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So I recently bought a 98 firebird 3.8 and was told it needed a thermostat. Well me being excited over getting the car didn't even think about it having other problems (stupid me) anyways, it's home and I've replaced the thermostat, water pump, some hoses and spark plugs. The oil is fine (no milky color) It doesn't seem to overheat if it's running and just sitting. But when you drive it, it overheats pretty quickly. When you have the radiator cap off while it's running, the coolant blows out everywhere. It's mis-firing on cylinder 3. Is this a messed up head?
 

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Take your radiator cap off and run engine at idle. Do you see air bubbles in the radiator? If yes, this is an indication of combustion gases escaping to the water jacket. Prime indication of a blown head gasket, cracked head, or cracked block. But blown head gasket is the most common suspect in this case.

We have the 98 service manual in the 4th gen tech section, for download. Get a copy and check to make sure all cooling hoses are in the correct location. I have heard of people having problems when hoses are in wrong place.

You can get a test done by most shops that will react if combustion gases are in coolant. Might even be a home test kit available at auto parts stores.

A compression test can spot a blown hg also. Will usually show 2 adjecient cylinders with low compression than other cylinders. Or just 1 cylinder with low compression.

Also using a vacuum gauge on engine can sometimes detect a blown hg.

3.8 engine is also known for severe intake manifold leaks. I do not know if that would cause combustion gas to get into water jacket, however.
 
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