do i add fuel cleaner with car on or off

Each treatment container has directions on the bottle which informs you how to put in the fluid, Most tell you to wait till you have a least a 1/4 tank of gas, pour it in then fill the tank. Please read the container of the treatment you purchased.

Add with vehicle off , if you add it while runing you may experience a check engine light, for the evap system large leak detection. like above 1/4 tank 1 bottle usually treats around 20-22 gallons.

Not to be contradictory, but I don't believe you will get evap leak error by putting additive in tank. If this were the case you would get one everytime you fuel up with car running.

wanna bet- anytime the car is at operating temperature the evap purge solenoid is continuously running, thus may set the check engine light as soon as pressure from the fuel tank is relieved . And no, additive does not set the light -pressure relief of the fuel tank does!

oh and by the way in many states it is also illegal to fill up while running at the pump.

I have a 300 M, a Dodge Ram, a KIA Spectra, a Gran Prix GTP, a Chevy Cavalier, and none of these give any errors on the OBD or check engine light if I fuel up while running. Yes I do break the law when I fill up my vehicles, but I don't know anyone that has never fueled up while leaving the car run. I bet you have at least once.........Also what are you trying to say when you say " And no, additive does not set the light-pressure relief of the fuel tank does!" ?

And I was a Daimler Chrysler Technician for 11 years- try how many people came in with check engine lights and they said, But All I did was put gas in it 20 miles ago, why is the light on. Now a days there is actually a light that comes on stating that either the cap is off or loose, rather than the CEL.

I'm sure your right. I just never had that happen.