Cessna 182 fuel vent issue

ytodd

ytodd

Pre-takeoff checklist
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182P with long range tanks with Monarch caps

For the longest time, after landing when you removed the right tank cap it would "gurgle" and it you pressed the tab in (under the Monarch cap) it would spit a little fuel on you. Did all the research and everything pointed to adjusting the vent on the left wing. Read through CPA Technote and adjusted it multiple times (the tech note has measurements in hundredths of inches) and never made a difference. Thought process was that the vent was in the air stream too much thus "pressurizing the right tank" so we moved it up father behind the strut the last time.

Flew the plane several times (but never with full tanks) and the "gurgle" was gone. Fixed right?? Saturday I flew over to KBMI (had 24 on each side) landed then added 12 a side. Flew up to KOSH, and topped it off. Normally when topped off during taxi I will get a little fuel dribbling out of the vent. This did not happen. Took off and about 30 minutes into the flight, the vent looked like it was frosted over on the last one inch of it, then there was a little blueish color above that. This has never happened before. (300 hours in this plane). Thought that was weird, but kept flying, tank gauges showed even flow. Landed at 3MY and put the plane away an then stuck the tanks. Right tank showed 28 (about what I was expecting). Opened the cap on the right tank, no "gurgle" but when I put the stick in, I felt something....... the bladder was partially collapsed up by the neck.

So it seems the tank was feeding fuel but was not being vented. Meeting my A&P tomorrow to look at it.

Any ideas?
 
ytodd said:
Opened the cap on the right tank, no "gurgle" but when I put the stick in, I felt something....... the bladder was partially collapsed up by the neck.

So it seems the tank was feeding fuel but was not being vented. Meeting my A&P tomorrow to look at it.

Any ideas?
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Never had a Monach cap in my hand, but it should have the same vent check valve idea that the OEM cap has, to be legal. There should be a small check valve in the cap that will let air in, but not fuel or air out. It's to make sure fuel keeps flowing even if the underwing vent plugs up with ice or bugs or something. That check valve sometimes ages or cracks and lets the low pressure atop the wing pull fuel out, or at least hold it back, and can suck the bladder up like you found. If the bladder is pulled up hard enough it also raises the fuel gauge sender float and gives an erroneously high fuel quantity reading.

Any trapped pressure or vacuum in the tank is a sign that the underwing fuel vent checks valves (inside the tanks) aren't working properly. They should let air in rapidly and out slowly.
 
ytodd said:
182P with long range tanks with Monarch caps

Hello, Just had the same problem today. Dont have the Monarch cups, but the problem matches exactly your description. Please let me know what did you find out!,
Thanks so much!!
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Same tanks, no Monarchs here. If the tank is "gurgling" when you opened it, it's had a vent problem for some time now. Which I think you know already.

Don't forget there's also a cross vent to the other tank, so something is seriously wrong there. Even a completely plugged up main vent, and cap vent on ONE side, should still eventually be creating enough of a reduced pressure to open the cap vent on the OTHER side, once the fuel level is below the cross vent on both sides... well before tank collapse.

At 28 gallons I would think you're below the vent line on the other side. What most folks report is the fuel gets "pushed" through the vent line when both tanks are full if the underwing vent is out in the airflow too much and not hidden by the strut partially. You'll start with both full and as you fly, the right tank will remain full and the left will draw down, until the upper cross vent is uncovered. Then pressure equalizes between the two and draw is pretty even, resulting in more fuel in the right tank after landing.

Ours does that a little bit, and our hangar floor also tilts slightly to the right wing side, so we see a little more fuel on the right almost always.

But a right tank collapsing means the right tank can't get vented via either the crossover vent or the cap. Either one should have done the job. If one is plugged and then the other stopped working, it would explain the weirdness seen with the "spitting" before the new problem started. They probably both need to be looked at.

The cross vent is a PITA to deal with if it has a problem, and your mechanic might not get after it. But you really want it working since there's no backup to the cap vent on the right side.

There's random thoughts for ya. The P model vent system is always somewhat problematic but if it's collapsing tanks, it has something quite wrong, and probably two somethings.
 
I had a problem with my 172 last year with the vent flapper valve that connects to the pilot side tank and under wing vent. Can test it by putting a bit of hose on the end of the under wing vent and blowing into it. If you can't put any air in it, there's a chance the vent line is plugged or the flapper valve on the vent line on the fuel tank is bad.
 
The vent in the left wing was plugged. Cleaned it out and no further issues.
 
ytodd said:
The vent in the left wing was plugged. Cleaned it out and no further issues.
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Still seems weird. Right bladder shouldn't have collapsed from that. The cap should've vented
inward. At least that's what the Cessna caps do. Can't speak for the Monarch, but they're the latest and greatest 1980s automotive gas cap technology! Haha.

That thing you would push on and it would spit on you, that moved inward right? it's hidden under the "umbrella" of the cap to keep water out, as I recall. But a tank with enough low pressure to collapse the neck should have popped that inward and relieved the suction.
 
I've always felt there should be published test procedures and disassembly and repair instructions for fuel caps. Especially for vented ones.

I've got an older very cheap set of monarchs and never did like them. I threw the monarch caps on the shelf and bought two new Cessna ones that fit the monarch tank adapter perfectly.

Monarch doesn't even make the old cheap ones anymore. The seal would get compressed and set in that shape then the cap would always feel loose with no way to adjust it. Monarch wanted $14 for a new set of seals and they only seemed to last a year
 
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Um... are you trying to solve a non problem? Slight pressurization of the tanks isn't a problem. The caps as mentioned are not supposed to vent at ALL in normal circumstance. The vent is only supposed to open if the tanks become under pressure (in case the other venting fails). The j tubes below the wings have check valves to keep fuel from leaking from them so they can indeed seal up at times.
 
