Purge Vent & Evaporative Systems

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Purge Vent & Evaporative Systems

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What are the evaporative emission system parts and where are they located? 

The evaporative emissions system keeps the gas in your fuel tank from escaping as vapor.  Gasoline evaporates at a fairly low temperature.  That’s good for combustion but bad for keeping the gas in your tank.  About 20% of vehicle emissions come from evaporation, so federal regulations have required cars to have systems to prevent gas from evaporating into the atmosphere since the 1970s. 

Before these systems came into being, vaporized gas could escape from the fuel tank.  The vapor escapes through a vent in the gas tank cap to a charcoal filled canister called the evaporative emissions canister.  The charcoal absorbs the gasoline.  Under certain pressure and temperature conditions, the vehicle’s computer sends a signal to open the vapor purge solenoid, sometimes called the vapor vent solenoid.  Vacuum from the engine pulls the vapor out of the charcoal, through a vacuum tube, into the intake manifold.  The fuel vapor can then be combusted to power the engine.  Now the gas that would have been lost to the atmosphere can be used by your engine. 

The evaporative emissions system relies on proper pressure being maintained throughout the system.  The leak detection pump (LDP) pumps air pressure into the system.  The computer receives a signal from the LDP to check the time it takes for air pressure to fill the system.  If it pressurizes too quickly, then there may be blockages in the system.  If it takes too long to pressurize, then there may be a leak.  Either case will result in the check engine light illuminating. 

How do I know if my ­­­­evaporative emission system parts need to be replaced?        

As mentioned above, the leak detection pump checks the evaporative system for leaks and will turn on the check engine light.  Evaporative leaks can also cause you to fail an emissions inspection.  There can be other signs of evaporative system problems, though.  As you might expect, leaking gasoline vapor can decrease your fuel mileage.  It can also lead to hard starting and rough idling if air gets into the fuel lines through leaks. 

Determining the exact source of an evaporative leak can be tricky.  You can start by visually inspecting the vapor canister and lines for cracks or holes.  A mechanic can run vacuum tests on the canister and the purge valve. 

What if the problem is with the leak detection pump itself?  Before running other tests with the LDP, the computer checks for electrical and mechanical problems in the pump.  Any problems with the LDP will result in the check engine light turning on. 

Can I replace the evaporative emission system parts myself?  

The difficulty of replacing any given evaporative emission system part can vary from part to part and from one vehicle to another.  The purge valve is located in the engine compartment, usually near the brake master cylinder.  You can disconnect the electrical connection and vacuum lines, remove the purge valve from its bracket, and install and connect the new one.  The process for replacing the canister itself or the LDP is more or less the same, although they are harder to reach.  The canister is found at the rear of the car near the fuel tank.  The LDP is found in the rear wheel well behind a heat shield.  You may have to remove some exhaust components to reach the LDP.  For that reason, you may prefer to have an LDP replaced by a professional mechanic.  

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