Hi, Brian,

I read your email with interest, since I've suspected substantial fuel losses (via vapor, not liquid) since I bought a 182 four years ago.  Problem is, I've not been flying a lot, so my measure of fuel loss over the years has been erratic and not well quantitated.

Have you received any response from other pilots, or from Monarch about this?

Thanks--
Lynn

I had the monarchs installed 3 years ago on my 1969 C185 and they worked fine, but in 2008 they started leaking out the top, I replaced one fuel cell but still they leaked out the top, i took off the safety chains from the aps thinking they were vibraing into the flapper and allowing fuel to escape and that worked for a couple of weeks and now i am spewing fule out everytime i fly, last evening over 15 gals in less than 30 minutes,, expensive and wastefull.  Has anyone modified th caps to not vent? if so how to?  Steve

I am pretty sure I have fixed my leaky caps. In my last post I said I thought the problem was the ratchet assy's in the caps. They ratchet to soon and therefore a person cannot screw the caps down tight.
I changed all of the o-rings to viton #326 o-rings, and greased them with silicon grease before I installed them. I also checked for and removed any plastic bumps etc. form where the o-rings sit. There seemed to be seams from the manufacturing process that have little ridges that might cause leaking. Then I checked and bent down, out of the way, the clips where the chain attaches inside the filler necks. One of these clips/tabs was scratching the very end of the cap, and stopping it from rotating. Finally I found that if you put pressure on top of the cap as you screw it in you can get the caps down so the metal part of the cap almost touches the skin of the wing. This means that the o-rings are down in the filler necks and not just barely touching them like before. I flew last night and have not streaks.

I had the earliest "killer" caps on my 55 180 and replaced them with the Monarchs when I replaced the bladders.  I wish I hadn't.  All the above occurred.  Slight weeping from the cap, at first.  A year later, with fuel selector on Both, there are heavy blue streaks on the wing from the cap back.  If I isolate the underwing vent (left wing only on my bird), it still vents out the Right tank's cap.  The cap check valves appear to work correctly, as described above.  So, it must be the gasket/ratchet issue as John states.  I'll try the #326 vitons.

I wish I could have left my 'killer' caps on.

I am a newbie pilot and cessna owner but a lifelong mechanic.  I posted earlier on this site about concerns I had regarding the venting system on my 172F. It's a poor design that wouldn't pass muster on your car or even your lawnmower.  Fuel caps wear out and are normally common cheap replacement parts on most equipment. But not on a Cessna. Your car doesn't spew gas when you drive up a mountain and the pressure changes. But not on a Cessna. The EPA wouldn't let your car or lawnmower spew gas all over the environment. But it allows it on a Cessna.  There is much posted about water in fuel problems on high wing planes and much can be laid on this poor venting system. 

As I am not an aviation mechanic or engineer I cannot propose a fix.  But I can certainly say that something needs to be done.  There needs to be a redundant fuel venting system besides the wing tube and cross tank tube.  Some Cessnas have a AD that involves a secondary vent on one of the tanks.  But when the caps are so prohibitively expensive that nobody replaces them, you wonder if that vent valve will work in a crunch.

Many modern engines have fuel systems that internally vent thru the fuel intake systems.  Is that a solution?  Or a vapor canister?

I don't know folks.  But a problem exists and an answer needs to be found.