Normal tank pressure is determined by the roll over valve. This valve is in the evaporative tank vent system. It is usually near the left, rear subframe mount. It blows at 2psi I think. Vapors blown past are accumulated in the charcoal cannister and under load air is pulled across the charcoal into the intake.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician