Niagara Gazette — SANDY FROM BOSTON: I own a ’06 Ford Taurus and the engine light went on. The last time this happened I replaced the gas cap and the light went out. However, this time when I replaced the gas cap the light stayed on. I had a mechanic look at it and he said something about the evaporation system? Is this just mumbo-jumbo?
TOM: The MIL (Malfunction Indicator Light) went on because the engine management computer detected a malfunction in the computer system and, in your case, the EVAP system. No mumbo jumbo here; your car has a genuine problem and it must be repaired in order to pass state inspection emissions laws.
I get a lot of questions relating to this topic. “Tom, I was told to replace my gas cap. I did and the light went out. Why?” “Tom, what is an EVAP code?” “Tom, can a bad gas cap cause my check engine light to come on?” Let’s explore answers to some of these questions.
What is an EVAP system?
The EVAP system is an emission control system in your car designed to trap and re-circulate fuel vapors back into the engine to be burned. It also ensures that harmful fuel vapors do not escape into the atmosphere. It is a closed and sealed system comprised of a series of steel and rubber lines, valves, and a canister. When the system is compromised, the engine management system senses it and trips a check engine light.
Can a bad gas cap cause the check engine light to come on?
Yes, absolutely it can. The gas cap has a large rubber O-Ring that seals the mouth of the gas tank inlet. When the rubber O-Ring goes bad, the sealed system is compromised and thus trips a MIL (Malfunction Indicator Light) or check engine light. Also, it’s a good idea to shut your car off during fueling. If you leave the car running, the system will sense a leak when the gas cap is off, and it can trip a check engine light as well. If when the tech checks the gas cap; notes that the sealing surface where the gas cap seals is marred or worn out, this will warrant replacing the fuel filler neck. This is the hose/metal tubing that goes into the gas tank, providing a means by which to fill the tank.