Today, after a very hot day, I saw that my coolant was almost reaching 100C/212, I pulled over, stopped and took a look over the hood. The very little water that remained in the coolant tank was all boiling. So, I went to a gas station, bought some water and refilled the tank. When I got home, almost the same problem, the water in the tank was boiling.

Now, my questions:

  1. I would like to drive the car to the nearest qualified service, but there is a fair distance to it (about 8km) as I'm living in a remote location. Is it safe to drive it that distance if I refill the entire coolant tank?

  2. What can the cause be? I know that is impossible to diagnose a car without seeing it, however, if you have any idea what could the problem be, I would really like to hear it.

  • 5
    @mindnose, I think this is a great question. I assume that you're talking about the overflow tank when you said you checked the "coolant tank". If you'd opened the radiator cap, I think you would have had terrible burns.
    – Bob Cross
    Jul 11, 2011 at 1:33
  • Yes Bob, indeed, I didn't touch the radiator cap. Even the overflow tank was very very hot because the water was boiling in it, so I had to use some tick cloth material to open it.
    – Rad'Val
    Jul 11, 2011 at 10:40

9 Answers 9


I would like to drive the car to the nearest qualified service, but there is a fair distance to it (about 8km) as I'm living in a remote location. Is it safe to drive it that distance if I refill the entire coolant tank?

I would say yes, you can drive the car. Load the car with much more water than you think necessary: I find that two liter soda bottles make excellent transport and pouring vessels for this sort of thing. While driving, watch your gauge: if you see a high reading, get out and add water to the overflow tank (never open a hot radiator cap). Repeat as necessary.

What can the cause be? I know that is impossible to diagnose a car without seeing it, however, if you have any idea what could the problem be, I would really like to hear it.

The water is clearly doing what it is supposed to do: carry heat away from the engine. Unfortunately, something is preventing the heat from leaving the water (e.g., sludge as previously mentioned, fouling in the cooling fins or just impeded airflow) or you have a leak.

I think that your instinct to go straight to a service station is a great idea.

If, however, you were to choose to try to diagnose this situation yourself (which I'm not recommending if you're not feeling confident), you could try running the car up to temperature, turning it off and listening and looking for a pressure leak (hissing and / or white steam).

For clarity, never never never feel around for a steam leak with your bare hands. Best case: you'll get a terrible burn.

Following up on that last paragraph to add this lovely quote from Dan:

Clouds, and the visible "steam" squirting out of a kettle or a steam locomotive, are liquid water droplets with a ceiling temperature of 100°C at sea-level air pressure. It's possible for actual invisible-vapour steam to be swirled in with condensed droplets as it mixes more or less chaotically with the outside air, but "pure" steam is invisible, and has no ceiling temperature. Put your hand in the visible portion of the steam coming out of the side of a locomotive and you may get scalded, but putting your hand in the invisible jet close to where it's exiting may flense the flesh from your bones.

The key phrase there is "no ceiling temperature."

  • Thanks Bob for this very good answer. Indeed I think it has a pressure leak as you said, because this morning I started the engine, fulled the tank, and after a while I heard the hiss (however saw no steam). More than that, it seems the water gets 'consumed' quite fast. Will go straight to a service station. Thanks for the answer!
    – Rad'Val
    Jul 11, 2011 at 11:01
  • @mindnoise, good luck, those radiator leaks are super annoying.
    – Bob Cross
    Jul 11, 2011 at 18:28

Your coolant shouldn't be boiling at 100C/212F if you have a proper mixture of water and anti-freeze (typically 50/50 ratio is recommended).

Water's boiling point is 100C/212F. Even though it is called anti-freeze, it also raises the boiling of the water.

You can buy a coolant mixture tester for a few dollars at a local auto parts store, you suck some of the fluid up into it and it will show what the freezing and boiling points should be (mine is about 270F).

Still attack the other items listed in the other answers, but it sounds like you are running none or almost no anti-freeze in your system and so getting some in there will allow your cooling system to continue operating at the higher temperature until you get the source of the problem sorted out.

