Yesterday, I filled the gas and forgot to close it, and I went to an auto washing house for cleaning. I think there may be a lot of water in my gas tank. What can I do?

 

Did you drive the car home from the car wash? If, yes, then you're good for now but if the temperatures drop below freezing, you're screwed.

I would go to the car parts store (Advance Auto, Auto Zone, O'Reilly's, or even Walmart, etc) and get some "Heat" or other brand of 'gas line antifreeze', which has methanol alcohol in it. This will chemically attach itself to the water and to the gasoline, and it'll get burned as you use the gasoline.

Again, this is an important thing to address ASAP before temps significantly drop and the water freezes in the gas line and then you're done until it gets thawed out (heated garage) and water-alcohol-gasoline mixture is used up..

 

Yep, methanol is your friend if there's water in the fuel.

Go for the standard HEET in yellow bottles, it's basically pure methanol (99% iirc). The red, ISO-HEET bottle are 98.5ish% Isopropyl alcohol. It's not recommended to use more than one bottle in 10 gallons of fuel, but it should be safe to run 1 bottle per tank through multiple consecutive tanks.

 

With that much possible water I'd pump it out of the tank or drain it from the fuel line.

 

How far did you get?