1996 Honda Civic overheating

An impeller broken free on the water pump is extremely uncommon except on some VWs that use plastic impellers. The more likely suspect is a leaking cylinder head gasket. If combustion gases get into the cooling system, they can pool under the thermostat and cause it to not open. Thermostats have to be hit with hot liquid. Hot air won't do it.

There's a chemical test that can be done at the radiator to check for a leaking head gasket. That involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.

Once the hood is open, the test takes just a minute or two. By the time the mechanic finds your car, drives it into the shop, does the test, writes up what he found on the repair order, and drives the car back outside, it's easy to use up a half hour.

At the very nice family-owned Chrysler dealership I used to work for, we had the owner's blessings to do this for free on our lunch hour, as a customer goodwill gesture. It doesn't cost the shop any resources other than the mechanic's time.

Usually not. The cooling system can only develop 15 pounds of pressure. That isn't enough to push coolant into the combustion chamber unless the leak is really bad. The combustion chamber develops hundreds of pounds, and it's real hard to keep that pressure contained. If it can sneak into the cooling system, it's going to.

The pressure tests is one method of finding an external leak in the cooling system, but it's not the best way for small elusive leaks. At issue too is if you're having to add coolant.