I have been slowly ironing out small squawks on the 180. Coming from the J-3 it is taking time to learn what is normal/acceptable and what truly needs fixing. So, I dismissed the first couple of times that a puff of pressure came out of the wings when I loosening a fuel cap. Then curiosity/self preservation got the best of me and I started researching the venting system and learned that a) it should not pressurize; and b) there are a number of things that could cause the system to pressurize ranging from mud daubers plugging the under-wing vent tube to a faulty tank vent check valve as outlined in the service bulletin SEB99-5. Following the troubleshooting procedure in the SB, it appears my issue is with the check valve (even though my valve probably isn't covered by the SB as the bladder was replaced in 1996 and the SB highlights valves shipped by Cessna between 1998 and 1999). So my questions at this stage:

- Considering the location of the check valve, what sort of ordeal are my mechanic and me in for?
- Any hints on the swap or additional troubleshooting ideas I should try before beginning surgery?

Thanks!

Have you checked the placement of your vent tubes? Cessna has some pretty stringent tolerances as to the placement of the tubes behind the wing strut. It doesn't take much movement to turn a "puff" into a "suck" with the fuel system happy with something in between.

Are you sure it's your check valve and not one vent hanging too low? I think if it's too low then it will cause it to pressurize. I would try adjusting it to the proper place before doing anything to major.

Tweak the placement of your vents. The Cessna Association has has a good article on this.

That is three votes by smart Cessna guys on adjusting the under-wing vent placement. I really want that the be the correct diagnosis since it is a simple and inexpensive fix. However, I followed the steps in the Service Bulletin referenced above and attached a length of tubing to the under-wing vent, inflated the bladder until it had the resistance of a filled balloon, then submerged the end of the tubing in a cup of water looking for bubbles. I didn't see any bubbles and ended up relieving the pressure in the fuel system by opening a fuel cap. Given that can I still be facing a vent placement problem? I've never seen a vent check valve, so I don't know how it is supposed to work or how it might fail. However, the flow of the SB instructions leads to "Replace S2359-1 Fuel Vent Check Valve."

Thanks again!

My mechanic and I spent yesterday swapping the fuel vent check valve, and I leaned some new curse words along the way. Even so, the job wasn't as bad as I had made it out to be in my mind.

After pulling the valve and it's fitting from the bladder we put it through a function check and compared it with a diagram in the "Guide for Changing Cessna Fuel Bladders" document that the Cessna Pilots Association has. The diagram calls for a one-way check valve that has an upstream hole, and a #60 hole drilled in the upper portion of the aluminum tubing. The check valve we removed from the plane had an indentation (as though made with a punch in preparation for drilling) but no through hole, and the old aluminum tubing did have a pin hole but it was plugged.

Old left, new right:



Closeup of "old" where hole should be:



New valve attached to newly-fabricated tube, ready for reinstall:



The system now seems to be venting appropriately. I'll sleep easier knowing my fuel bladders aren't trying to blow my wings apart!


Sent from my iPhone using Tapatalk

Felix wrote:My mechanic and I spent yesterday swapping the fuel vent check valve, and I leaned some new curse words along the way. Even so, the job wasn't as bad as I had made it out to be in my mind.

After pulling the valve and it's fitting from the bladder we put it through a function check and compared it with a diagram in the "Guide for Changing Cessna Fuel Bladders" document that the Cessna Pilots Association has. The diagram calls for a one-way check valve that has an upstream hole, and a #60 hole drilled in the upper portion of the aluminum tubing. The check valve we removed from the plane had an indentation (as though made with a punch in preparation for drilling) but no through hole, and the old aluminum tubing did have a pin hole but it was plugged.

Old left, new right:



Closeup of "old" where hole should be:



New valve attached to newly-fabricated tube, ready for reinstall:



The system now seems to be venting appropriately. I'll sleep easier knowing my fuel bladders aren't trying to blow my wings apart!


Sent from my iPhone using Tapatalk

But think of all the extra lift from fatter wings that you're giving up!

Cary

Ummmm, if the vents aren't working, your fuel tanks should present a vacuum when a cap is opened, and that is not good for fuel bladders. The check valves allow air INTO the tanks, as fuel is drawn from the bladders.

If you're actually getting pressure in your tanks, you have a very unique problem......

But getting those vents to work will save you from an unintended stop one day.

MTV

mtv wrote:Ummmm, if the vents aren't working, your fuel tanks should present a vacuum when a cap is opened, and that is not good for fuel bladders. The check valves allow air INTO the tanks, as fuel is drawn from the bladders.

If you're actually getting pressure in your tanks, you have a very unique problem......

But getting those vents to work will save you from an unintended stop one day.

MTV

This is what I thought. I am getting air into my tanks as well, but it's because my left vent is a bit low and needs adjusting.

Felix wrote:My mechanic and I spent yesterday swapping the fuel vent check valve, and I leaned some new curse words along the way. Even so, the job wasn't as bad as I had made it out to be in my mind.

No blood on the knuckles, couldn't of been too bad!

Haha, the blood is on my elbow and forearms from reaching though the top skin into the tank bay.

Interestingly, I looked up the part number on the check valve we removed from the plane. It is in fact the wrong part. Somebody along the way decided to modify a Beechcraft fuel line check valve (hacked off the downstream end of the unit showed below). That person started to drill a pressure relief hole but evidently stopped, thinking the pinhole in the tube would be sufficient pressure relief for the whole system. The bladder is 20 years old, so I guess it was adequate for about that long...



