Difference between regular and improved EVAP systems.

by Zacharh Hui
(Covina, CA USA)

Older vehicles that had an evaporative emissions system, prior to the enhanced systems we see today, usually would use engine vacuum to purge the evaporative vapors in the fuel tank. By doing so, this would prevent those vapors from escaping to the atmosphere and cause pollution. This system had great intent, but was not fail-proof. The vehicle's computer or brain could not determine if the vapors had actually been purged into the engine's intake system to be burned off. These systems were "dumb" because there were no computer parameters in place to verify if purging had happened, and also because the system was not robust enough or equipped with any sensors to check for any escaping vapors that could result in pollution.

On newer vehicles, equipped with the improved EVAP systems we see today, the vehicle's computer now has the capability to detect leaks of vapors to the atmosphere, and to detect the systems operation and capability to purge and vent vapors. These new systems use various sensors and solenoids, along with computer software, to determine the condition of the evaporative system and verify whether or not the system has any vapor leaks and also verify if the system is correct working order. These sensors usually include a EVAP purge valve that's on or near the vehicle's air intake system, a charcoal canister to filter vented vapors before they are released to the atomstphere, a vent solenoid of sorts- which usually used to close off the vent at the canister so the computer can check and test the integrity of the EVAP system sealing, and sometimes a Leak Detection Pump, which serves the same purpose- to determine if there are system leaks and correct purge and vent flow.

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