fuel tank/vent/line/valve?

[Paul B]

My plane is a 2000 MXT7-180. I have two symptoms, don't know if they are related:

First, small amounts of fuel frequently come out of the vent for the right main tank. The tank can be less than full and it will still vent fuel. The vent itself is discolored from fuel stains and the wing immediately aft of the vent is slightly stained due to venting in flight. The left tank never vents fuel.

Second, when the fuel selector valve is in the "both" position, significantly more fuel is used from the left tank. Sometimes twice as much fuel will be used from the left tank compared to the right tank. The engine runs fine when any position is selected on the valve and the fuel pressure is the same for all positions.

Should I be concerned about this and does anyone have ideas about what could be the cause?

[MAU MAU]

Hello Paul -

I have the same plane with similar characteristics.

The main control valve for the two fuel tanks are behind the left kick panel, and as the left tank is closest to that junction, it will tend to feed the engine more if the selector is on BOTH.

I now only select the left tank or right tank individually and switch back and forth every 30 minutes. Your GPS may have a function for setting up a reminder to switch tanks.

The fuel vents under the wing are another story.

Hopefully Jeremy will see this post and lend you his knowledge.

[maules.com]

If you have the later single entry air vent system, because of dihedral where the aux tank is higher than the main, an over filled aux tank fuel enters the air vent line and flows across the airvent drain and into the main tank. If the main tank is already full, the fuel will come out of the main tank vented cap, flow rearward and inboard and can in worst case, enter the wing root under the fairing and because some inspections are not replaceing the closure tape, will then enter and stain the headliner.
If you have the individual tank vent lines and the tanks are overfilled, some fuel will come out until the fuel level drops below the air vent line position in top of the tank.
In a taildragger, fuelling on ground, the tank is tilted and sometimes when initiating descent in the air with a full tank on one side fuel may exit the vent.
Fresh staining can easily be cleaned away with fuel damped rag but the brown stain (varnish) is more stubborn and a quick wipe with lacquer thinner followed immediately with a dry rag will remove the stain.
It all boils down to careful fueling levels and getting to know your airplanes levels and fully studying the fuel system. Next annual ask your mechanic to trace out the full fuel system for you.

[Paul B]

Thanks Mau and Jeremy for responding. It sounds like using fuel faster from the left tank when the valve is set to "both" is not unusual. I will just get used to it.

The right main tank will sometimes vent fuel on the ground even when it is several gallons from being full, the selector valve is set to "left" and the plane is parked in a level area (it is a nose-wheel plane). There has never been any fuel in either of the outboard aux tanks since I have owned it. The right tank never vents fuel.

The plane is being annualled right now. Any thing I can suggest the mechanic should look at?

EDIT: I meant to say the LEFT tank never vents fuel, only the RIGHT tank.

[maules.com]

Several gallons from left main when on LEFT or RIGHT indicates a leak somewhere. Open tank top wing skin and investigate.
Overflow at vent tube can look like alot but is usually no more than a quart.
If parking overnight on OFF or BOTH, fuel will transfer to lowest tank, and if one tyre is softer than the other it will depress that tyre relative to the other and more fuel will transfer.
Never leave the tanks on OFF or BOTH when fuelling up as it can transfer even in the time it takes to close the first tank move the ladder, chat with gas attendant or co-pilot and move nozzle and ladder to second tank. Thus tanks are not full. Make a habit of LEFT at fuelling.
In flight get a habit of flying on LEFT first as that fuel burned helps lateral balance as pilot weight is on left.
No need to switch to RIGHT until an hour and a half later.
If low on total fuel at landing or takeoff switch to BOTH.

[Mountain Doctor]

FWIW...

I have a similar model Maule.

I have also found unequal feeding in flight while on 'Both' so I just switch manually between 'Left' and 'Right' to keep them more or less balanced.

If I have heavier people on one side or the other I will use the fuel to compensate somewhat.

I leave the plane pretty much all the time on one side or the other, never 'Off' or 'Both'.

I once filled all 4 tanks to the top and had significant overflow out of somewhere (fuel cap vent?) when parked. No damage but I had some blue slime to clean off the paint. No harm no foul but I don't fill the tanks all the way, especially before parking on a hot day.

[MAU MAU]

Thanks Jeremy for the great information.

[Mr. Ed]

I had similar issues with the vent.

The Aux tanks have a check valve in the vent line. It's in the line right where it enters the tank. Make sure that is not stuck. You can put a plastic tube on the aft vent tube and check to see if it is working properly by sucking and blowing into the tube. Don't do this with the forward vent tube, no check valve and you can start a siphon out that line. Remember, there's a hole i the back of the vent line that you need to plug with your finger.

