing, the solution is simple, don't put in every last drop of fuel possible on a hot day, leave a small gap at the top.
Dan Thomas
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And that thin aluminum tank wall can also be wallowed out easily by some guy torquing on the nut if it's seized on there tight. The flats in the hole can get torn out. The aluminum is only 5052, soft stuff, to make forming the tank possible.
Dan
Dan Thomas
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Same if you are parking on a ramp that's sloped so that the vent is on the low side. In a 150 the fuel lines are teed together before the shutoff valve, and fuel will crossflow to the low tank and out the vent.
Or if the airplane is filled right up on a cold day and then put into a heated hangar. That will guarantee fuel on the floor and an appalling fire hazard.
Dan
N3368K
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My Swift's original vent was on the right tank top. It was a tube, coming straight up, turned 90° toward the front and crimped closed on the end. It had three small holes drilled on each side near the crimped end. When full, if making a right turn, it would squirt out in six tiny streams. It has subsequently been rerouted through the fuselage out the top behind the rear of the cockpit bulkhead. Few retain the original vent for this reason.
My Starduster Too would "pee" through the inverted vents if filled to the neck. Just always made sure I filled it myself and left it down an inch or two.
Airplanes sitting on hot tarmacs after fueling tend to vent fuel overboard if filled to the max. Sitting at an airshow several years ago after filling a B-25 and S2F Tracker I crewed on. We fueled early in the morning and the planes sat in the sun. After lunch, both started peeing...
Jim R
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N7155H--1946 Piper J-3 Cub
N3368K--1946 Globe GC-1B Swift
N4WJ--1994 Van's RV-4
My Starduster Too would "pee" through the inverted vents if filled to the neck. Just always made sure I filled it myself and left it down an inch or two.
Airplanes sitting on hot tarmacs after fueling tend to vent fuel overboard if filled to the max. Sitting at an airshow several years ago after filling a B-25 and S2F Tracker I crewed on. We fueled early in the morning and the planes sat in the sun. After lunch, both started peeing...
Jim R
Collierville, TN
N7155H--1946 Piper J-3 Cub
N3368K--1946 Globe GC-1B Swift
N4WJ--1994 Van's RV-4