by Jim Kerr

When the “check engine” or MIL (malfunction indicator light) shines brightly on the dash display and the vehicle seems to run fine, there is a good chance you have just become the latest victim of what many technicians are calling the “gas cap code”. Most 1996 and newer vehicles are designed to meet On Board Diagnostic Level 2 (OBD2) emission regulations. Some of the regulations deal with evaporative emissions, and that is where the “gas cap codes” come in.

Evaporative emissions refer to all the hydrocarbon vapours that evaporate from our vehicles. In the 1950’s and 60’s, evaporative emissions were high. In fact, a 1960’s vintage automobile produces more total emissions just sitting in the driveway than a new vehicle does on a 300 kilometre drive! In the past, unsealed gas tanks let fuel vapours out to the atmosphere. Draft tubes let crankcase oil fumes escape. Oil leaks put more oil vapours into our environment. Since then, evaporative emissions have been reduced to almost zero!

Testing for evaporative emissions is done in a “shed”. This shed is a sealed garage where the vehicle can be driven into, parked for twenty four hours at a controlled temperature, and all the hydrocarbons in the shed’s air are measured. To control these emissions, any system that contains oil or fuel must be sealed tightly.

Crankcase fumes are now fed back into the intake manifold of the engine through the PCV (positive crankcase ventilation) system. Gas tanks now have sealed filler caps. All the gasoline fumes that evaporate inside the tank are stored there or in a carbon canister (filled with activated charcoal) on the vehicle, and are burned in the engine the next time the vehicle is driven.

Before the days of computer controls, the carbon canister system was controlled by vacuum switches and temperature sensors. Today, the engine computer uses all its inputs to calculate when to activate solenoids that control flow to and from the canister. Starting in 1996 with OBD2 regulations, the parameters for testing evaporative emission systems changed. Before OBD2, the computer did not verify the system was fully functional. The computer could set lean or rich exhaust codes if it saw a fuel mixture problem, but these codes could be related to other components other than the evaporative emissions system. Now the system undergoes eight specific tests to verify its operation.

Two solenoids are used to control the flow of fuel fumes from the carbon canister into the engine. A purge solenoid opens the canister to engine line, and a vent solenoid opens the canister to atmosphere so clean air can be drawn into the canister to aid purging the fumes. The computer tests each solenoid electrically, and then tests it functionally. If the purge solenoid is open and the vent solenoid is plugged, vacuum builds up in the canister and fuel tank. A pressure/vacuum sensor inside the fuel tank signals the computer of a vacuum build up and sets a code. The MIL engine light comes on if the test fails twice.

The so-called “gas cap code” occurs because there is a small vacuum leak in the system. To test to see if the system is intact, the computer is programmed to place a vacuum inside the fuel tank by opening the purge solenoid and closing the vent solenoid. It then closes both solenoids and measures how long a vacuum is held in the tank. If the vacuum does not hold for several seconds, then a code is set. Any leak that flows more air than a .040 inch hole would, will set a code. Loose hose connections, leaking solenoids, or a loose gas cap are common causes of this code.

Simply tightening the connections or gas cap may be all that is needed, but it is not that simple. Once the MIL light has come on, the system must pass its tests three consecutive times before the light will be turned off. It can be also cleared with a scan tool at the dealership. Conditions to run the test are very exact, so it can take a lot of driving to turn the light off again. For example, on a specific vehicle to run the test once, the engine must be started cold, the fuel tank level must be between 24% and 75% full, the outside air temperature must be above 7 degrees, C, and the engine must be operated for several minutes. If all the conditions are met, the test runs. Pass the test three times and the light goes out.

Many drivers do not understand why sometimes the light comes on with a loose gas cap, and the next time it will not. It all has to do with the conditions programmed into the vehicle’s computer to run the diagnostics.

It may be a nuisance to have that MIL come on brightly on the dash, but it helps us protect our environment. Keeping that gas cap tight will help keep that light off too.

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