You have a fault within the system that is indicating a large leak in the evap system. The reason it says check the gas cap, is that it thinks that since the leak is fast enough, it is probable cause. An evap system leak is not real friendly for the do it yourselfer. If nothing is obviously damaged, like a line, canister, or fuel tank, you could have a possible leaking vent solenoid, or a faulty tank pressure sensor. Ideally, you need to be able to read the sensor data, and perform special functions that would be done with a scan tool. To diagnose the system, you need to have the ability to close it off, and either pressurize it, or place it under vacuum, while monitoring the pressure sensor reading. Upon viewing the pressure sensor reading, it may be very obvious(faulty sensor), but without that ability it is guesswork.
Oct 18, 2010 at 9:44 PM
