Fuel Tank Pressure

[NSFIAT]

I'm hoping I can get some feedback on an issue I've been having. . .

'81 FI that suffers from negative pressure in the gas tank (ie, implodes) if the gas cap is tightened. If I leave the cap loose then I have no problems at all.

I haven't had a chance to start pulling things apart and am wondering if there is a common culprit or a place where I should start.

I don't know if this is related, but the charcoal canister appears to be sealed shut - there is a line coming from the tank to the bottom and a line from the top to the plenum. Is there supposed to be some sort of release valve?

[RRoller123]

The tank only gets make-up air through the vent lines, so there are a few possibilities:

The charcoal canister could be plugged or the vent hole in its top fully closed off. It has a cap (or should have) that meters the proper amount of air resistance through the canister.

The lines back to the tank could be plugged,

The two-way valve (circled) could be plugged.

The merger union (where 3 tubes merge to the one from the charcoal canister could be plugged.

Should be pretty easy to isolate where the blockage is. Maybe try disconnecting from bottom of canister and see if you can blow air back to the tank?

Good luck!

Pete

[NSFIAT]

Thank you for the diagram! Nice and clear and the best I've seen to date.

The charcoal canister has definitely been altered - the vent hole you mentioned has been replaced with what looks like a plumbing fitting. That in turn has been glued in place.

Can a small hole be drilled in the top? Or is that going to cause me grief elsewhere?

[76was124]

Thank you for the diagram! Nice and clear and the best I've seen to date.

The charcoal canister has definitely been altered - the vent hole you mentioned has been replaced with what looks like a plumbing fitting. That in turn has been glued in place.

Can a small hole be drilled in the top? Or is that going to cause me grief elsewhere?

If the plumbing fitting is in place of the cap (vs. in place of the hose connection), then it needs a small hole in it. Something like .062"

[NSFIAT]

Duly noted. Thank you for that! I'll start there then work my way backwards using the diagrams.

[RRoller123]

One more thing, the flange on the circled part (2-way valve) faces back towards the tank. The pressure required to operate the valve in either direction is not equivalent. The low pressure release direction is back towards the tank. i.e if you blow air through it, it is noticeably easier to blow air back towards the tank through this valve, than from the tank up towards the engine.