I thought vented fuel caps were to prevent fuel starvation of the engine if fuel tank vents become obstructed. Do they work? Have they been checked?
 
bnt83 said:
I thought vented fuel caps were to prevent fuel starvation of the engine if fuel tank vents become obstructed. Do they work? Have they been checked?
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Since he said he gets air and fuel blowing OUT of the tanks, the problem is in the opposite direction that the vent is there to solve.
 
The rubber seals on Cessna fuel caps get hard over time and will not open to allow air into the tank in the event the under wing vent gets clogged. Saw this recently on a 172 a friend flies. After two emergency landings due to loss of power and 4 different shops working on it with no problems found. We concludeded it had to be a vent problem and sure enough it was plugged with a mud daubers nest. The cap vents should have opened to prevent fuel starvation and did not. When we checked the caps the rubber was stiff from age. We cleared the vent and replaced the caps and now no more problem. Personally I think the caps should be rebuilt or replaced every 8 years or so as a sqfety measure. He now blows into his vent line on every preflight.
 
Most 172's by the way should only have the vented cap on the right size. Not that the fuel system on Cessnas are similar but different, so you need to understand WHAT caps are called for on what models. You just don't want to go willy nilly putting vented caps (or the wrong sort of vented cap) where they don't belong.

However, again, there doesn't seem to be a venting issue here. He has pressure in the tanks, this will typically occur when it's hot and the tanks are fairly full (which they'd have to be to have fuel shoot out as he describes). This is not an abnormal condition on some 182s.
 
flyingron said:
Um... are you trying to solve a non problem? Slight pressurization of the tanks isn't a problem. The caps as mentioned are not supposed to vent at ALL in normal circumstance. The vent is only supposed to open if the tanks become under pressure (in case the other venting fails). The j tubes below the wings have check valves to keep fuel from leaking from them so they can indeed seal up at times.
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flyingron said:
Since he said he gets air and fuel blowing OUT of the tanks, the problem is in the opposite direction that the vent is there to solve.
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That was his "normal" state. The "spits" when he pressed the monarch vent open. That's normal. A little pressure in the tanks happens. You try to keep it to a reasonable amount via location of the J-tube being slightly behind the strut.

Then he reported that his J-tube looked "frosted over" and his bladder tank and "collapsed at the neck" which would indicate there was no vent feeding air to the tank in-flight. Then he said the main vent was plugged. That was the change.

But this also means the main vent has been fixed, but the Monarch vent did NOT open as designed, as the bladder collapsed.

So he really has two problems, but the main vent repair is going to go back to covering up and making the Monarch cap vent problem, unnoticeable.

He needs/wants those cap vents working properly, besides the repair to the J-tube vent that was already accomplished.
 
flyingron said:
Um... are you trying to solve a non problem? Slight pressurization of the tanks isn't a problem. The caps as mentioned are not supposed to vent at ALL in normal circumstance. The vent is only supposed to open if the tanks become under pressure (in case the other venting fails). The j tubes below the wings have check valves to keep fuel from leaking from them so they can indeed seal up at times.
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That vent check has a tiny hole in the flapper to let air out slowly, such as when the tanks heat up on a hot day. Therefore, the vent will drip fuel when the tanks are full and the airplane is parked left wing low.

Cessna's service manuals call for testing of that system every 100 houras, which most never do. Attach a hose to the vent tube, blow into it, then hold the hose in a jar of water and watch for a stream of bubbles. You can also feel it if you hold the end of the hose up to your ear. If there are no bubbles (or draft on your ear), that little hole is plugged with something, and overpressuring of the tanks is going to cause trouble sooner or later, such as cracking the welds in them.
 
flyingron said:
Most 172's by the way should only have the vented cap on the right size. Not that the fuel system on Cessnas are similar but different, so you need to understand WHAT caps are called for on what models. You just don't want to go willy nilly putting vented caps (or the wrong sort of vented cap) where they don't belong.
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Vented caps may be used on both tanks. Just try buying a new non-vented cap and see. The AD doesn't forbid the use of vented caps in both tanks, and in fact says that in airplanes with two fillers in each tank (as in some 180s and 185s, for float operations), a vented cap shall be installed in the outboard filler of each tank.

http://www.airweb.faa.gov/Regulator...225F45DF0B489ACE86256A34006AB0A2?OpenDocument
 
I'm curious what the outside air pressure is near the fuel cap in level flight. Can fuel gravity feed against that low pressure above the wing and do vented caps really work?

I have had a vent plugged and only noticed it after takeoff when my fuel pressure went to zero when feeding on that tank. This 177 has one vent per tank and no interconnect, both caps were the monarch types that are supposed to be vented. With the fuel selector feeding from "both" one would only notice a fuel imbalance an hour or so after departure.

I'm pretty sure my 150 had one lone vent total and one vented fuel cap (who knows if it works), makes me think a bit about what ifs and launching into IMC.
 
I'm pretty sure my 150 had one lone vent total and one vented fuel cap (who knows if it works), makes me think a bit about what ifs and launching into IMC.
As do the 172's. There's a cross feed between the two tanks. The j tube is on the left side, the vented cap should be on the right (to cover if the cross tube blocks).

The 140's can for example can be really messed up if you screw around with vented caps that aren't supposed to be there. Find out what is right for your plane and don't screw around.
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Dan Thomas said:
...that little hole is plugged with something, and overpressuring of the tanks is going to cause trouble sooner or later, such as cracking the welds in them.
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My mechanic sees failed welds in bladder tanks all the time. ;-)

(Someone not notice he said he flies a P model Cessna 182? No welds in these tanks, unless you're welding rubber.)
 
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