Evaporative Codes and Natural Vacuum Leak
Most everyone will at some point have a check engine light or (MIL) on our vehicles during the duration of ownership, everyone by now has heard of the (gascap) code and or evaporative emissions leak codes. This article explains the basics of how it works and why it is so difficult to diagnose and how it will continue to get more difficult as time goes on.
First off what is an evaporative leak? Alot of people think this has to do with air conditioning, of course this is far from the truth, it is related to the fuel system including the fuel tank, basically when fuel is introduced into the fuel tank in a liquid form it by its nature wants to evaporate, this system is designed to control and test the evaporation rate and prevent it from being introduced into the atmosphere. a few basic components are the fuel cap, tank itself, purge solonoid and vent solonoid, canister and in some sort of pressure/vacuum sensor. These all work together to allow venting when necessary and to purge these vapors into the engine where they are burned in the combustion event. When any of these components fail or leak you can get any of a series of evaporative codes, testing these parts individually takes time and testing for leaks takes time, leaks are difficult as most of them are testing at a vacuum equal to about 1.5psi in air pressure, so you can imagine they are very small.
Now the newer systems are using new methods of testing including NVLD or natural vacuum leak detection. A very basic explanation is that anything with air inside it, once sealed will contract as it cools which creates a natural vacuum. Like taking a coke bottle and putting hot water inside it and sealing the top, as it cools it will contract and suck the bottle in some, this is the basic priciple of NVLD testing. The biggest problem is that most of this automatic testing occurs when the vehicle is not in motion and wont run every single time, so in essence it could take days or weeks to get this test to run by itself.
Now with this information you can see why it takes more time to diagnose and confirm evaporative codes as in our shop we prefer not only to test with our methods to verify a repair but to allow the computers own test to run and pass to verify the repairs, this is the most accurate way for us to guarantee a quality repair although it does take more time and sometimes requires more diagnostic time as well.
Please keep this in mind when having diagnostics performed especially when having evaporative system codes stored, they are by far the most misdiagnosed system on a vehicle today and with the patience and right testing methods can normally be corrected right the first time.