1992 348 Gas Tank Pressure!

Hey guys I've got a 1992 348 TS well maintained and runs perfect.... Until today! Took it for a one-hour ride and when I got to where I was going when I shut the car off I could hear a loud hissing sound coming from the engine bay. Couldn't find anything but I could smell fuel vapor. I decided to open the fuel tank cap and it was so hot it scalded my hand! That in and of itself was very worrying to me! As soon as I loosened it, the pressure equalized and the hissing sound stopped. Just for the heck of it I started the car and it started fine. I'm baffled on this one any suggestions would be greatly appreciated. I'm thinking maybe the charcoal canister could be clogged? But I'm not that familiar with the system.

 

PS. I did search the topic and there's not much out there most folks say this is normal but I got to tell you there was more pressure in this than a radiator and the fuel cap was just as hot if not hotter than a radiator cap which again is very worrying to me..... then again the fuel fill is directly above a header! It's just unbelievable how things are shoehorned in there!

 

Awesome guys.... will search tonight and report back! The pressure was unbelievable in this fuel tank I guess on a positive note there was no fuel dripping anywhere...everything was dry only Vapor. On the downside, vapor is more explosive than liquid gas!

 

Excellent write-ups by both Ernie and Jason! My problem is more like Jason's rather than Ernie as I have positive pressure. I disconnected the tank rollover valve and it blows through freely, unrestricted. Ufortunately I haven't tried it when the engine bay is hot as I realize that may affect its function so I'll do that asap. In the meantime a quick Google search and I found what looks to be a pretty awesome drop-in rollover valve replacement which actually is from the Mercedes 124 class!
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Just an update: My charcoal canister valve is clear as well as the tank rollover valve... and they're both clear even when the engine bay is hot and at operating temp. One interesting thing, and I think Ernie touched on it, the 2 purge valves, when they are not energized with engine off, are normally open and when the engine is running (at start up cold) they are energized and closed. If you unplug them while running they open and the amount of vacuum from the engine is remarkable! I'm so stupid I forgot to check if they were open when the engine was hot at normal operating temp I'll try that and report back. But again, I was stunned at how much vacuum is created.... certainly enough to implode a flimsy fuel tank or charcoal canister. Again in my case I had massive pressure but I was stuck in bumper-to-bumper traffic because it was an accident and I have a feeling that contributed to the problem because I've never really had the pressure in the fuel tank like this before so I'm having trouble recreating it. But evidence looks like my Purge valves are not opening for some reason....ie not energizing.

 

This system is so ridiculously engineered... with catastrophic consequences like what happened to Ernie If a purge valve gets stuck open. Im actually considering just leaving my gas tank lid slightly loose. This will keep too much pressure from building up and will also prevent vacuum from building up in the event of a valve failure.