If the pressure sensor does not see an increase in vacuum pressure, a code will be set. If the purge valve and canister valve are closed, there should be a vacuum in the tank. Under these conditions, the fuel pump is pulling fuel from the tank. Part of the EVAP monitor for some Hondas is completed when the engine is idling. These tests are part of the EVAP monitor. But, most codes are generated by an incident when managing the fuel vapors.Ī series of tests ensure the integrity and operation. Testing is carried out all of the time if the conditions are right. The two valves and pressure sensor are used to test for leaks in the system. The reading from the pressure sensor should change only if the fuel is heated and cooled, or fuel is removed from the tank by the fuel pump. If both valves are closed, it means that the system is sealed. Cooler fuel means the charcoal canister can be downsized and the purge valve doesn’t have to work as hard. Heated gasoline produces more vapor and evaporative emissions. On older systems with a return line and a regulator on the fuel rail, the fuel would be heated. Interesting fact, the main reason why manufacturers switched to returnless fuel systems, with the regulation of pressure and volume in the tank, was not for safety or performance it was changed to improve evaporative emissions. The same strategy is used to detect leaks. If the opening or closing of a valve does not produce the desired result, a code will be set. The pressure sensor also checks the operation of the valves. This valve is called the canister control solenoid or valve.Ī sensor measures the pressure inside the tank. The other valve allows air into the tank to equalize pressure, or it can direct the vapors into the charcoal canister so they can be burned off later. One valve vents the vapor into the intake manifold it is called a purge valve. The EVAP system controls the pressure inside the tank with two valves on most Honda models. If there were no EVAP systems, the gas tank would collapse or expand as fuel is burned or with changes in temperature. Instead, the vapors are burned in the engine and outside air is allowed to replace the volume of fuel consumed. During these changes, the fuel produces vapors that can’t be vented into the atmosphere.

The fuel in a gas tank changes in volume due to temperature and consumption. But, if you concentrate on the basics of EVAP solving some of the most difficult EVAP problems, diagnostics gets a lot easier. These two mandates are what makes EVAP diagnostics difficult because to pass the test, automakers are left on their own to design the EVAP system and software to control it. Like the test chamber, the EPA does not mandate any specific technology or monitor readiness procedures. The other part of the EPA’s mandate is that the vehicle has to be able to detect a leak as small as 0.0020” or as large as the gas cap being left behind at the gas station. If the sensors detect fuel vapors above a certain level, the car fails. The EPA test does not care about the size of the charcoal canister or the placement of the purge valve.

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