The gas cap can cause this. The system is seeing an evap leak of large rate, and in this case it assumes the fuel cap, as it is the most obvious. Other conditions that can cause this are a faulty fuel tank pressure sensor, evap vent solenoid, charcoal canister, or lines. Ideally, to efficiently diagnose this, it requires a scan tool. You can view data, and execute tests, that would normally take some time as the on-board tests run infrequently.
Monday, November 22nd, 2010 AT 12:13 AM