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more than likely the heater core is leaking
Water is leaking on passenger side floor board when heat is on
Your heater core is leaking. This is the part of your car that creates heat for the passenger compartment of your car. There is no fix for a leaking heater core except to replace it.
The heater coil is leaking. Water from the cooling system runs through the heater coil to heat the passenger compartment. Antifreeze is corrosive, some heater coils are thin.
It sounds like the heater core is leaking. Check for the smell of antifreeze and that will confirm the bad heater core.
Check AC drain and clear the blockage. Also look for firewall sealer skips.
It can be one of 3 things. My first guess would be a leaking heater core considering the area where it is leaking. Are you sure this is water and not coolant. If it tastes sweet or is slick to the touch, it is coolant and you have a leaking heater core that must be replaced. Secondly it can be the windshield seal that is leaking. And 3rd it may be that the drain holes below the windshield cowl are stopped up allowing water to enter the passenger compartment.
cracked water jacket in the head
Head gasket, timing cover gasket, or hose leaking.
Passenger's side? Could be a leaking heater core
have your heater core checked its probually leaking
Water on the floor that is not coming from the heater core is coming either from rain water leaking around a windshield seal or one of the firewall seals where hoses, wires or other components enter from the engine compartment.