Overheating,leakage in overflow tank and under car

Overflow tank for coolant

If you are losing coolant then you have either an external or internal leak.
If you can find no indication of an external leak from the radiator, the radiator overflow tank, any of the hoses on the system or around the water pump fitting then you may well have an internal leak. Check the radiator overflow tank carefully and the hoses to it for any signs of external leakage.

An internal leak of coolant can be via a failed cylinder head gasket or a crack in the cylinder head or both. Coolant can then leak under pressure into the cylinders or into the engine's lubrication system and will then mix with the oil. Check the condition and colour of your oil for evidence of coolant contamination. Check your exhaust for a constant flow of white exhaust emissions which is evidence of coolant entering the combustion chamber.

If you have run the vehicle with the engine overheating this will generally cause failure of the cylinder head gasket and possibly a crack in the cylinder head.

There are 2 tanks on a radiator. One on top and one at the bottom. They are what seal the top and bottom of the radiator core. You will also find an overflow tank connected by a small hose near the radiator cap that connects to the overflow tank. The overflow tank is where you top up the coolant. One some coolant system, the overflow tank is pressurised and the "radiator" pressure cap is on the overflow tank, on others the radiator pressure cap is on the radiator and the overflow tank is unpressurised.

Have a cylinder leakage test performed to check your head gasket. It dosn't have to overheat to have a head gasket problem. A leakage test checks if looseing combustion gases to the cooling system which would presurise the system higher than normal which causes the tank to overflow.

Change the cap for the overflow tank. The seal has gone bad and can no longer hold pressure, therefore it has no where to go but leak from the overflow. New cap $10. This should fix the problem.

Whenever you overheat once, then the thermostat must also be replaced. It is operated by the expansion of a contained wax pellet, and overheating causes permanent loss of wax.
However, you may also have caused a head gasket leak now from heat warpage. If the thermostat does not help, then borrow a cooling system pressure checker from a place like Autozone, and see of it holds pressure for 5 minutes after pumping it slightly above cap rating. Then start engine and see if it is steady or oscillating from 1 cylinder leakage.

If the water is flowing into the overflow tank then it is probably running a little hot. It is normal for a little bit to flow into if as the water heats and expands but there shouldn't be enough to overflow the tank. Have you tried changing the thermostat? If the thermostat isn't opening properly this will have all the effect in the world on how hot your engine runs.

about 100K miles? sounds like a blown head gasket is allowing coolant to be pushed out the exaust,and combustion pressure is pushing fluid out the overflow.have your compression tested.

It can be added to th overflow tank..
Do NOT open the radiator cap whaile hot..

When fluid s put into the overflow tank - it can be a bit slow going into the radiator..

Good luck !!

Hi mpullan,
You have a leakage. The tank that you're referring to is the expansion tank.... When the system gains operating temp the coolant exchanges between the tank you're referring and the radiator. When you replace the damaged parts, keep in mind that the system will need to be bled free of air....To do this remember to open the cabin heat controls.

Regards Johngee10