Article on The Internet Mini Encyclopædia
It could well be a failed head gasket, but there are other possibilities, and you hope it will be one of the cheap and easy to fix ones.
So I'll be the one to ask the real dumb questions:
Is the fan belt still there? (No don't laugh, and don't get all insulted, this
has been known)
Is the fan turning? (See above) (Seized water pump).
Is it losing a lot of water?
If so, where is it going?
Start it up and let it fast idle until hot, look for pools and drips underneath.
If you find drips, could be just a hose, favourites are the joint between the
heater hose and the bottom hose, and the little bypass hose between the water
pump and the head.
Other common leaks are from the heater control valve on top of the block, and the water pump.
Could be blowing water out of the radiator cap. You can check that by taking the overflow hose from the cap and diverting it into a plastic bottle to collect the overflow.
If it does pump water into the bottle, the cheap easy possibility is a tired radiator cap, but it is often a symptom of a blown head gasket. Let it cool down a bit, then cautiously remove the cap, and run the engine again. If the head gasket is gone, you often get bubbles of exhaust gas blowing into the rad, and an exhaust smell. Not to be confused with the surge of water from the top hose, but if it is blowing gas bubbles, you will recognise it. You would also expect to probably find water in the oil (forming dirty mayonnaise under the rocker cover).
If it is losing water, but no puddles on the driveway, is it blowing out of the exhaust in billowing clouds? You do expect some vapour on start-up on a cold day (I live in Scotland, we do get the odd cold day) and a few drops of water even after warmed up, but not a steady trickle of water, and not steam train clouds. Sniff for the smell of anti freeze. Another sign of gasket failure.
Incidentally, starting from cold with the radiator cap off lets you look for signs of the thermostat opening. Little movement in the water to start with, then a surge from the top hose as the thermostat opens. It that doesn't happen, the stat isn't open or the water pump isn't working.
With the engine is hot, and the radiator cap on so that there is pressure in
the cooling system (squeeze the top hose, you should feel the pressure),
switch off and remove the plugs, leave it for a couple of hours. If the head gasket has indeed failed and exhaust gas has been getting into the waterways, the pressure in the cooling system will now force water into the cylinder.
When you came back after it has cooled down, put the car in gear and roll it forward to get one pair of pistons at tdc.
Poke a pencil or screwdriver or something in the plug holes (try not to let it fall in), and it may come out wet due to a puddle of water in the piston bowl.
Roll forward to get the other two pistons at tdc and repeat.
The wet pencil is a sure sign of gasket failure, and it tells you which
cylinder is affected.
However, absence of water in the cylinders is not a complete guarantee that the gasket is OK, because the gasket might fail between waterways and oilways, but not round a cylinder. However, that seems to be less common.
Good luck, hope it is an easy one.
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