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The Parts Of The EVAP System

Vapor goes from the tank into the charcoal canister, then the purge valve feeds it to the engine and the vent valve lets fresh air back in.

How fuel vapor moves from the gas cap and tank through the canister to the engineFuel TankEngine IntakeFresh AirGas CapFiller NeckTank Pressure SensorCharcoal CanisterVent ValveLeak Detection PumpPurge Valve

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Each box opens the guide for that part.

Gas CapIf the OBD II system can tell a leak is from a missing or loose cap, the maker does not have to turn on the MIL if the cPurge ValveFord's test opens the canister purge valve to pull about 8 inches of water vacuum on the sealed system.Vent ValveFord closes the canister vent solenoid to seal the system for the leak test.Charcoal CanisterStores fuel vapor. With the engine running, the computer opens the purge valve and pulls the vapor out of the canister. Fuel Tank Pressure SensorFord's leak tests use the fuel tank pressure sensor and the fuel level input along with the vent and purge valves.Leak Detection PumpAudi describes the leak detection pump as a vacuum driven diaphragm pump run by the engine computer. It needs the engineFiller NeckHonda says a filler neck leak on 2016 Pilot and 2017 Ridgeline is fixed by replacing the mis fuel inhibitor and main fla

How The System Works

  • The OBD II system must check that vapor purges from the EVAP system and must watch the whole EVAP system for vapor leaks to the air, except the tubing between the purge valve and the intake manifold. (source)
  • ORVR systems are carbon canisters in the car that catch gasoline vapor pushed out of the tank when you fill up. (source)
  • Toyota describes two sides of the system. The workhorse side stores vapor in the canister and purges it into the running engine. The regulatory side checks that the system is sealed. (source)

When a shop finds the leak, it is often a hose or a seal rather than the costliest part. See a smoke test we ran on a 2005 E55.

Anthony Limpert in a Supercanic shirt
By , co-founder of Supercanic in San Jacinto, California, a shop with ASE certified mechanics. Updated November 12, 2026.

Author Source EEAT Authors · Anthony's Author Page

Sources For This Page

  1. 13 CCR 1968.2, Malfunction and Diagnostic System Requirements, OBD II, California Air Resources Board (via Cornell LII), read 2026-11-12
  2. Fact Sheet, Final Rule Determining Widespread Use of ORVR and Waiver of Stage II, US EPA, read 2026-11-12
  3. Diagnosing Toyota's Key Off Evap System (February 2016), MOTOR Magazine, read 2026-11-12
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1685 S State St Suite D, San Jacinto, CA 92583. We test EVAP leaks with a smoke machine.

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