Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Fuel Evaporation Systems: Operation
When fuel evaporates in the carburetor float chamber or fuel tank, fuel vapors pass through vent hoses to the charcoal canister. Fuel vapors are held on the activated charcoal surface until they are drawn into the intake manifold when the engine is running. Charcoal canister purging is done by engine vacuum drawing fresh air through the canister. This fresh air mixes with the raw fuel vapors in the canister, and is burned in the combustion process.
The liquid check valve incorporates a float and needle assembly. If fuel enters check valve (when fuel tank is above canister), float will rise and force needle to close vent passage, preventing liquid fuel from entering vapor hose to canister.
The rollover valve consists of a plunger and a stainless steel ball. When valve is inverted (as in a vehicle rollover), the stainless steel ball pushes the plunger against its seat, blocking fuel flow through valve.
Vehicles with large fuel capacity (over 25 gallons), a two-canister system is normally used. On the two-canister system, fuel vapors from the primary canister are purged through the carburetor port. Vapors from the secondary canister are purged through the PCV hose to the carburetor using a distributor vacuum signal applied to the purge switch. See Fig 1 and Fig 2.
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.