I drive a 2002- Chrysler PT Cruiser. I hear this bubbling sound when I turn the engine off. It occasionally registers at hot so I put more coolant in and it seems OK for a while. The coolant is not on the ground. I don't see any leaks. What could this be?
Bubbling probably means that air pressure is building in the coolant. This is often caused by a blown head gasket, where coolant leaks into one or more cylinders in the engine, and is simply burned off, so you don't see any leaks or coolant on the ground, as it's leaking internally. Compression from the cylinder forces air back through the head gasket into the coolant system, and the air makes its way up to the overflow tank where you hear the bubbling and may see foam on top of the coolant. The engine will not run as well, as the compression will be lower as the cylinder has lost its seal, the coolant seeping into the cylinder will affect the burning of fuel, and the loss of coolant will increase risk of overheating. The coolant lost into the cylinder can also damage the exhaust components, particularly the catalytic converter, which can further hinder performance and efficiency of the engine.
If you're lucky like I was, it simply means there's a leak in one of your pipes or the coolant bottle is cracked. This would allow air to escape, meaning you lose pressure, which in turn allows the coolant to boil. Look for a tiny leak in the pipes or bottle somewhere. When the engine is hot and the coolant is boiling, open your hood and have a look around. DO NOT try to open the coolant bottle, it will blast you in the face with hot, poisonous gases.
When you start your engine and it runs up to operating temperature most of the air in the system is let out of the rad cap. When the engine cools down some air is drawn back into the cooling system. The amount of involved is accomodated by the volume of the cooling system in total. The full mark is used on a cold engine to minimise this air activity to a once only event. If you are having to top up your system every short while then you do have a water leak. This can be an external leak, and can be seen by water/antifreeze stains or drips on the engine or cooling system parts. The engine can also leak internally, and this is more usually the case when you have a boiling and bubbling event in the water reservoir that you fill with coolant. Never remove the rad cap on a hot engine. A further possibility is the water pump on your engine has a failed impeller and is not circulating the water through thr rad for cooling. Have the vehicle checked at your favourite repair shop to avoid any engine damage.