No heat in 97 mercury villager

heat gauge indicate hi temp, coolant leaking on ground below engine, coolant over flow tank constanly low, noise coming from pump body

Sounds like an air pocket in the coolant system. Those can be a pain, some cars have bleed valves to help. If there is an air pocket, it will seem to be full of coolant, but when the air pocket gets to the heater core it gets stuck there and guess what? No heat, because you won't get any heat unless hot coolant is in the heater core. Old fashioned way is to open the system and keep pouring coolant in at operating temp with heat on full until all the air is pushed out. If it has a bleed valve that can be helpful. Have seen this problem on many vehicles.

air trapped in cooling system. Loosen cap, cold engine, fill rad & res. run engine at 2500 rpms to spin pump fast and it will purge out rad cap. tighten cap and keep res full until engine cools down and sucks coolant into rad.

This sounds like a thermostat that is fully open all the time. It allows the engine coolant to warm when sitting still (no air being forced thru the radiator), but even that goes away when moving, since the air through the radiator will remove that heat from the coolant. Does you van have a temp gauge? Is so, at full operating temp. most vehicles have coolant temps of 180-195 degrees F, depending on the thermostat installed. Anything less indicates a thermostat that is starting to malfunction.

Did you check your coolant level? If it gets low you won't get heat. Is the temperature getting completely up to the normal operating range? If the thermostat sticks open your engine won't get to normal range and you won't have good heat.

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It sounds like you have low coolant in the engine, If your coolant is low you may intermittently have no heat, the rear heater probably gets coolant first, and that is why it seems like it is performing better. With a cold engine add about a pint of coolant above the full line, then warm the vehicle up to regular operating temperature and shut it off. As the engine cools the coolant from the resevoir should be drawn back into the engine. If after the engine cools the coolant level is lower than where you had filled it to, try this again. The reason for bringing the engine to operating temp is that as the coolant ciculates, if their are any air locks in the system they will be circulated out to the resevoir.

you have air trapped in the system.
Park facing uphill (on car ramps if you have them).
with cold engine, remove radaitor cap and fill radiator.
Fill reservoir.
leave cap on loose.
Turn on heater.
start engine and run until upper radiator hose gets hot (meaning thermostat has opened).
Check coolant level and top off. Repeat until temp gauage reads normal and heat is coming from heater ducts.

go here for villager/quest help.

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if you replaced the thermostat, you have an air bubble trapped in the engine. So, you don't have any coolant running through the heater core, therefore no heat.
sometimes hard to "burp" air bubble. Try this. Park park facing uphill and jack up front of car too. You want to get radiator opening higher than top of engine. Then remvoe radiator cap (cold engine), fill radiator, then fill coolant reservoir to full point. Install radiator cap but leave it loose so air can escape. Start engine until upper radiator hose going into radiator is hot. This should burp air. Repeat a few times while adding coolant mix (50/50 antifreeze) to keep radiator full. Be careful not to remove radiator cap when engine is hot.
lwt us know how ou make out. There is a more complicated procedure but you need tools and knowledge of where air bleed is on top of engine

you have air trapped in the engine. very common. park van uphill, fill radiator & reservoir, turn on heater, leave cap loose. wait for engine to warm up to burp air.
also see

http://autos.groups.yahoo.com/group/villagerquest/