Very good to have the manuals and asking questions. More should do that.
You should not need to drain the coolant. Check to see where the thermostat is in relation to the top of your radiator. If the thermostat is lower than the radiator cap, then when you unbolt it, coolant will go everywhere, in which case it would be best to drain some coolant to prevent that.
That said... you might consider doing a coolant flush anyway depending on how long it's been since you changed your coolant.
Few questions: Have you determined it's not the water pump or cooling fan?
If you have a mechanical fan, you might see if you can see any fluid leaking from the fan clutch, and also spin it a few times with the engine off, to see how freely it spins and if it wobbles at all. If it does either, spin freely or wobbles, the clutch is bad, and at least part of the reason you are overheating.
If it's an electric fan, spin it while off, see if there's any resistance. Resistance is futile. *cough* Then see if the fan comes on when it should. First start the car and turn the AC on. The fan should always run when AC is on. Then with the AC off, see if it kicks on when the coolant gets hot. If not, you either have a bad fan, or fan relay, or coolant sensor. (there's normally 2 sensors, one that runs your coolant temp gauge, and one that turns the fan on)
Water pump is a bit harder. Remove the belt and see if the pulley has any play in it. Also check is water is leaking from it. Finely with the pulley on, remove the cap to your radiator, and start the engine. When it warms up, if the thermostat opens as it should, you should see coolant flow in the radiator. A PT cruiser I looked at, the water pump had worn away all the vains resulting in overheating. Coolant flow was almost non-existent at idle.
Mechanical fan clutches and water pumps wear out at nearly the same rate. If one is bad, the other is normally not far behind. If you do pull the thermostat off, you can check it pretty easy by putting it in a pan of water and boiling it. It should open pretty fast when it hits 190 degrees. (btw, do not replace a thermostat with anything less than 190 degree)
I'm having some overheating issues with my '94 Jeep (140K miles, straight-6, 4.0L). I know that, although I live in TX, it hasn't been this bad before. Although I'm probably going to get a new vehicle by the end of the year, I'll need to keep this one running until then...
The issue is either the thermostat (probably never replaced) or the fan (which may have run out of viscous fluid). I'm replacing the thermostat first, but I'm wondering:
Is is a necessity to drain the radiator first when replacing the thermostat? And...I've been told I may have 2 thermostats (some Jeeps are set-up like that) - how can I tell without taking my cooling system apart?
I'm not afraid to work on my Jeep, but I'd also like to prepare I'm ready to tackle some fixes. My Chilton/Haynes manual says it's gotta be done - it seems like a hassle to me, but based on the plumbing, I don't see away around that task.