It will cause the tank to rust.
Mixing bleach with gas can produce toxic fumes, such as chlorine gas, which can be harmful when inhaled. It can also create a fire hazard due to the combustible nature of gasoline. It is important to never mix these two chemicals together.
Bleach
pancake sryup is my gas tank, what do i do?
the tank will leak
Bleach, or sugar
What happens when chicken bone gets stuck in gas tank
pancake sryup is my gas tank, what do i do?
You will likely need to drain the gas tank.
add bleach to the gas tank and let it sit for 2-3 days. this allows the gas/bleach blend to distill out the water in the lines
It has been shown on the TV show MythBusters that bleach will oxidize the car's fuel system and engine, rusting everything up slowly over time and causing it to seize.Bleach is a particularly effective oxidant so it will do this in a matter of days.
This would depend on how much petrol is in the tank and how much bleach. The bleach might be very diluted. Short term bleach won't do much damage, the car may not run, but the whole fuel system should be flushed out. In the long term bleach can be corrosive.