Is250 Coolant Boiling In Expansion Bottle

[james250]

I've got a big issue with my IS250 on holiday and driving down and temp gauge started climbing higher than half way. I stopped and noticed steam coming and i could see coolant boiling in the expansion tank. I let the car cool down then checked everything but no leaks anywhere to be seen. Topped up coolant and tried to drive down slowly and within minutes the same thing started happening and the expansion bottle was empty again so called breakdown service. Been towed to a garage which won't be open til Monday now. But the breakdown guy said he suspects the thermostat. Water pump looks fine and apparently no signs of bad head gasket. After a few hours of waiting for the breakdown guy he suggested putting some water in and see how it goes and at this point it didn't overheat. So maybe a sticky thermostat? The leak was definitely coming from the expansion bottle which to me seemed like it was because the bottle was boiling.

Any ideas what this can be? I won't know anything til Monday now

[ChumpusRex]

A number of things can cause overheating

1. Thermostat

2. Water pump

3. Head gasket

4. Clogged radiator

5. Faulty radiator cap

6. Leaks

Leaks will result in loss of water from the system, so you will usually end up with less water in the main cooling circuit (it won't necessarily pull in water from the bottle)

Head gasket could look like water in the bottle boiling as exhaust gas circulates through the cooling system and escapes through the bottle. Can be reasonably accurately tested on the spot with a coolant analyser tool.

If the coolant is clean then stuck thermostat, faulty water pump or clogged radiator are possible. Clogged radiators don't happen overnight and should be uncommon, as Lexus use a 10 year anti-corrosion coolant which is very effective against sludge or corrosion (unless someone has topped up with a generic coolant, which will react with the toyota stuff causing sludge to form).

Thermostats don't often get stuck closed, but it happened to me once before, so it does happen.

[james250]

Problem solved. The pressure valve within the radiator cap had broken off and fallen inside the coolant system. The garage dismantled the pipes and drained coolant to take off the broken spring like valve and then re-filled and put in a new cap. Now the temperature sits very nicely below the half way mark.

Because the radiator cap was faulty, the coolant system wasnt pressurizing and the cap wasnt able to hold pressure. The system needs pressure to increase the boiling point of the coolant. Because the pressure wasnt there, the coolant was boiling much earlier and overflowing out of the expansion tank.

From this experience, I would highly recommend inspecting this cap for those who have not done so for a while, specially if the cap is over 3 years. If there is corrosion on it then might be worth changing it coz mine corroded and broke off. Then again my cap is over 6 years old. Catching it early will save you the hassle and money required if it were to break like mine did.

[ChumpusRex]

Thanks for the feedback. I was planning on changing mine in due course, but I might just speed things up a bit.

I'm tempted to get the thermostat changed at the next coolant change too, for the same reason.