If you had to add 1 gallon of coolant, you definitely have a leak somewhere (external or internal), or your radiator cap is not holding the pressure.
Pressure test your cooling system. You can buy some dye; it should help you to detect the leak.
Make sure you check everywhere, from the hoses, water pump, intake gasket etc.
If you still can't see any external leak, you may have an internal one; do you have a lot of white smoke coming out of your exhaust?
Do a compression test on each cylinder. A low reading would indicate a leaky head gasket.
You may also have a cracked head or block.
Friday, June 12th, 2020 AT 12:09 PM
(Merged)