COOLANT/WATER LEAKS FRONT BOTTOM PASSENGER SIDE. CAR WILL GET HOT IN STOP AND GO TRAFFIC. BUT WILL STAY IN THE MIDDLE TEMP WHEN ON FWY OR STRAIGHT ROAD WITH OUT STOP.

No your car will not overheat, but it is advisable to my 50% coolant on the water to prevent deposits of rust and other debris that may affect the effectiveness of the cooling system.

there are 2 sources of water, one is good, one is bad.
the good is pure water from the A/C condensate tray, tube. drain.
the bad is the Green/ org kind called AF coolant.
antifreeze coolant, (only YOU know the color)

the acid test for AF coolant leaks is.... (hot ,never cold)
is:
1; drive till fully hot, 20 miles, (engine temp reaches normal on dial)
2: idle until rad fan comes on,
3: shut off engine, (safety)
4: the system is now at 14PSI pressure
5: any leaks are now in full view (assumes not in engine oil side)
all you do is stand there, and look for 10mins.
all hoses, the RAD , all coolant lines. (hot lines are coolant)
and the block -head split , no leaks.
on the passenger side carpet (heater core leaks)

if you look , you will see the leak. and the exact source.
it is very easy to do this, hands i pocket,

I'd have to see it in person to comment. The water pump is on the passenger side of the van.
A mechanic would fill the cooling system then pressure test it for leaks.

Yes, the temperature gauge should normally stop in the middle, between the cold and hot points on the gauge. When the engine is cold the thermostat will be closed and will keep recirculating the coolant in the engine block until it warms up sufficiently. Then the temperature of the warm coolant will cause the thermostat to open the channel to the radiator to get rid of excess heat, which should stop the coolant from over heating. So the effect of the thermostat keeps the coolant at a fairly stable temperature. When the coolant runs low, it can't transfer enough heat to the radiator and the coolant and engine then over heats. Watch for radiator leaks under your car after you have been driving it. If the coolant goes low again fairly quickly you could have a leak in a hose or the radiator itself.

CAR OVERHEATING FIRST CHECK FOR LOW COOLANT IN THE RADIATOR COOLANT OVERFLOW JUG.IF COOLANT LEVEL IN OVERFLOW JUG LOW REFILL ADD COOLANT BACK TO THE FULL COLD MARK.PUT RADAITOR CAP ON COOLANT OVERFLOW JUG START ENGINE LET IT IDLE UNTIL IT OPERATING TEMPERATURE.MAKE SURE TOP RADIATOR HOSE GETTING HOT WHILE ENGINE WARM UP.IF TOP RADIATOR HOSE DONT GET HOT WHILE ENGINE IDLING YOUR THERMOSTAT COULD BE STUCK CLOSE CAUSING CAR TO OVERHEAT.IF TOP HOSE IS GETTING HOT.CHECK FOR COOLANT LEAKS CHECK TOP RADIATOR HOSE AND BOTTOM RADIATOR HOSE FOR LEAKS.CHECK FOR LEAKS AT THE RADIATOR CORES AND PLASTIC SIDE CONTAINERS.IF ALL IS GOOD ENGINE OVERHEATING WHILE IN A LONG TRAFFIC LINE, COOLANT FANS NOT COMING ON.COULD HAVE FAULTY ENGINE COOLANT TEMPERATURE SENSOR OR BLOWN COOLING FAN FUSE OR RELAY.COULD HAVE PCM FAULT.MAKE COOLING FAN ELECTRICAL CONNECTORS AND WIRING NOT DAMAGE OR BROKEN.

Both fans don't necessarily have to start and stop together but you do need both of them to adequately cool the engine. If you plugged the harness from the drivers side fan into the passenger side fan and it worked then I think you've ruled out any electrical problems with the wiring. It's probably 1 of 2 things. Either the fan has gone out, or the temperature sensor has gone bad. I would start with the temperature sensor first. They are inexpensive and easy to replace. The sensor just threads into the radiator, usually at the bottom.

Good Luck!
~Brandon

Known head gasket problems on those cars. Suburu dealership have a good product called stop leak. Use that first add a bottle into the radiator run the car to operating temp. Recheck the belly pan for leaks. hope this helps.

waterpump houseing/shaft seal is on the way out hence the water/coolant, running along the block. when hot the seal does so, when cold and depressurising seal doesnt.

Sounds like the water pump, it will fling coolant as it leaks when the engine is running, also check the heater hoses and if coolant seems to come from the bottom of car and not the engine then it is a heater core leak.

look for leaks on the passenger side!!!! also this car has a "air pocket" you need to find the bleed off screw on the upper hose fitting on the engine and bleed it when filling up with coolant