Anyway, glad I listened to the plane and found the cause of this issue (even if it did cost me some elbow skin!). Thanks to the guys who sent me hints and tips along the way!

Nice work, Felix!

mtv wrote:Ummmm, if the vents aren't working, your fuel tanks should present a vacuum when a cap is opened, and that is not good for fuel bladders. The check valves allow air INTO the tanks, as fuel is drawn from the bladders.

If you're actually getting pressure in your tanks, you have a very unique problem......

But getting those vents to work will save you from an unintended stop one day.

MTV

They go both ways. You can also build pressure with cold fuel that warms in the sun, so you need a way to relieve that or bad things can happen. The tanks will always try to equalize with the outside air pressure through those vents, pushing or pulling air as necessary.

I know at least on 182's the FAA recommends vented caps to relieve pressure as well, as a secondary fallback in case the primary system fails. The 182's tank vent is placed slightly differently from the 180 due to how the wing sits on the ground, so I'm not sure if the same recommendation applies in this case.

FYI that valve is actually what Cessna appears to have installed. Either that, or you and I got bladders installed by the same guy. Maybe Cessna raided the Beech factory one night, or ran out of stock and decided their valve was just the better half of the Beech version anyway. Mine looks like it was cut in half as well, right through the labeling that was on it.

My bladder came with the same vent with a written-on date of 1957. Somehow that writing survived being fuel soaked for 60 years which is pretty remarkable. The little spring on the flap wasn't functional any more. If anyone reading this finds themselves with a similar valve and situation, just google search for "S2359-1 valve" and you'll find one. Don't use the part number engraved on the valve or else you'll wind up with the dual-threaded variant pictured above.

Interesting, Colopilot. Out of curiosity did the valve you removed have a relief hole drilled upstream of the flapper valve? Or did it look like mine with just the dimple?

It has a dimple, however the hard line has a small hole a couple inches back from the valve in the middle of the bend. Guessing that serves the same purpose.

Old airplanes often have cobbled installations, inappropriate "good enough" parts. My IA has found a few on my airplane. A few years ago, for instance, I had my IA install a new headliner--the old vinyl one was ripped and torn in places, and while the upholstery wasn't quite a 10, it was easily a 9, and the headliner was more like a 2. So when he took the old headliner out, he discovered that the connecting vent tube between the left and right tanks had apparently developed a leak at some time in the distant past, so instead of replacing it with either a new part or a proper "owner manufactured" part, someone had instead cut it at the leak, placed a piece of rubber hose (fuel line? I don't think so) in place, and clamped it with a couple of hardware store hose clamps. No telling when that was done, as it doesn't appear anywhere in the logbooks (probably for good reason), but from appearances, it was a long time ago.

Incidentally, on 172s with only one below-the-wing vent, the right tank is supposed to have a vented cap and the left one isn't supposed to. It's no biggie if they are both vented, but if along the way, both have been replaced with non-vented caps, or the vented right one and the non-vented left one get swapped, it is problematic, as the tanks are aluminum and will oil can excessively. They oil can enough anyway, so that repairs are common as they get older (been there, done that, with both my tanks). Failure to vent properly will increase the probabilities of cracking.

Cary

The new valve arrived a little while ago, this one has a functional spring! Maybe now my tank won't deposit a gallon of fuel on the tarmac every time I turn after filling up.

This one has a dimple for a hole in front of where the hinge sits, just like the old one. What's different about this piece is there is an actual hole around the side, on the hex-machined area. I imagine this takes the place of the hole in the tube, so I will need to get a new tube made up. This valve is still old (mfr date of 1963) but is NOS and the parts do what they should. Note there is another one like this on eBay now for $170 if anyone needs one.

Here's a pic showing the dimple on top, and the actual hole down on the side:

OK...this post is right up my alley since I replaced the fuel bladders in the 1958 182 I bought back in June of this year. I have pressure or vacuum issues in my tanks when I take the cap off to refuel and push the flapper open on my Monarch caps. I assumed it was pressure in the tanks as I get a blast of fuel when pushing the flapper open. Never thought of vacuum...? Maybe air is being sucked in on opening and causing fuel to blast out? Gotta look closer. But I’m wondering what good a vented Monarch cap is if the flapper seals up the inside of the neck? There are no chains inside my caps.

My vent tube under the left strut is 3/8” lower than when I bought the plane and installed the bladders. Somehow it got pushed out during that process. I can tell because there’s 3/8” of tubing with no paint on it. I need to get that pushed back up but I have to loosen a couple clamps inside the wing to do that. When we got the left bladder out we found the check valve and it looked exactly like the photo above with the bent tubing with the exception that Felix’s new valve is red. Mine is the same as his old one. The bladders were repaired the the mid 70’s and I imagine it was put in then. We got the valve reinstalled with the hinge on the top of the check valve and it seemed to open and close easily when we had it out. I don’t recall seeing any vent hole drilled upstream of the valve. It seems as though if I have either pressure or vacuum build-up in my tanks then that would happen “downstream” of the valve and the above pictured drilled hole wouldn’t make any difference since those holes look “upstream” of the valve.

Either way...a little “blast” of fuel when I open the flapped into my tanks doesn’t seem right. I’ll report back after adjusting the vent tube and also send this post to the Hartwig/Monarch folks.