Last week the disco threw P0455, the mass evap leak code. I checked the gas cap and sure enough, the wife didn't fully screw it down when she got gas the night before... so I tightened it down properly until it clicked, cleared the code, and on we went.

Then today the code reappeared.

I went back and checked the gas cap again, mind you, I was the one this time who last closed it... and sure enough! It was almost completely unscrewed just like before. What the heck is happening? Is it possible for the gas cap to become unscrewed and basically thread itself off?

We have a spare gas cap from a donor disco, so I'll swap that on and see if it helps. Weird though...

 

The cap needs to "click" at least 3 times.

I've had this happen a few times with LR's & other cars...especially Honda!
In my experience 9 out of 10 times the ratchet mech in the cap was bad.
Grab a new fuel cap and you should be good to go.

 

I've only seen something like this happen on a Abbot and Costello movie...sorry...that's all l've got...lol.

Brian.

 

It also could have been cross threaded in and vibrated loose.

 

I replied in your other thread but here it is again. Might also be your problem. My cap seemed to swell in the heat of the day then contract when it got cooler in the evening etc. What feels fully screwed in when hot will be loose when cold.

I got a new cap for 5 bucks on Amazon and problem is gone

 

I just get the 5.44 STANT gas cap at Wal-Mart, take it apart, use the newer threaded piece, and all internal parts, and then re-use my original cap with the lanyard on it. I've seen the threaded parts of the filler neck get nasty causing the cap to click prematurely. If you look from the underside you'll see the gasket isn't seated against the filler neck. Either use some scotchbrite pads on the threaded filler neck, or replace the gas cap.

 

Here in Oregon they pump your gas (totally stupid), I usually tighten my own cap on the Rover because they (the fuel jockeys) can't seem to tighten it correctly to where it seats. I cut off the lanyard to make it easier for them, cleaned the threads, but it's still difficult I guess.... I may just break down and buy a new cap so I don't have worry about telling someone they might be too stupid to correctly tighten down my gas cap....lol