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My 1996 5 liter commodore has started using water, about a radiator full in a week, but there is never anything under the car. Any advice would be appreciated. Also are any of the radiator stop leak products any good? there seems to be a lot of conflicting information on the web.

Thanks
Nick

A loss of coolant indicates either leak out to the atmosphere somewhere (perhaps on the ground or even inside the car from the heater), a leak between the radiator and oil cooler (in which case, your transmission will have coolant going through it - causing damage to the trans) or go through the combustion chamber and out of the exhaust.
Unfortunately, your situation sounds like the latter, which usually indicatates a blown head gasket.
Stop leak products are a band-aid fix, and can in some instances cause more problems. It's important to find out where the coolant is going so I'd suggest getting a mechanic to carry out a pressure test and a Tee-Kay test.