My husband and I recently purchased a 96 Isuzu Trooper. A new radiator had been installed just prior to our purchase, but we’ve had some overheating problems since. My husband has replaced the thermostat and water pump to no avail. The reservoir remains full even though the radiator seems continually low on coolant. We’ve topped it off with water on several occasions. What’s happening?
Coolant is leaking out somewhere. If not in the cabin or on the ground, then into the engine and, hopefully, out the tailpipe. Check the engine oil to make sure it is not leaking into the crankcase. This will destroy the engine.
A cooling system pressure test can help find the leak. With the engine cold, this will pump up the cooling system to the operating pressure. Any drop in pressure shows a leak. Usually, the leak can be found by looking for the coolant as it pushes out.
It sounds like this engine has been overheated several times, probably before you bought it and since. Based on your description I’d guess you have a head gasket leak.