Evaporative Emissions Systems

  By Brandon Baldwin

              This is the first in a series of emissions related Ask-A-Tech articles.  We are starting with EVAP which is short for Evaporative Emissions Systems.  EVAP began in 1971. The first simple systems have very few parts: a charcoal canister, a method of directing the flow of vapors, and lines to carry the vapors.  Remember how we used to see the fuel vapors coming out into the atmosphere as we filled the gas tank?  Have you noticed that doesn’t happen anymore?  It has been said that current vehicles emit fewer emissions while running, than the old cars did while sitting there not running.  I believe it.  I’ve lost an entire tank of fuel on some of my old machines that haven’t run in a year because of the venting.  

In old systems as well as current systems, as you fill your gas tank, the air gets displaced in the gas tank by entering fuel (recall that an empty tank is actually full, just full of air/vapor).  That air that gets displaced is full of fuel vapors.  These vapors are “caught” or absorbed by the charcoal in the charcoal canister as they pass through it and only the air exits the vent, which normally open.  The line that runs to the intake for the engine terminates at a purge valve.  The purge valve is normally closed.  If the valve was stuck open when it shouldn’t be, the result is a hard start after refueling due to the overly rich condition created by the fuel vapors in the intake manifold.  

At some point, the charcoal canister must be evacuated of it’s vapors so  that  it  can  be refilled the next time you fill your tank.   Therefore, the vapors will be drawn into the intake along with the intake air to be burned in the combustion chamber as part of the normal air/fuel mixture.  The method of draw into the intake will be one of a few methods.  For some, you will see a small port just above the throttle plate on carbureted systems.  Once the throttle plate opens enough to have the intake charge pass by the port, the vapors will be drawn in.  In other systems, the port is on the port vacuum side (constant vacuum) where a purge valve will open to allow the vapors to pass  into  the  intake.  The  timing  is determined  by  the  PCM  (current) or heater control valve (old).  This  processed  wasn’t  monitored (to see if it actually happened) until 1996.  It’s been monitored ever since.  And, it’s why 80% of check engine lights are on for.  The new systems also have a vent valve  that  can  be closed.  If  the  vent  valve  is  stuck  closed,  you  will have trouble  getting fuel into your tank because the air has trouble being displaced.  You will find that the automatic nozzle will keep shutting off or you actually have fuel spill or spray on you.  Of course, you guys that have tow trucks  or other trucks with a fill neck that is horizontal may also find this problem.  And, hey, if your system is without problems, don’t fill beyond the first shut-off click.  If you fill your system enough to overflow liquid fuel into the charcoal canister, the only way your vehicle can be cured is to replace the canister (not cheap).

So, we are old car buffs.  Why keep the EVAP system?  For those concerned with our environment, we don’t want the gas fumes to contribute to any unwanted problems.  For those concerned with the cost of fuel, we don’t want to waste any that aren’t making our car move.  For those that are concerned  about  engine  performance, it doesn’t hurt engine performance in the least, especially when it is working correctly (remember we need a vacuum to have the vapors pass into the intake, and there is no vacuum at wide-open throttle.)

Happy Engining (because a motor is electric).

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