So my coolant expansion tank developed a leak. So I replaced it but forgot to bring the cap from the old tank. Anyway I thought that I would try filling it up and running it warm anyways.

So I turned on the internal heater and and started her up. Temperature started rising and all seemed fine. As the water was getting hotter the I think that it expanded some with a little overflowing as a result, nothing unexpected.

So getting ready I turned of the inner heater, and after some minute or so water just started overflowing a lot. I turned of the engine to not introduce any more heat.

After a few minutes of cooling down I filled up the tank again. It swallowed about 1.5-2l water, so quite much. Expansion tank takes 1.5l.

Now I'm just wondering if this is a expected behaviour since I din't have a cap. Or does it suggest that I have an air pocket in the system, and need to find some way too bleed it properly?

(It's a Range Rover P38 V8 4.6l)

  • What year is your Rover? May 20, 2017 at 0:32
  • It would be the 1997 model
    – Markus
    May 20, 2017 at 11:14

1 Answer 1


Expected behavior. The cooling system is a sealed, closed loop and becomes increasingly pressurized as the coolant heats up and its vapor pressure increases. Due to this increased pressure, the coolant is able to remain liquid at temperatures higher than its normal boiling point.

The expansion tank and radiator are both under pressure at different times in the cycle: when the pressure in the radiator gets beyond a certain limit, an overflow valve releases coolant into the expansion tank - this lowers the pressure in the radiator, and increases the pressure in the expansion tank (assuming it is properly sealed and the cap is on tight).

Once the pressure in the expansion tank is higher than in the radiator, the coolant drains back into the radiator.

If there is a leak in the expansion tank, or if the cap is not screwed on tightly, once coolant temperature exceeds its boiling point at normal atmospheric pressure, any coolant entering the expansion tank via the overflow valve will immediately boil - some will be vaporized immediately and some will be forced out as liquid.

On occasion I have had to (very slowly and carefully) remove the cap to the expansion tank when the system was hot - and it was like opening a bottle of coke that had been shaken very well. Even opening it just a crack caused boiling coolant to spew out the sides. And people have been severely burned who did not know about this hazard and removed the cap too quickly when the system was under pressure.

Think of a stew pot with a lid on it - with the lid on, it is merely simmering, take off the lid and a few minutes later it will be boiling over. The temperature has not changed - but by reducing the vapor pressure, you are lowering the boiling point. This is the same reason that water boils at a lower temperature in Denver than it does in San Francisco.

  • So I guess the conclusion is just to try again with the cap on?
    – Markus
    May 20, 2017 at 11:39
  • @Markus - Yes. Try it with the cap on. May 20, 2017 at 12:23
  • What's the usual working pressure in the expansion tank?
    – FarO
    Jul 4, 2018 at 13:09

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