Evaporative Emission System

What does it mean when we have an evaporative emission code, often called the "gas cap code"? These would be the codes from P0440 through the P0450's.

Gas hose valve

Gasoline evaporates rapidly when heated up from driving or in hot weather, releasing volatile compounds (bad stuff) into the air.
There are more than 150 chemicals in gasoline, including benzene and toluene, and the fumes are one of the leading causes of smog and air pollution.

In the 1970's it was mandated that the fuel tanks in our cars and trucks be sealed to keep these toxic fumes out of the atmosphere. These evaporated fumes have to have a place to go to keep the pressure in the fuel tank from getting too high. The fumes are stored in a charcoal canister and as you drive they are slowly released into the engine to be burned as fuel. This system is called an evaporative system. It also keeps moisture from getting into your fuel and keeps the fuel from losing key ingredients over time.

A typical system consists of a small canister full of charcoal, valves, hoses and vents in the fuel lines and a sealed fuel tank cap. When fuel evaporates inside the gas tank, the excess vapors are transferred to the charcoal canister until they can be transferred back to the engine to be burned with the normal air-fuel mixture.

The early systems were mostly ignored and would often fail and vent gasoline fumes. In 1996 it was mandated that the on-board computer needed to be able to test the system for leakage and proper operation to prevent this from happening.

This required solenoids and diagnostic units to be added for the engine computer to test the system periodically for leakage and proper flow. When the computer detects one of these problems it will set a code. It can set a code for a large leak and a different one for a small leak. There are also codes if it detects it is not purging or other problems. A loose gas cap often sets a leak code but not one of the other codes.

Evaporative System

Simple Evaporative System


Common problems with evaporative systems include sticking or bad purge and vent solenoids, leaks in vent and vacuum hoses, and loose or bad seals on gas caps. The code we don't want to see is a P0442. This indicates the system has detected a very small leak, possibly as small as a pin prick. Small leaks can often be very tedious to locate and require quite a bit of time to trace down. An intermittent sticking solenoid can also be very frustrating to diagnose.

There are times when an evaporative system problem can cause mileage, performance or fueling problems, but most of the time it doesn't cause any noticeable problems.
The big downside of not fixing an evaporative system, besides the environmental issue, is if the check engine light is already on, you will not be warned of new problems cropping up.