Over heating.. leaks water out back of engine..

turning the heat on lowers the temperature of the water running through the engine which in turn lowers the pressure in the radiator

Fine the engine is heating and you say leak under van. What is the leak, is it water/ coolent/ oil or what?
is it possible to know where the leak is coming from?

Change the radiator.

i would suggest a new thermastat and maybe even a new radiator cap but dont forget to check your hoses they maybe collapsed

The water pump is in the front of the engine, so probably a heater hose or some ther part of the cooling system. Also, you didn't say which engine but some have a cover in the back of the cylinder head that can leak.

It appears that you have a minor leak at where a hose attaches to an engine or radiator pipe. When the engine is hot, any small leak will evaporate quickly and therefore not be seen. As the coolant heats up it will expand into the overflow tank but once the engine is stopped the extra heat build up will force the coolant out the leaking joint. As the engine cools, the leak prevents the coolant from being sucked back from the overflow tank.

Check all hoses and clamps including heater hoses and those pesky little bypass hoses in the cooling system for a good tight fit and replace any hoses that have gone hard or have become soft and stretched. Those spring type hose clamps tend to be less effective in clamping as the hoses age. Worm drive ones are the best to use.

If unsure take vehicle to a cooling system specialist and have them do a pressure test and more thorough diagnosis.

If problem persists, then it could indicate a problem with the head gasket (Usually caused by allowing engine to become excessively overheated when cooling system has run dry) allowing very hot high pressure combustion gases into the engine water jacket, which super heats the coolant in the engine block, which then boils off, and is released via the cap or any leaks. If the head gasket is really bad it will leak water into the cylinders and into the oil in the sump. This is indicated by an emulsion of water and oil mix on the dipstick. You will also see white smoke (water vapour) from the exhaust.

CHECK WATER PUMP WEEP HOLE SEE IF ITS LEAKING ANTIFREEZE IF SO WATER PUMP SHAFT BEARING GOING BAD CAUSING WATER PUMP LOCK UP HARD TO TURN.MAKE SURE DRIVE BELT NOT SLIPPING DUE TO BAD BELT TENSIONER.CHECK TO SEE IF COOLING FANS TURN ON WITH AIR CONDITIONER ON HIGH.IF NOT CHECK HEATER + AC IGNITION FUSE.IF FUSE GOOD.YOU HAVE FAULTY COOLANT TEMPERATURE SENSOR.IF ALL LOOKS GOOD.CHECK ENGINE OIL IF LOOKS LIKE MILK SHAKE YOU HAVE LEAKING HEAD GASKET.IF OIL LOOKS GOOD.YOUR ENGINE CYLINDER BLOCK WATER CORES CLOGGED UP WITH RUST RESTRICTING COOLANT FLOW THROUGH THE ENGINE BLOCK TO REMOVE HEAT AWAY FROM THE ENGINE.BLOCK PROBABLY NEED FLUSHED OUT AT RADIATOR SHOP.

rear cylinder block core plug its behind the bell housing.It has been caused by running the engine with no antifreeze which also acts as an anti corrosion inhibitor .

It actually sounds to me like you're low on coolant. My Jeep leaks coolant and whenever it gets low the temp gauge will go real high and then drop, and the heater doesn't work well. This is because the cooling system of your car is supposed to be a closed system, full of coolant and no air. When coolant leaks out, the space it used to occupy is now occupied by air, which does not transfer heat well. When 'air' is passing through your cooling system, no heat can be transferred from your engine to the heater and radiator, resulting in a hot engine and no heat at the heater. Then when a pocket of water passes through the system, the temperature gauge quickly falls as the water absorbs the heat from the engine. The hot water that cools the engine is where the heater gets it's heat from as well, so when water passes through the heater core, the heater works, but when it's filled with air, it doesn't.

Hot water runs through the heater core regardless of whether or not your thermostat is open or closed. That's why your heater works in the winter even before the engine is at normal operating temperature. The fact that the heater stops working is a good sign that the thermostat is probably not the culprit.

As for the water pump. If the water pump was bad, your temp gauge would go into the red and stay there. Water pumps generally don't work intermittently. Either it's good and ot works, or it's bad and it doesn't.

However, if you are in fact low on water, as I suspect, it means you probably have a leak somewhere. The leak could be in the water pump housing gasket, so depending on where you take it for repairs, they may try to sell you a new water pump anyhow. So just beware of that.