Overheating
 

Imagine the scene. I'm driving along the motorway at about 80 m.p.h. and start to notice the temp gauge going up a bit.

"That's ok", I think, "after all, I don't normally go this fast"!

However, it didn't just go up a bit higher than normal, it went all the way and at some points it was even nudging the red.

When I arrived at my destination I checked and discovered that the engine seemed to have dumped the majority of the coolant into the coolant reservoir instead of keeping it circulating around the engine and radiator where it was needed!

Also, I had to wait at least half an hour before the car would start again even though the temp gauge by this time was reading normal.

Now, this had also happened several months ago but I thought I had fixed it by changing the thermostat and repairing a split hose. But now it's done it again.

What problem could be causing this strange effect? It shouldn't be the thermostat because that is new, and as far as I can tell I don't have a leak.

Any ideas?

water pump maybe?

Check the seal on the radiator cap, if thats good then you might have a BHG. Mine did the same thing when the headgasket went. One of the pistons was pressurizing the coolant system and would send the coolant out the cap when the pressure overcame the cap pressure.
Fill her up and start the car with the cap off and see if water shoots out of the rad, if it does then more than likely a bhg.

The only reason the overflow "overflows" is from a combustion to coolant passage leak, better known as a blown head gasket. Your symptoms are dead accurate.

Many thanks for that.

I did what you said and nothing came out of the top but I suppose that doesn't mean it isn't a bhg!

Looking through the rest of this forum it does seem that this is the most likely cause of overheating and if the only reason for the overflow filling up is this than that's that!

Do you think I could manage this myself or is it a garage job?

I would drive my car home from work everyday and my overflow would take a massive leak all over the ground as soon as I would turn off my car and get out to see it. Well turns out the problem was my radiator cap. The seal was messed up and wouldn't hold the pressure thus sending fluid back into the overflow reservoir. This happened however, after replacing the water pump and thermostat which I believe both had gone bad.

Great, I'll try this first as it's the cheapest and easiest to do :)

Also, thinking about it the radiator cap does look a bit dodgy..

Thanks.

Yeah, a bad radiator cap will have your coolant system purging all day long (or until it reaches the point where there\'s more steam than anything in the coolant system.

The radiator cap is a relief valve. The one our cars come with are largely dependent upon a preset amount of elasticity in the spring. As the spring ages and anneals from the constant heat cycling if begins to open at lower and lower pressures.

BTW - The temperature representation provided by the temp gauge is logarithmic. The temp can go pretty high with little to no movement, even up to arguably hazardous temps. If your temp gauge jumps into the red stop the car ASAP.

Just did a head gasket repair and mine dosent do it anymore, The water jakets on the head were eaten up small leaks in the gasket!! replaced everything that could have caused the problem, but it was a head gasket:mad:

Yup it happens. I just wanted it made clear that there are other things that can cause this problem besides a BHG.

Let\'s not forget that a faulty rad cap can cause a BHG.


For the benefit of those interested: The radiator is a pressure valve. The higher the pressure in the cooling system the lower the boiling point. Reduce the pressure the boiling point rises. So when a bad radiator cap starts venting coolant too soon it can release enough pressure that your coolant actually starts to boil. Water vapor takes up a lot more room than condensed liquid and the excess volume has to go somewhere. Once this happens both the coolant mixture and water vapor begin purging out of the system through the overflow.... they\'ve got to go somewhere. As this continues the coolant mixture begins boiling more and more rapidly, and as such the purging gains momentum. You can actually end up running on less than half of your coolant in a couple minutes. If you\'re not paying attention you can overheat big time and then... you guessed it: BHG.