The International Cessna^® 170 Association

Hello all, I noticed when removing my fuel cap the fuel blew out at me as if it was under pressure. I do have newer vented caps. I mentioned it to my IA and he thought I might have a blocked vent. Is it difficult to clean out the "gooseneck" top of the wing vent? I also noticed an increased uneven fuel burn between tanks. Anyone experience this? Thanks...

The gooseneck itself just unscrews at the joint on top of the wing. If it has anything in it it will be easily removed. If nothing is in the gooseneck you could blow into the gooseneck with your breath to see if it is stopped up. Others may suggest a better method of checking the vent line if nothing is in the gooseneck. Let us know what you find.

As an aside, our '56 172 is the only year model that the left fuel tank had the gooseneck on top of the wing. 1957 models. going forward, had them behind the lift strut.

The gooseneck on my 1955 model (sn 26541) is welded to a flat circular plate which is in turn riveted to the top of the cabin. Inside the cabin, the gooseneck tees left and right and is connected via short, clamped on hoses to aluminum tubes heading toward the wing root, where they connect via two more hoses to the vent lines going to the tanks.

It sounds like dirt daubers or some other small critter may have nested in the gooseneck. If the blockage is in the gooseneck you wont be able to blow through the gooseneck into the tanks. If the blockage is past the tee, you'll have to drop the front of the headliner and disconnect the vent lines on the inside to clear them.

Uneven fuel flow with the BOTH position selected is common, and most have learned to live with it.

How could a blocked vent pressurize the tank? I understand if the vent is blocked and the tank is pressured up somehow it will blow when the cap is removed but how could it pressure up to start with? If the vent is blocked, there should be less pressure in the tanks as gas is used, not more. Are you filling up the mains from an aux tank of some sort with a transfer pump?

As the airplane either sits (especially in the sun) or is agitated by towing, etc., the fuel is encouraged to evaporate and build a slight pressure in a sealed tank (sealed because the vent is blocked.) Vented caps vent INwards but have checkvalves to prevent out-flow. ()At least you know your fuelcap checkvalves are working.)

Be careful blowing into a vent system. Make certain the caps are off the tanks if you do that and don't use more than about 5 psi of pressure.

Another method to check the vent system is to use a siphon-blow-gun to apply suction to the gooseneck while listening with an ear close to the open fuel cap for air.

(And don't listen to Frank whose vent is not the only thing "screwy" about him....
Your 170 gooseneck should be a welded unit.)

George, reread PEL22's post. He references his 56 172, not 170! The first (56) 172 moved the gooseneck from the middle of the cabin to the left fuel tank and screws onto a fitting . The crossover vent line still goes across the front of the cabin as in 170's.

Thanks guys for all your replies, I'm hoping to look further into this in the next week or so, it does appear that you can unscrew it, ill start with that. I will report back.

170C wrote:George, reread PEL22's post. He references his 56 172, not 170! The first (56) 172 moved the gooseneck from the middle of the cabin to the left fuel tank and screws onto a fitting . The crossover vent line still goes across the front of the cabin as in 170's.

Yep, Thanks, Frank! I did indeed miss the fact he's asking about a 172 instead of a 170. (I made the same mistake to a caller one time, who wanted info on how to remove the check-ball system from his fuel vent. We spent almost a month talking about his gooseneck and visiting with his IA and his FSDO...all of them also missed the fact that I was continually referring to a Cessna NL regarding the 170.... So it's an easy miss to make.) In regard to that, Frank... is your vent tube check-ball system operational still? Do you have the ball, the pin, and the boot still? If so, don't lose them. You'll be shocked to see what Cessna wants for that rubber boot, and apparently there is no basis to remove it on the 172 like there is on the 170. Go figure.

George, it's really easy to mistake text when reference is made to 170/172 . I've done it several times. Re: the fuel tank vent gooseneck, it's been several yrs since I had my fuel tanks out an that would have been the last time I really looked at my vent closely (gotta do so again after your post), but I am pretty sure there is no longer a ball In the gooseneck and without referring to my IPC I don't know what rubber boot you refer to. Maybe it is to prevent water intrusion around the vent into the tank area?

The rubber boot is actually what retains the ball from spitting out onto the top of the wing and rolling off into the grass. It is slipped onto the opening of the gooseneck. It does double-duty as the "seal" to prevent gasoline from regurgitating onto the top of the wing.

My '56 hasn't had the boot in the 27 yrs I've been the caretaker and I'm pretty sure all that's in the gooseneck is that cotterpin, no ball.

I know a certain IA one of our Members hates because the annual was not approved for return to service until that $350 rubber boot was replaced. It looks like an automotive vacuum-plug with a hole in it. Maybe that's why your airplane is so slow....??

It's that excessive drag due to that big gooseneck sticking up out way out there on top of that left wing (which they eliminated on '57 & subsequent models)

cessna170bdriver wrote:The gooseneck on my 1955 model (sn 26541) is welded to a flat circular plate which is in turn riveted to the top of the cabin. Inside the cabin, the gooseneck tees left and right and is connected via short, clamped on hoses to aluminum tubes heading toward the wing root, where they connect via two more hoses to the vent lines going to the tanks.

It sounds like dirt daubers or some other small critter may have nested in the gooseneck. If the blockage is in the gooseneck you wont be able to blow through the gooseneck into the tanks. If the blockage is past the tee, you'll have to drop the front of the headliner and disconnect the vent lines on the inside to clear them.

Uneven fuel flow with the BOTH position selected is common, and most have learned to live with it.

I ran across this in my archives and thought I would share.

Your cap vent is inoperative. Repair or replace it. Also check to make sure you have pliable gaskets on the cap. Fuel problems are generally associated with rubber that has deteriorated in the valve and the seals.