What actually happens when there is not enough coolant in a car engine?

Water flow will stop and the heat will not be able to be transferred out of the engine block. Engines cooling systems are very intricate and feature "pockets" depending on how much cooling is needed for a given area. A lack of coolant means that the system will not be able to be pressurized by the pump and the coolant will not flow.

Disadvantages:

• Overheating

• Water pump running dry and damaging itself in the process for a lack of lubrication and resistance

You get less cooling. If the radiator is only 1/2 way full you have water in contact with cooling fins for 1/2 the time and 1/2 the cooling. The fluid is pumped from bottom so it will continue to flow. Eventually the water is hot enough that the engine turns it to steam. Once you empty the radiator then zero cooling as nothing is pumping.

NOTHING will happen if that lower amount wont cause overheating. For example in my country winters are very cold about minus 32 degrees Celsius. My car to get warm it takes about 30 minutes. It's Italian car and it very bad at warming up in winter. So i use less amount of coolant so this amount heats faster. And impossible to overheat car because when thermostat opens cold coolant from radiator makes coolant cold again and car never overheats. People here just makes worst scenarios. witch is not possible if your coolant temp is in normal range - 90c. Also fairy tales about water pumps is stupid. Water pump is just bearing that is driven by belt. Bearing rotates plastic stuff that makes water flow. Maybe in other cars it's different in mine there is no actual pump. SO if there would be no coolant, which is impossible based on your question - it wont broke.

My sons 99 LS1 Trans Am started to lose power after running fine to the last gear but then it would lose power and run the rpms 2000 at 35 mph. Yesterday, he stopped on the side of the road because it had a smell like something burning and a little bit of greyish smoke was coming from the back. He let it cool and made it home. No more smoke. When the mechanic looked at it today, there was very little coolant/water in the radiator. He told him that with the low coolant it was telling the transmission it was getting too hot and therefore it wouldn't go faster. He said that smell was burning coolant. He put coolant in it and they took it out up and down the highway and let it warm up and he got on it and it went through all the gears and had plenty of pickup. The mechanic said it should be ok now but keep an eye on it. I told him to watch for any leaks under it when he parked it because the radiator or a hose could have a leak. I am hoping he dodged the bullet with it and will keep up with making sure it has coolant. Im guessing that if the head gasket or block were damaged, there would be some signs. The transmission is shifting smoothly through all the gears.

As the coolant gets lower and lower things will progress as follows:

1. There won't be coolant in the coolant expansion tank even when the engine is warm.
2. There will often be insufficient fluid to circulate coolant through the heater core and the heat won't work. A water cooled throttle EGR / body could suffer too.
3. The water pump will pump most of the water out of the radiator and there will be insufficient coolant flow, and the temperature gauge will show higher than normal coolant temperature under heavy load.
4. #3 will get worse
5. There will no longer be sufficient coolant to circulate coolant. The temperature gauge could show too low of a temperature due to lack of coolant coming out of the engine. The coolant will start to boil and then it will condense on the temperature sensor and it will show hot. The engine will over heat.
6. There won't be enough coolant to fill the cylinder head. The temperature gauge will report too low of a temperature. The cylinder head will over heat and it could lead to damage like a warped head. Eventually the block will heat up and it will boil off coolant and the temperature gauge will show high temperature, but it could be too late. The high head temperature can cause pinging or knocking from pre-ignition. Shut it off now and the head can be saved.
7. No coolant - the engine will be severely damaged in a matter of minutes if run under load. The temperature gauge will report too low of a temperature since there is no coolant, as the temperature sensor will likely not heat up fast enough to show the true engine temperature and let the driver know to shut off the engine until it's too late. Noticing that the temperature gauge is too low, or hearing pinging can be an indication to shut off the engine to prevent more than minor damage. Some vehicles have a cylinder head temperature sensor that isn't dependent on coolant. The vehicle may go in to limp mode.

Some engines require purging to remove air pockets from the cylinder head after the coolant level is restored. This may require revving the engine up momentarily.

Engines with aluminum cylinder heads on iron blocks are highly susceptible to head gasket damage when over heated, due to the differing rates of expansion of the different metals. Avoid running them even if the temperature gauge shows a minor overheat.