Gas tank building up pressure

[82DMC12]

I had an interesting problem crop up suddenly a week ago. I was driving around town and at stop lights I would start to smell raw fuel. I pulled over and looked the car over and could find no visible leaks and the smell was not in the engine compartment, so I proceeded the short distance home and parked in the driveway. I opened the gas cap (repro from DMCH which I've had for years and never an issue) almost popped up into my face there was so much pressure behind it.

I removed the rollover valve since that's the first likely thing to jam from the gas tank back and it seemed to be working OK. The inside valve is loose and I can blow through both ways when it's right-side-up but can't when it's upside down. I then removed the line from the vapor-in port on the charcoal canister and I blew compressed air through it, confirming it's clear. I also checked and confirmed I have vacuum when the engine is running at the other two ports (valve open port and fume suction to engine port). However I don't know if the charcoal canister is OK internally. I removed the rollover valve and bypassed it, reassembled everything else, and the next few days went on a 1,200 mile road trip with zero pressure build up issues. I think I will reinstall the rollover valve and watch for problems moving forward, but I'm wondering if there's a good way to test the charcoal canister? If I use a vacuum pump on the valve open port , should I be able to blow through the other lines clearly?

One note is when this was happening, I noticed the larger hose that connects to the oil filler cap / PCV was NOT connected , I think I forgot to connect it after an oil change two weeks ago. Perhaps this being disconnected allowed the intake manifold to be vented to the atmosphere and I didn't have enough vacuum to open the purge switch??

[David T]

I had an interesting problem crop up suddenly a week ago. I was driving around town and at stop lights I would start to smell raw fuel. I pulled over and looked the car over and could find no visible leaks and the smell was not in the engine compartment, so I proceeded the short distance home and parked in the driveway. I opened the gas cap (repro from DMCH which I've had for years and never an issue) almost popped up into my face there was so much pressure behind it.

I removed the rollover valve since that's the first likely thing to jam from the gas tank back and it seemed to be working OK. The inside valve is loose and I can blow through both ways when it's right-side-up but can't when it's upside down. I then removed the line from the vapor-in port on the charcoal canister and I blew compressed air through it, confirming it's clear. I also checked and confirmed I have vacuum when the engine is running at the other two ports (valve open port and fume suction to engine port). However I don't know if the charcoal canister is OK internally. I removed the rollover valve and bypassed it, reassembled everything else, and the next few days went on a 1,200 mile road trip with zero pressure build up issues. I think I will reinstall the rollover valve and watch for problems moving forward, but I'm wondering if there's a good way to test the charcoal canister? If I use a vacuum pump on the valve open port , should I be able to blow through the other lines clearly?

One note is when this was happening, I noticed the larger hose that connects to the oil filler cap / PCV was NOT connected , I think I forgot to connect it after an oil change two weeks ago. Perhaps this being disconnected allowed the intake manifold to be vented to the atmosphere and I didn't have enough vacuum to open the purge switch??

Check the hose coming out of the bottom of the vapor canister. It gets plugged up by insects.

[Timeless]

When I had a fuel smell in the cabin I found the vent hose for the bottom of the canister missing. New hose installed - smell gone. I did not have tank pressure build up. (DMC repro cap)

[DMC-Ron]

Andy,
Some gas tank pressure is considered normal according to this reference in the workshop manual:
Workshop Manual D-01-16.jpg

Your tank pressure does sound excessive. I will sometimes notice a small pressure release when I stop to fill up.
Ron

[82DMC12]

So after removing the rollover valve I haven't noticed any pressure build up in the tank at all, not while idling, not while getting gas, and not even after my car sat for a week in the garage, untouched, and I just now slowly removed the cap to see if there was any pressure. None.

I have a repro fuel cap installed.

I had removed the charcoal canister from the car while I was struggling with this and shook it up a bit. Maybe that loosened some packed up charcoal or something. I have the hose at the bottom poking out through the pontoon but I didn't look inside to make sure it's clean. I will remove the charcoal canister again and run a scope up it and make sure there's no visible trouble. I might also try to figure out how it works using a vacuum pump. If all is good I guess I'll put the rollover valve back on.

The only thing that makes sense to me is I was driving around without the hose to the PCV / oil cap connected and once I reinstalled it, at the same time I had the canister out, and when everything was back together, no more problems.

[82DMC12]

Still don't really know how the charcoal canister works. I removed it from the car and pulled vacuum with a mityvac at the PCV port , first to 15 psi, then 30 psi. The switch holds vacuum. While under vacuum, there still doesn't seem to be a way to blow compressed air through any other port. I guess I'm just blowing air into the thick charcoal bed. I removed the hose at the bottom and blew it out. There was some debris in there but not much.

Buttoned it all up again, assuming it works as intended.

I'll reconnect the rollover valve and see if the problem returns another day.

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