  • 1
    You are correct: "With proper antifreeze a wide temperature range can be tolerated by the engine coolant, such as −34 °F (−37 °C) to +265 °F (129 °C) for 50% (by volume) propylene glycol diluted with water and a 15 psi pressurized coolant system" from en.wikipedia.org/wiki/…
    – Bob Cross
    Jul 11, 2011 at 18:26

You shouldn't be losing fluid - did it all boil off?

If you are ending up with low coolant, I would suspect a leak.

If the level is okay, I would suspect either a restriction in a pipe or the radiator itself, or possibly a damaged thermostat restricting the flow.

Check for a bent or twisted radiator hose or damage to the radiator.

If nothing is obvious, I would make sure the coolant level is where it should be then drive slowly to your garage - you did say this only happens at higher speeds.

The radiator could have a build up of rust or sludge, so having a garage flush it could be useful, and they can get a full check of the colant circulation path.


When I've had overheating problems and unable to get it fixed or checked out immediately, driving with the heat on full blast (yes, in the summer) is an excellent way to reduce the engine temperature and get you to where you need to go without damaging your engine.

  • 1
    Yes, I had to do exactly this when I had my radiator hose blew. Driving around in big clouds of steam while roasting alive inside the car is not super duper fun.
    – Bob Cross
    May 10, 2013 at 19:46
  • open windows, point the vents out the car. Apr 9, 2014 at 11:05

Hey be carefull but not overly worried.

Probably everything was fine, the cap on de radiator keeps the water under pressure so it only boils above 120 C. I was not boiling, only when you opened the cap and the pressure was gone. Very dangerous to open that cap on a hot engine.


When you first start the car check the exhaust pipe. Some condensation is normal if you get clouds of steam you may have a cracked head. You can get it tested at any reputable repair centre. Some manufacturers have increased engine capacity by boring out the cylinders this in some cases has left very little metal between the water jacket and the cylinder head if you don't run glycol with an alloy motor this can corrode allowing water into the cylinder hence the steam on start up of the engine. MAKE sure if the car boils you don't just top up with water add glycol or you can have a major problem in the future.


Main causes for an overheating engine:

  1. Fan belt loose or broke
  2. Faulty water pump
  3. Split Hose pipe
  4. Faulty radiator cap
  5. Seized thermostat
  6. Head gasket blown
  7. Blocked radiator
  8. Air lock in coolant system (with engine cold and shut off, remove coolant filler cap (fill to water level if required) then squeeze bottom radiator hose several times until no air bubbles come from the place you filled with water.
  9. Badly worn engine (have you checked oil level? is there any sign of water on dip stick? is oil level too low?
  10. Electric fan or temperature switch faulty (Main cause of overheating on modern engines (The fan should start as soon as the temperature goes above normal)

I Would NEVER attempt to drive a car that is overheating (you may cause a lot more damage e.g. seize the engine or crack the cylinder head). If you can't fix the fault then get someone who can and knows what they are doing.

NEVER NEVER NEVER put cold water in a hot engine, chances are you will crack the block and/or cylinder head, also it will blow out of where you are filling it like a volcano and may well scald you.

If you DON'T know what you are doing, then it's best to get someone who does.


Buy a bottle of chemiweld, pour it into the radiator without the radiator cap. Let the motor heat up let it cool. Drive as normal flush radiator and heater etc. When the problem is solved. I know this is an old thread but for anyone new looking to solve the same problem this dude has had head gasket seepage .... water flowing into head and boiling cause gaskets stuffed between cylinders a bottle of chemiweld there are others it works best fixes that in moments.


Check for a blown head gasket. Get a combustion tester.

I had same problem, it was a leaking head gasket and I had no oil mixing with coolant.

  • 1
    Can you provide some additional information to the OP. What is a combustion tester? How do you do the test? etc. Thanks, welcome to stack exchange. You can take a tour to see how the SE work by going here mechanics.stackexchange.com/tour Dec 24, 2015 at 17:20

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