Blown head gasket. what is the cause? there is very high pressure in the radiator and eventually a lot of air and little coolant. it then starts over heating because all the coolant gets pushed out at the reservoir. new thermostat and radiator cap fitted.

You m?y have a blown head gasket letting exaust into the cooling system. The pump cannot push exaust. Try a liquid head gasket sealer before replacing gasket. But first open the radiator cap and start the engine. Watch for the coolant to bubble. Top engine and add coolant or water. Some times after doing work on a cooling system you may get air in the system and need to *burp* the systems.

Hello Malcolm , A cracked head, cracked block, or a blown head gasket can cause your problem. Put a pressure tester on the cooling system to determine if the pressure is too high.

Thermostat is bad or the head gasket is blown, Test on cold engine, first remove radiator cap, start engine and observe coolant in radiator, you should not see any air bubbles being produced by the engine as it warms up, some coolant may slosh out so locate a drain pan under car to catch any overflow, keep pets away from coolant!
If air bubbles are present the head gasket is suspect, further testing with a cooling sytem pressure checker is required to confirm the diagnosis. I had a 22RE in a 94 Toyota Pickup that had a warped block causing a blown head gasket. Not a cheap repair if it is.

Bad thermostats do not let the coolant into the radiator hence the engine will be hot but the radiator will feel cold when running.

Be careful with radiator caps, never remove one while the engine is hot, scalding is a most unpleasant experience.

Your radiator cap may not be holding pressure, or there may be a head gasket failure that's allowing compression pressure to get into the cooling system. The overflow is supposed to have *some* coolant backup in it normally; I assume yours is pushing a lot of coolant into the tank. That's usually a sign of a blown head gasket (and sometimes a cracked head), which is distressingly common on these at high mileages - particularly when the radiator fan connector burns out or the intake gaskets have failed and let the coolant escape to cause an engine overheat.

SOUND LIKE BLOWN HEAD GASKET.TAKE VECHICLE TO A RADIATOR SHOP OR GARAGE. LET THEM DO A COOLANT PRESSURE TEST.THEY WILL PRESSURISE COOLANT SYSTEM SEE IF THERE IS LITTLE AIR BUBBLE IN RADIATOR.OR COOLANT LEAKING DOWN IN OIL PAN.YOU CAN LOOK AT OIL DIP STICK.LOOK LIKE MILK SHAKE YOU HAVE A BLOWN HEAD GASKET.

U have a leaking engine cylinder head gasket, very common on this van.

could be that it was hot or could be that you have a blown head gasket. If the engine got hot once it could have been enough to blow the head gasket. Does it blow it out after it warms up every time?

The three most likely thing's cause overheating are faulty thermostat,electric fan or clogged radiator. Only at has been overheat so much it boils dry that it will cause engine trouble. Most common problem's are blown cylinder head gasket or a cracked cylinder head.
mixing coolant and water depends on where the gasket has blown or head is cracked. You will first need to check if it actually is overheating. The reason is. If the head is cracked or gasket blown then you can get a false boiling which is compresion from the cylinders blows through the crack in the head or gasket causing the coolant to blow out of the radiator cap.Fill the radiator to the top leave the cap off then start the engine.If the water blows out then you will need to remove the cylinder head check the gasket is ok. If its damaged replace if not get the head checked for crack's. If no water blow's when you start it check the thermostat is working properly. To check the thermostat put it a in a saucepan cover it with water bring the water to the boil. It should open before the water boil's but near boiling-point. If its ok then start the engine again leave it running until it boils and check the fan is running. If that's ok then its radiator problem. Their is no way to check if the radiator is ok you will need to get a new one. One last point. If the cylinder head or gasket are faulty it was caused by overheating so by just fixing the cylinder head problem is not a cure it will overheat again. Check the three things I mentioned at the begining and told you how to check them. Hope this help's. Cheer's

i can give you a general answer. the cooling sys is a closed system. as the engine warms up pressure is also increased. kinda like how a boiler works, or a pressure cooker. i dont know the figure of how much pressure is produced within an engine that is operating normally. the engine's thermostat. the thermostat of many car engines opens at 195 deg coolant temp, allowing the coolant to circulate. high pressure when considering a car's cooling sys to me is relative. if you try to remove the radiator cap when the engine is at/near/above operating temp you will be burned by the hot coolant under pressure rapidly escaping out the radiator. most pressure that is above normal is caused by an obstruction somewhere in the system. a t'stat that is stuck shut is an example. air pockets within the system will also lead to high pressure within the system. air pockets occur when you refill the system with coolant/water. pouring water/coolant back into the radiator traps air that can circulate throughout the system and you must ensure that these air pockets escape out the radiator, or you risk damaging the engine from excessive heating.

Twice in two years now my Saab 9-5 has had a pin-hole in the oil cooler, which has caused oil to flow into the reservoir. A way to tell is if coolant/oil is coming out of the reservoir cap. The reservoir cap is rated for the pressure of the coolant system, which is significantly lower than the oil pressure. When there is a hole in the oil cooler, oil will be forced out at relatively high pressures, causing the reservoir to fill and then over pressurize and spill out of the cap.