While reviewing customer vehicle repair paperwork recently, I came across a recurring problem. I found a number of check engine light codes for “Large Evaporative System Leak”. The gas tank is a sealed system that prevents fumes from escaping. The fuel in the tank needs room to expand as temperatures rise and so it is circulated through a charcoal filter and is burned in the engine. Gas fumes create ozone pollution when combined with sunlight. Before the 1980s, the fumes were vented to the air by a vent hole in the gas cap.
The system that controls the gas fumes is known as the evaporative control system. It also monitors itself by periodically checking for leaks. When a leak is found the check engine light comes on. The code for a large evaporative leak is P0455. While certainly not the only cause of an evaporative leak, a common one is the gas cap. There are three main faults with a gas cap:
- Gas cap seal may be worn and cracked.
- The cap may not tighten sufficiently. This is evident by the fact that it doesn’t take much pressure to get to the click.
- Even a good gas cap sometimes doesn’t get tightened all the way to the click (operator error).
You would think that if you didn’t tighten your gas cap when you filled up that the light would come on right away, but the system only tests itself between ¼ and ¾ of a tank. By the time the light comes on you probably forgot about filling up.
When filling up periodically check these items:
- Look at the rubber seal on the gas cap. If it has any cracks, it’s time to replace it.
- Be sure it takes pressure to make the gas cap click. If it turns easily before clicking, it is probably not tightening up enough and time to replace.
- Be sure the cap threads on correctly and you hear it click before you stop tightening.
When replacing the cap, we always replace with the original equipment part from the dealer. The ones from the parts store don’t always fit correctly.
If the gas cap is the cause of the check engine light coming on, installing a new cap won’t make the light go off immediately. It takes at least 2 drive cycles of 20-40 minutes before the system runs enough tests to shut off the light.
