The check engine light caused by a loose gas cap is the equivalent of a Big Foot sighting for consumers and shops — everybody talks about it, but very few drivers have actually seen it in the flesh. Many drivers come to a shop complaining of a check engine light and say, “I checked the gas cap and it is still on.”
It all started in 1996, when every new vehicle sold had an Evaporative Emission Control System (EVAP) that could detect leaks thanks to OBDII.
Modern Diagnostic EVAP Concept
Modern EVAP systems are selective when they vent vapors into the engine. The PCM will select the right engine temperature and throttle position to purge the system. They are also very selective when system checks are performed. A computer is constantly monitoring the results of the commands sent to the valves with the help of sensors mounted in the tank and EVAP canister.
The system traps the vapor and moves the vapors using pressure differentials between the components and engine vacuum. Think of the components of the EVAP system as locks in a canal. Instead of using gravity, the EVAP system uses engine vacuum to move the vapors from the tank to the engine.
The start of the journey is in the fuel tank. Inside the fuel tank is a barometric-style sensor that senses changes in air pressure. This is the main EVAP sensor used to detect pressure changes caused by leaks and actions of the solenoids in the system.
The system regulates this flow with a series of valves. The valves come in two style: vent and purge. Some systems combine the valves into one component. Vent solenoids allow outside air to enter the system to control the pressure differential (some vents even have small air filters). Vent solenoids can be found connected to the tank and on some EVAP canisters. Purge solenoids typically move the vapor between the tank, canister and engine. The ECM/PCM controls the valves, and some vehicles have dedicated EVAP modules that connect to a serial database.
The valves and pressure sensor can also be used to perform leak tests. The valves can isolate the fuel tank, and the pressure sensor can measure changes in pressure over time, as either the fuel pump removes fuel from the tank or the fuel cools down when the car is parked. This can be performed with the vehicle running or with the key out of the ignition.
This is where it gets complicated with evaporative emission monitors and testing. In order for some operations and testing to be performed, certain criteria must be met. These conditions can range from how much fuel is in the tank to how long the vehicle sits overnight.
Fuel Level
The fuel level in the tank is critical for some vehicles to test the integrity of the EVAP system. If a vehicle is driven around empty or full, chances are it will not be able to test the system. This is important for key-on and key-off testing because a certain amount of air volume in the tank is required.
If a vehicle is having difficulty resetting the EVAP monitors, try asking the customer to run at least a ½-full tank of fuel for a while. Also, if there is an issue with the fuel level sensor, it will also prevent proper operation.
Temperatures
EVAP systems need know the temperature of the engine to be able to operate and test the system. Most systems will not purge vapors into the engine until the vehicle has reached a specified operating temperature.
For some key-off tests, some systems will look at ambient air temperatures. The system typically wants the fuel to be at an ambient temperature so there is a diminished possibility of releasing excessive amounts of fuel vapor.
Some vehicles will require a period of eight hours below 90 degrees F before performing some tests. Check the service information for specific procedures.
System Voltage
A weak battery can prevent some systems from carrying out EVAP testing. For example, GM needs the system voltage between 10 and 18 volts to perform EVAP testing. For some vehicles that perform key-off testing, battery voltage is even more critical.
Drive Cycle
EVAP monitors are some of the most difficult to reset because not only do they require certain environmental criteria to be present, but specific drive cycle events must take place. Also, to reset the monitor may require multiple cold starts