Also check your fuel caps. They should vent one way only. Clean the cap and then suck on the inside part of the valve. Air should come in but when you blow on it it should seal. I replaced my caps with new ones from Maule. The new ones come with a very thin rubber gasket that does not seal properly. I would see fuel vapor off the trailing edge of the wing when the tanks were full or it was turbulent. It was being sucked out from around the cap gasket. I got new rubber gaskets that come from American Champion. The combo of the new fuel caps and new gaskets and cleaning the check valves fixed my venting problems.

Good luck.

[Paul B]

Thanks for the response Mr. Ed. Here is what I found after flying today: I parked the plane in the hangar (very level) with the fuel tank selector valve on the right tank. A drop of fuel would come out of the right wing vent about every 20 seconds. I measured the fuel in the right main tank with my accurate dipstick at 15 gallons (21.5 gal tank). I sumped the left aux tank and it was bone-dry as I expected it to be. The right aux tank had fuel in it. I stuck the dipstick in and there was a lot of fuel in it (at least a couple of gallons, my dipstick is not calibrated for the aux tanks). Both of the aux tanks were dry when I bought the plane less than 2 months ago, I checked this myself. I have never put fuel in either aux tank and I am the only one who has fueled the plane. So this tells me I have a check valve problem. I am not clear yet on how this is related to my constant venting of small amounts of fuel from the right side (maybe it is not) but I will try some of the things the Mr. Ed did and see what I can see. I think I will also call the factory.

I thought I was using more left aileron than normal the last few flights but it was easy to tell myself it was the way the plane was loaded or wind conditions or my imagination. I will pay more attention to how the plane handles from now on!

[aero101]

Sounds like you've got a check valve problem in the Aux sys... The transfer pumps also have some kind of check valve / anti siphon valve installed as well, if you have correct pump installed? There are several of these pumps out there that look identicle, but only one (Facet 40131E) that has the correct anti siphon stuff internally? There is also a kit available from Maule to install check valves on the Main Tanks vent lines... But if fuel appearing in Aux tanks for no reason, this is a problem and probably related to why your vent line leaking as well...

[maules.com]

The only way fuel can get from main to Aux if the fuel valve is always only on one tank, is by backing through the Aux pump or less likely through the vent system and impossible through the vent system if down to 15 gallons on the Main.
The vent line tubes are on top of the tanks and thus if fuel in tanks is well below the necks, fuel cannot get into vent lines if aircraft is parked level and not moving.
If Main is at 15 gallons and Aux is 2 gallons, fuel cannot get into the vent lines except via osmosis or incorrectly installed tubing.
If the plane had been flying with full Main, some fuel could get into a sagging Main tank vent line and be traped there until such time as warming of the air in top of Main tank pushes the trapped fuel along to the vent line inlet from where it could drip.
Looks like 1. sagging vent lines and 2. bad Aux pump check valve.

[aero101]

Agreed!! But we don't know when the 2 gallons xfer'd into the Aux sys? In any case, I'd suspect the transfer pump first, don't think the check vlv is line replaceable, but pump can be ordered from NAPA or Aircraft Spruce and doesn't take big effort to change. Have you popped off the lower cover of pump to check the pump screen? This tends to get overlooked at 100hrs, and sometimes find good deal of stuff living in screen... This could also render the anti siphon system inop...

[Paul B]

A little more info after flying today: I drained the R aux tank yesterday morning and left it overnight on a level surface with 18 gal in the R main, the valve set to "right" and no fuel present at the R wing vent. This morning I confirmed there was absolutely no fuel in the R aux tank, still 18 gal in the R main and there was no evidence of fuel coming out the R vent.

I went for a 1.1 hr flight, mostly straight and level sightseeing, one take off and one landing. All ground ops prior to take off were with the valve set to "right" and all ground ops after landing were with the valve set to "left." About 1/3 of the flight the valve was on "right" with the rest of the flight on "both." At no time was either aux fuel transfer pump turned on.

After landing I parked in exactly the same position and measured 14 gallons in the R main tank. There was now 6 ounces of fuel in the R aux tank. Fuel was dripping out of the R wing vent, about 1 drop every 20 seconds. No fuel in the L aux tank (there never is) and no venting from the L wing (there never is).

I really appreciate the responses from you guys. If you think this info narrows things down, let me know!

[aero101]

Sounds to me like you have 2 problems which are probably not related... Unwanted slow xfer to Aux tank, I'd suspect pump anti siphon system, or check valve, which could just be trash, rust, etc.... If inspected and found clean, I think I'd just replace them both... Problem 2 is venting at RH Main tank. Do you have the correct gas caps? vented or unvented? Plugged up vent line? Drooping vent line?

[a64pilot]

Just for the heck of it put some fuel in the aux tank and run the aux pump. I'm betting the check valve or whatever there is that prevents fuel flow backwards in the system may be freed up by running the pump, sort of like a leaking fuel tank drain, often they can be "fixed" by simply actuating the drain valve and flushing whatever trash out.
Either way it can't hurt to try.