Fellas,
I'm having a serious amount of fuel siphoning from one of my Monarch Caps on my 61 Cessna 182.

I've noticed the problem before on my left wing and called Monarch. They told my it was the vent tube out of alignment pressurizing the tank. Didn't buy it. Measured the vent tube per instructions. perfect. Did a quick diagnostic by switching caps. Yep, the siphoning now occurs on the right side.

Lost 8 gallons of fuel on a flight where I actually burned 10 gallons. (fuel flow monitor)

As I was flying into the backcountry, I saw a friggin contrail off my right wing and had to return to base.

Right now I hate these things. They are sending me a couple of new o rings but this should not occur after only a few years of use.

Anybody experience this? (I'm sure its been posted before but I'm done for today. Sat in an FBO for 6 hours waiting for weather to clear before discovering problem).

Wish I could post a picture of my 1990 originals to see how different they are than yours. Mine has a spring loaded pressure relief deal on the underside of the little flapper deal at the bottom of the neck. I flew mine one time when I forgot to put the cap back on and didn't lose a drop. The flapper has a rubber seal it mates with to make a very good seal. Do you think supercub.org would frown on me using their website to link to the pictures? :

Just curious, Last time o- rings replaced? Has worked ok with no leaks since install for a couple of years? Some peeps have no problems, others have nothing but problems. Sure would like to find out what causes the problem.

One of the things I noticed about my monarch caps is that I could get them a little tighter if I applied down pressure on the caps as I was turning them on, rather than just turning until they clicked.

Something to check anyways, might cure your problem.

I put a bit of lubrication on the o ring and got it to seal but the problem has always been a bit intermittent.

I have noticed that it sometimes screws down tighter than other times.

I'm thinking its an o ring problem more than anything.

Idahomike wrote:Fellas,
I'm having a serious amount of fuel siphoning from one of my Monarch Caps on my 61 Cessna 182.

I've noticed the problem before on my left wing and called Monarch. They told my it was the vent tube out of alignment pressurizing the tank. Didn't buy it. Measured the vent tube per instructions. perfect. Did a quick diagnostic by switching caps. Yep, the siphoning now occurs on the right side.

Lost 8 gallons of fuel on a flight where I actually burned 10 gallons. (fuel flow monitor)

As I was flying into the backcountry, I saw a friggin contrail off my right wing and had to return to base.

Right now I hate these things. They are sending me a couple of new o rings but this should not occur after only a few years of use.

Anybody experience this? (I'm sure its been posted before but I'm done for today. Sat in an FBO for 6 hours waiting for weather to clear before discovering problem).

You may have a look at these other threads:

https://backcountrypilot.org/forum/monarch-fuel-caps-20643

https://backcountrypilot.org/forum/skywagon-monarch-caps-20305

I thought that the Monarch leaking issue was solved, some Monarch necks have the chain retainer punched out a little too far and it interferes with the cap.
However, since your leak moved with the cap, and didn't stay with the filler neck, the problem might lie elsewhere.
I'd still double-check the chain retainer on both sides to make sure that isn't the problem.

180Marty wrote:Wish I could post a picture of my 1990 originals to see how different they are than yours. Mine has a spring loaded pressure relief deal on the underside of the little flapper deal at the bottom of the neck. I flew mine one time when I forgot to put the cap back on and didn't lose a drop. The flapper has a rubber seal it mates with to make a very good seal. Do you think supercub.org would frown on me using their website to link to the pictures? :

I guess the Monarch cap design has evolved over the years?
Dunno when the Monarch kit I had (and sold) was purchased,
but as I recall it was different from how you describe yours--
no "pressure relief deal" on the flapper,
and no rubber seal for that flapper to seat against.

Hotrod will you private message me your email? I could send pic's and maybe you could upload so people can see the difference

Here's the pics Marty emailed me showing his Monarch caps.
They look a lot different from the ones I had (and sold).

Update: I received new o ring seals from Monarch and it seems to have fixed the problem. I tried to order two pair to have a pair as backup but I got a bit of pushback for some reason. I'd love to find the o rings from another source.

I figure I lost 20 gallons of fuel between all the intermittent problems.

Would be nice to find the seals from another source. $50 for a pair is ridiculous. Problem is that -u-cup design, fills a pretty big gap, more than just a standard o-ring.

Yup. New o-rings should solve it. Also, as noted in other threads, you can extend their life by flipping them over. They guy at Monarch suggested this to me. He proudly told me he was the only source for those special made o-rings, which sucks because of the extortion prices for them. The design did change and the part number that is in the installation instructions refer to the original ring. The new ones are much different. I'm limping mine along but will need new ones before long. I was told to replace them every 5-6 years by the guy at Monarch.

FYI, if you're thinking of ordering these, my mechanic was recently told to expect the better part of two months to get them.

I have 180 with the caps.did have leak after a few years was torgue on plate srews.lost a cap once could not get new one right away. My 97 chev pickup had ratcet cap tried it worked prefet. I had a locking cap it worked.looks like the same o ring.i carry it as spare.

I just looked at my invoice after receiving the new o rings. Geez.
I'll be looking for other sources that will work. I don't think there is any magic in the Monarch Special O rings.

Update:
So I borrow my brother's truck to go to the dump and I filled up his tanks for the help. Low and behold, he had a Stant Replacement Cap on his 95 Ford F250 with a VERY similar O-ring as the Monarch Cap O ring. Same odd 2 lip design. I went by my local Napa auto parts and their brand name replacement cap also had the same style 0 ring.

I'll be bringing my old o ring to compare sizing. Hopefully, it will match up.

120+ bucks for 2 O-rings to be fedexed to me doesn't cut it.

Cool! If it works out, please post a part number or a pic of the label! Thanks!


Sent from my iPhone using Tapatalk

So, the o-ring I flipped a year ago or so started to leak. Tried looking all over for just the o-rings with no luck. Then went to Autozone to look at gas caps. Sure enough, very similar o-rings used on a lot of caps. Most have an inner diameter of 1.57” or 1.62”. I chose the first one and it fit fine. With a fresh o-ring on there, the fuel cap that wasn’t setting well and was actually vibrating loose during flight, now snugs down flush and tight like it should. $11. Better than $20! Hope this helps.

Part number Duralast 6817A.

Thanks for the tip. Funny how the ~28 year old ones I have still work and it is an effort to unscrew my caps. Hadn't even given vibrating loose a thought.

Just wanted to post an update from @grassstrippilot rec above. Had one starting to leak after 5-6 years in service, and another starting to crack, and didn't want to wait for more from Monarch. Tried the Duralast 6817A as mentioned above, and no leaks after a few flights. Checked out a few other caps as well, and most seem to have pretty similar geometry.

Might still get a few Monarch ones, but will run the Duralast for a while to see how it continues to go.