I have an 07 5.4 3v, my check engine light will come on if I use regular unleaded gas, but if i run the tank to almost empty, then fill with E85 gas the check engine light will go off after about 50 miles of running the E85. The truck runs great with either regular or E85 gas, just don't know what would be causing the check engine light to come on when running regular gas.

Any input will be appreciated.
Thanks, Boone

 

why don't you have it read while it's on ? it will tell you...