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Old 02-05-2005, 08:00 AM
  #8  
rparkssos
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Join Date: Jun 2004
Location: Groveland, MA
Posts: 1,047
Default RE: Failed emissions test in NJ

The reason they are testing the cap is to try to check the integrity of the Evaporative Emmissions System. The Evap System is basically an activated charcoal canister that traps fuel vapors and then routes these vapors into the intake manifold where they are burned as you drive. If the cap is leaking, vapors that should be contained by the system would be venting to atmosphere. In most 1996 or newer vehicles, or what are called OBDII for On Board Diagnostics II, the cars computer checks the system by routinely applying a small pressure or vacuum to the system and then checking to see if the vacuum or pressure hold. If not, the "Check Engine" light or equivalent will eventually be lit. That's why if you leave your gas cap loose after filling, or having it filled in Jersey , the light will come on after a couple of what are called drive cycles. The whole idea is to prevent vapors from leaking which are a huge source of pollution.
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