There's many causes of overheating (on all cars..).
The first and most obvious thing I would do after checking the coolant level in the radiator expansion tank was at the maximum level, would be to check for coolant leaks while the engine is running with a 'few revs' on a warm engine.
Hoses can split internally and also become soft and collapse internally through age. When this happens the hose becomes blocked and prevents the coolant from passing through easily. If a hose feels 'soggy'/soft and is easily squeezed flat by hand, it's suspect.
Coolant can also leak from the heater hoses, the heater unit, the bearings/seal on the water pump and of course the radiator. Nor is it always easy to see a leak let alone find it. If the in-car heater unit is leaking the carpets may be wet at times and sometimes, the windows may mist up when the car is standing - this is the coolant condensing on the interior of the glass.
Ok... you can't see any leaks while the engine is running. Is the car losing coolant when it stands? Or is it losing coolant when the engine is running? ( a split hose may only leak when it is pressurised with warm coolant) Or is it just losing coolant when it overheats?
Remove the cap from the radiator expansion tank when the engine is cold. Make sure that the water level is at maximum. Leave the expansion tank cap off. Leave the car standing overnight and next morning look at the coolant level. If it has dropped there's probably an unidentified leak somewhere. If the coolant level hasn't dropped, it points towards a problem that is caused when the engine runs.
With the expansion tank cap still off (get a flashlight to help you) start the engine and peek into the expansion tank. Watch what happens to the coolant (though do keep your face out of harm's way). As the coolant begins to circulate air bubbles will probably appear within the coolant. This is quite normal - air is 'bleeding out' of the coolant as it circulates. The air bubbles should stop after a couple of minutes.
As the coolant warms it will rise up in the expansion tank (keep your face out of the way ..). If the air bubbles continually appear or, there's a constant and continual stream of bubbles or a 'violent bubbling' then this may point to problems with a warped/cracked cylinder head or leaking head gasket (or both).
A defective cylinder head/gasket can allow exhaust gasses to be pumped into the water jacket (the coolant system) simply by the compression action of the pistons. Just like a hypodermic needle can inject air into your bloodstream. When this happens - pressurised exhaust gasses being forced into the cooling system - the cooling system itself becomes pressurised.
The coolant itself can find its way into the cylinders where it is vapourized and pumped out of the exhaust along with the exhaust gases. If you can't find any leaks - the missing coolant may be going out of the exhaust as steam (though you may not see any steam as such). A classic sign of cylinder head/gasket problems is overheating. Check your oil - if there's a yellowish/creamy mayonnaise/sludge that's another sign of head problems. The sludge is caused by coolant finding its way into the oil.
No leaks, no bubbling expansion tank - and if you're happy that there isn't a head/gasket problem, turn your attention to the radiator, thermostat and water pump.
With age, cooling fins on the radiator can corrode and crumble away, reducing its cooling ability. Whilst coolant still passes through the radiator it isn't being cooled sufficiently. Check the condition of your radiator. Radiators can also suffer from an internal blockage. With a warm engine that is switched OFF, feel the top radiator hose - it will be hot. Then feel the bottom radiator hose. If the bottom hose is cold it indicates that coolant is not finding its way down/being circulated maybe due to a radiator blockage or failed thermostat. Flushing may cure blockages.
A thermostat can fail in the 'closed' position. When this happens coolant is prevented from getting into the radiator via the top hose. The coolant in the block then overheats causing the temperature gauge to hit red. The thermostat is located (usually - it depends on make/model variations) on the cylinder head where the top radiator hose joins. They're very easy and cheap to replace.
The water pump can leak water when the bearings/seal fail. Coolant that slowly drips onto a warm engine soon evaporates making detection difficult. Way back, some water pumps had plastic impellers (perhaps they still do). The plastic vanes on the impellers used to wear away with age and use, leaving a spindle spinning uselessly in the coolant - not pumping it. Think of an airplane with a propeller. If the propeller blades wore away the 'nose cone' would just spin uselessly and no air would get moved ..
Before jumping to any conclusions and replacing parts unnecessarily, get a workshop to look at the car. A workshop will be able to test the coolant for exhaust contaminants within minutes (or pressure test the coolant system. If there's contaminants present - there's a head problem. No contaminants present - the fault lies elsewhere.
Back to the cylinder head:
It's a 2-3 day job to do the work yourself. A cylinder head must be skimmed prior to refitting. Refitting an unskimmed cylinder head back onto an engine cures nothing.
Years ago, here in the UK, there was a liquid additive called 'head weld' (and one for the radiator called 'radweld') which provided a TEMPORARY get-you-home fix. Head-weld was a liquid added to the coolant system. It contained fine particles in suspension ... these particles were carried to the crack/leak in the cylinder head and formed a 'dam' that was held in place by the water pressure until they (the particles) hardened.
Recently I noticed an advert for a product called 'steelseal' - here in the UK. The advertising blurb claims that it uses new technology without particles to form a permanent fix for cylinder heads/gaskets. It's a clear liquid that you just pour into the cooling system and then run the engine until its fixed. I've never tried it. At around 45 dollars a bottle it isn't cheap, though if it does what is claimed then it's a hell of a lot cheaper than having a cylinder head/gasket fixed. No doubt there are similar products on the shelves of car accessory and parts shops near you.
Aug 12, 2008 |
1992 Volvo 960