C182 fuel system mystery

flyingcheesehead

flyingcheesehead

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Twice this weekend, I had something odd happen - I landed for fuel, heard a gurgling noise coming from the fuel vent (I have a single vent on the left tank with a crossover to the right) and when I took off the fuel cap I got a whooshing sucking noise.

At first, I suspected a blocked fuel vent (nothing visible blocking it though). The engine seemed to run fine, though, and I fueled both sides equally (close, anyway: 30 gallons left, 33 right). So, I suspected some sort of one-way valve in the fuel vent, but nobody I talked to had ever heard of such a thing and neither have I.

Then, when it happened again, I got another surprise: It wasn't sucking, it was blowing! So now the mystery expands: Where the heck is high-pressure air coming from??? :dunno:

This happened twice over the weekend, but not every flight. Here are the details:

Happened:
1) Short flight, KOSH-KCLI (37nm), 4500 feet. Fairly low tanks - 79 useable, added 63.
2) Longer flight, KCAD-MBL-MTW-63C, 12,000 feet over the lake. Fairly low again, ended up putting in 52 gallons (again, very close between tanks - 25.7 left, 26.3 right).

Didn't happen:
1) KCLI-KMDW. Started with full tanks, flew about 1.5 at a max altitude of 7500. No gurgling noise. Wasn't present for the 10 gallons that were added.
2) KMDW-KCAD. Started nearly full, flew a couple of hours at 7500 feet. Did not add fuel, but no gurgling noise either.
3) 63C-KMSN. Short flight, 3500 feet, no gurgling, did not add fuel.

I fly this plane a LOT and this has not happened before. Any ideas?
 
Maybe partial blockage of the vent line between the tanks. 'High' pressure coming in thru the under/over (don't know what year yours is) wing vent, 'pressurizing' the tanks. The gurgling and rush of air you get is the residual pressure coming back thru the restriction in the inter-tank vent line??? Another thing to check for if it is relevant (can't remember 182 system) If the fuel tank caps are vented, make sure the vent holes are not blocked. Just guesses. Howard
 
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flyingcheesehead said:
Twice this weekend, I had something odd happen - I landed for fuel, heard a gurgling noise coming from the fuel vent (I have a single vent on the left tank with a crossover to the right) and when I took off the fuel cap I got a whooshing sucking noise.

At first, I suspected a blocked fuel vent (nothing visible blocking it though). The engine seemed to run fine, though, and I fueled both sides equally (close, anyway: 30 gallons left, 33 right). So, I suspected some sort of one-way valve in the fuel vent, but nobody I talked to had ever heard of such a thing and neither have I.

Then, when it happened again, I got another surprise: It wasn't sucking, it was blowing! So now the mystery expands: Where the heck is high-pressure air coming from??? :dunno:

This happened twice over the weekend, but not every flight. Here are the details:

Happened:
1) Short flight, KOSH-KCLI (37nm), 4500 feet. Fairly low tanks - 79 useable, added 63.
2) Longer flight, KCAD-MBL-MTW-63C, 12,000 feet over the lake. Fairly low again, ended up putting in 52 gallons (again, very close between tanks - 25.7 left, 26.3 right).

Didn't happen:
1) KCLI-KMDW. Started with full tanks, flew about 1.5 at a max altitude of 7500. No gurgling noise. Wasn't present for the 10 gallons that were added.
2) KMDW-KCAD. Started nearly full, flew a couple of hours at 7500 feet. Did not add fuel, but no gurgling noise either.
3) 63C-KMSN. Short flight, 3500 feet, no gurgling, did not add fuel.

I fly this plane a LOT and this has not happened before. Any ideas?
Click to expand...

Research and read all the ADs on Cessna fuel systems......I'd put a vented cap on both sides of the fuel tanks. You didn't tell us what year of 182 it was .
 
Howard Wilson said:
Maybe partial blockage of the vent line between the tanks. 'High' pressure coming in thru the under/over (don't know what year yours is) wing vent, 'pressurizing' the tanks. The gurgling and rush of air you get is the residual pressure coming back thru the restriction in the inter-tank vent line??? Another thing to check for if it is relevant (can't remember 182 system) If the fuel tank caps are vented, make sure the vent holes are not blocked. Just guesses. Howard
Click to expand...

Under-wing vent on the left side only, and this was opening the left tank, so it's not the inter-tank vent.

NC19143 said:
Research and read all the ADs on Cessna fuel systems......I'd put a vented cap on both sides of the fuel tanks. You didn't tell us what year of 182 it was .
Click to expand...

