348 vapor canister purge valve removal

So obviously this was an older thread, but it was a good read and educational for me. Being a new owner of a 348 I noticed that I had pressure when taking the fuel cap off after driving. I initially couldn't tell if it was positive pressure or negative pressure, but I suspected positive pressure since the cap wasn't difficult to remove and I could smell fumes that seemed to have "blown out" from the filler. So I put a balloon over the filler instead of the cap (secured with rubber bands) and ran the car. Sure enough the balloon was swelling out so I knew it was positive pressure. I figured this was a good sign, better than negative pressure which could obviously cause tank implosion as discussed in this thread.

So after reading here....I decided to investigate a bit. Another owner told me that he too had positive pressure and it may possibly be by design. One thing that wasn't real clear to me after reading this entire thread was whether or not the anti-tilting valve was also the 2 way valve discussed in the system, but after a visual inspection of all the components I decided it had to be.

I first ran the car to build the aforementioned positive pressure. Then I took the left rear wheel off and removed the inner fender (front portion) which gave me access to the anti-tilting valve, I disconnected the hose from the valve to the evap canister. No air released (a good sign). I then took the hose that leads to the fuel tank venting off of the anti-tilting valve and sure enough air rushed out of the tank (the positive pressure) and out the now disconnected hose that was attached to the valve. This confirmed that the anti-tilting valve was holding the positive pressure in the tank.

Next I used an air compressor to blow air into the disconnected anti-tilting valve to test its operation. When blowing air through the valve in a direction that would mimic air coming from the evap canister through the valve and into the fuel tank venting hoses, it flowed easily and smoothly at any pressure given. This confirmed to me that the fuel tank had no problem replacing any displaced air from fuel flow. Then of course I tried blowing air through the disconnected anti-tilting valve in the opposite direction (as if fuel tank venting to the evap canister). What I found was that at low pressure the valve did not let air through. At a bit more pressure however, it opened and let air though. At too high a pressure applied it closed back up. The last thing I did was try air the same direction (tank venting to canister) with the valve inverted and sure enough it stayed closed and didn't let air through.

So after all of that, I came to the opinion that my anti-tilting valve is functioning as intended and that positive pressure in the tank is by design (due to thermal expansion). So my conclusion is that a tank imploding must be from an anti-tilting valve malfunctioning. There doesn't seem to be any redundancy in the system (I come from the airplane world where there are lots of redundant systems!), so I'm thinking about putting an extra anti-tilting valve in parallel with Y fitting on each side of what would be 2 anti-tilting valves. Thoughts?

I do want to check my evap canister at some point just for good measure, as I suspect it is original. Matter of fact, if anybody has a bad/junk canister laying around and want to send me, I would take it with the goal of reverse engineering it and creating a 3D printed replacement that has a seal-able, serviceable top/bottom, since some of you discussed the readily available charcoal material. If it worked out I could make the file available.

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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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