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My wife's Mini has a leak in the coolant recovery tank, and she has been told by the dealer that her car is at risk of overheating without the coolant recovery tank.

Frankly, I don't believe it. Cars decades ago didn't have a coolant recovery tank, and there was always some amount of air mixed with the coolant. The coolant recovery tank serves two purposes: it prevents coolant from dripping on the ground (and thus being washed into the environment and eaten by wildlife), and keeps air out of the engine (thus making the cooling system more efficient at absorbing heat), but I don't believe it is absolutely necessary.

So, will modern engines overheat if there is no coolant recovery tank?

  • Been awhile, good to see you back. – DucatiKiller Mar 16 '16 at 15:24
  • I check most of my SE communities daily. But I avoid writing duplicate answers, as it's just plain rude, and I've learned the hard way not to supply an educated guess as an answer. Thanks for the kind words. – BillDOe Mar 16 '16 at 22:32
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tl dr; Not directly, but yes.

The purpose of a recovery or overflow tank is to allow for expansion of the coolant, then to recover it back into the radiator as things cool off. If the tank is not there (or it all leaks out), the radiator is pushing it out onto the ground without the ability to recover any of it. When things cool off, it sucks air back into the system. The air can cause hot spots with in the engine, same as if you haven't completely or properly burped the coolant system after you refill the antifreeze.

It may take some time, but sooner or later you'll have a great enough lack of coolant and your car will overheat. The only way around this is if you are checking and refilling every day before you drive your car.

As an alternative, you can seal the crack in your recovery tank until you can purchase a new one. There are plenty of sealants on the market which will do the job for you.

  • Thanks, Paulster. My wife is having the car serviced by her local mechanic next Monday (not dealer). – BillDOe Mar 16 '16 at 5:19
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The coolant recovery tank serves two purposes There is another function on newer cars. They maintain pressure in the system. Back in the day there was a spring-loaded radiator cap and the tank was not under pressure. On newer cars the overflow tank has a pressure cap on it.

The system has to be pressurized because pressure raises the boiling point of the coolant. Without the pressure there can be boiling in the system. Although the whole sytem may not boil over there can be localized boiling in the engine (usually near fast moving parts like the water pump vanes). These tiny steam bubbles cause two problems. First, steam has way less thermal conductance than liquid water so the parts where the bubbles are forming don't get cooled properly. Second, the steam bubbles form then collapse. They collapse with extreme speed and force, This is called cavitation and it causes rapid wear.

Here is a photo of cavitation on a marine propeller. Cavitation damage

And here is the kind of damage it causes on moving parts. Cavitation damage

Images from Wikimedia commons

  • I never considered the effects of cavitation. There was a special I saw some years ago about the Badlands of Washington (yeah, what's that got to do with cars), that said the collapse of cavitation bubbles can generate pressures of over 100,000 psi. Probably not so much in an engine's water pump, but still probably pretty respectable. Good additional info. Thanks. – BillDOe Mar 17 '16 at 5:10

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