By Jim Kerr

If you drive a 1998 or newer vehicle, you may have had the “Check Engine” light come on for no apparent reason. The vehicle seems to be running fine except for that glaring warning light on the dash. Some of the newest vehicles may display a warning message on a Driver’s Information display: “Check gas cap”! These warnings are often the result of a fault of the vehicle’s evaporative emissions system.

Evaporative emissions are all the vapours from a vehicle as it sits or drives. While there are some emissions from rubber, plastic and vinyl components, most evaporative emissions come from either the fuel tank or the engine crankcase. Regulations have limited the allowable amount of evaporative emissions and the systems auto manufacturers have developed to contain them have been very successful. For example, a new vehicle produces less total evaporative and exhaust emissions during a 200 km drive than an early 1960’s car does just sitting in the driveway for a day. To eliminate these emissions, the fuel tanks and engine crankcase have been sealed.

When a vehicle is operated, gas is pulled from the fuel tank. Air must be allowed to enter or the vacuum created would cause the tank to collapse. A sealed fuel system must not only be able to contain the vapours but also allow air in. The charcoal canister, electrical solenoids and fuel injection computer accomplish this task.

When the vehicle is sitting, the fuel system is sealed. As gasoline evaporates, the fumes are stored in the activated charcoal of the canister. Crankcase fumes from evaporating oil are also routed to the canister or contained inside the engine. When the engine is started, fumes from the crankcase and charcoal canister are pulled by engine vacuum into the engine intake manifold and cylinders where they are burned. The fuel injection computer also switches a solenoid to allow air back into the fuel tank.

The injection computer tests the fuel system sealing and will turn on a warning if it detects any potential vapour leaks. Currently, auto manufacturers are using two methods to detect fuel system leaks. With the engine is running, the computer operates the solenoids to allow the engine to pull a slight vacuum in the fuel tank. The system is then sealed and a pressure/vacuum sensor on the fuel tank signals the computer how long vacuum is held inside the tank. If a leak is detected, the computer sets a trouble code.

The second method of testing for vacuum leaks monitors the vacuum in the fuel tank after the vehicle has been parked. The vehicle must have been driven for several kilometres and outside temperatures have to be within a specific range before the test will run. As the vehicle is driven, unused fuel returning to the fuel tank from the engine fuel rail heats the fuel in the tank. When the vehicle is parked, the fuel cools, causing a slight vacuum to develop in the sealed fuel system. The computer monitors the vacuum level for 45 minutes or longer after the vehicle is parked, and again, if there is not enough vacuum, a trouble code is set.

Often, the evaporative emissions warning is the result a fuel cap not tightened enough. Before the warning light will be turned off however, the computer has to test and pass the system test three consecutive times. Depending upon outside temperatures, this could take several days or more! Many drivers will take it to the local repair shop where a computerised scan tool is used to clear the codes and turn the light off.

There are many other parts that can leak however. One-way check valves inside the gas cap could leak. Hose connections between the filler pipe and the gas tank may be slightly loose. Even the rubber 0-ring used to seal the fuel sender unit in the fuel tank can allow air in. It is difficult to find these leaks, as the computer will set a code for any leak caused by a hole .020 inch or larger.

Technicians often use a special smoke generator to fill the system. They then look for smoke leaking from joints and fittings with an ultraviolet light that highlights any visible smoke. Even then, they may not find a leak because the materials have expanded with temperature and the leak is temporarily sealed. Perhaps that is what makes finding these leaks so frustrating. Diesel owners, don’t be concerned. Your vehicles don’t have evaporative emission controls on them – yet.

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