2000 xj 4.0. I'd say about one in five times when I go to fill up when I remove the gas cap it gives a loud depressurizing sound with a little bit of mist. Wondering what would be causing this?
I have been doing the poor man's prime for almost a year. Maybe this has something to do with it? Not sure. Thanks guys.

 

I dont know about the mist, but the hiss is fine. Every car I've had has done it even my mom's 2013 accord does it. The only thing I can think of for the mist is maybe there is some water built up around the cap.

 

My '92 does the same, only when I fill up when I'm under a 1/4 tank. My guess is the vent hose that runs to the evap canister or the canister itself.

 

Only when your under a 1/4 tank. That's interesting. When i filled up today I was under a 1/4 tank and it did it. Ill have to keep an eye on that.

 

Dollars being needlessly sucked from your wallet, if only we had our own oil........

 

It is the evaporative emissions system working properly. Your tank is stored under a slight vacuum, the vapors from the gas are stored in a charcoal canister and burned while running dowen the road at cruise speeds. When you take the cap off you are letting the vacuum out/ letting atmospheric pressure in...

 

May I ask what the "Poor man's prime" is?

 

Cycling the ignition to get the fuel pump to turn on and prime the fuel system.

 

Sounds like you've got evap/purge system issues.
This is possibly due to a failed gas cap, evap solenoid, a thoroughly pizzled charcoal canister or a cracked vacuum line.

Have you run your codes? Check for enhanced DTCs.