1971. The only AD I know of right offhand was the replacement of the original flush fuel caps with the "new" style that stick up - I think the issue was water leakage into the tanks. That one was done long ago.
 
flyingcheesehead said:
...

Then, when it happened again, I got another surprise: It wasn't sucking, it was blowing! So now the mystery expands: Where the heck is high-pressure air coming from??? :dunno:
Click to expand...

The fuel will expand just due to temperature differential between the storage tank and the ambient air but I would think you'd use up the fuel at rate faster than the rate it expanded in volume so you shouldn't have an overpressure.
 
Have you taken the right fuel tank cap off before the left one in any of the scenarios you've described, or always the left one first?
 
mikea said:
The fuel will expand just due to temperature differential between the storage tank and the ambient air but I would think you'd use up the fuel at rate faster than the rate it expanded in volume so you shouldn't have an overpressure.
Click to expand...

Right - And especially on the CAD-63C leg, if there was a blockage, I'd have expected it to "leak" during cruise and trap low pressure inside the tanks on the way down from 12,000.

Problem aside, I just don't get where the high pressure could even be coming from in this scenario. It's carbureted, so nothing's really being put into the tanks. The only thing I can think of is if there were a one-way valve in the fuel vent (in only).
 
I'd suggest a look at the maintenance manual for your airplane - it should have detailed information on the fuel system and venting that your AFM won't. It's quite possible that the underwing vent is an inlet to keep pressure down, and high pressure would be vented through the caps. But I don't know this for certain on your airplane.
 
flyingcheesehead said:
Twice this weekend, I had something odd happen - I landed for fuel, heard a gurgling noise coming from the fuel vent (I have a single vent on the left tank with a crossover to the right) and when I took off the fuel cap I got a whooshing sucking noise.
Click to expand...

I ain't no A&P but it seems clear to me that you have a vent problem.

Then, when it happened again, I got another surprise: It wasn't sucking, it was blowing! So now the mystery expands: Where the heck is high-pressure air coming from??? :dunno:
Click to expand...

No mystery there. Two things happen in a fuel tank - one, the engine sucks out fuel which tends to create a vacuum. Two - fuel evaporates which tends to create a pressure. Which one wins? Depends. If the fuel gets hot (sun load, ambient temperature) you tank will have a positive pressure, on the other hand, if the engine is pulling a lot of fuel, the tank stays cool, you get a vacuum.

Get enough of a vacuum and the fan will quit working.

I fly this plane a LOT and this has not happened before. Any ideas?
Click to expand...

I would stongly suggest that you quit flying it until you get it fixed.

So far you have been lucky - either you are getting enough of a vent, or enough vapor generation to keep the engine running. Personally, I wouldn't count on being that lucky the next time you fly.
 
Kent,

I'm not sure if your '71 model has bladders or wet wings but, if you have bladders, I'd definitely quit flying until the problem is corrected. The vacuum could collapse a bladder and then you'll have big problems.

I, too, would suspect that you have a blockage in your one and only underwing vent. But, then again, I DKS.
 
Kent, was it because it didn't happen for a few flights after the KOSH-KCLI leg that you were willing to go over the lake?
 
gprellwitz said:
Kent, was it because it didn't happen for a few flights after the KOSH-KCLI leg that you were willing to go over the lake?
Click to expand...

And because it didn't happen in CAD. And because I'd been flying quite a few hours with no problems.

If I was smart, I'd have started opening the right tank first just to see what happened. :dunno:

Oh well, off to the shop. :(
 
timwinters said:
Kent,

I'm not sure if your '71 model has bladders or wet wings but, if you have bladders, I'd definitely quit flying until the problem is corrected. The vacuum could collapse a bladder and then you'll have big problems.

I, too, would suspect that you have a blockage in your one and only underwing vent. But, then again, I DKS.
Click to expand...

I can't imagine how it would be worse than having to actually switch to a single tank, which is how us Piper drivers fly all of the time (try it when you have 4 tanks). ;) That, and having to deal with a an unbalanced fuel load which could get serious if you ran a long time that way.
 
mike you have no idea. with about 200 hrs in the 421B im starting to see why people love the C models so much with their simple left/right fuel systems. main,aux, and wing locker tanks, ive got 7 or 8 fuel pumps on the bird.
 
A positive pressure in the fuel cells after a quick desent in a cold soaked aircraft is pretty common, the fuel mass will stay cold for a while but as it warms up the tank vents can't keep up with the expanding fuel.

when you pull a cap while it is in the positive pressure mode you will get a whoosh.

It happens quite often here when we desend from crossing the mountains.

Remember the tank vent under the wing has a one way check valve in it. and it faces to the tank, and the pressure must be vented thru the tiny vent in the cap.
 
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