EF 6cyl.

it constantly stays on 'R' when driving ... and when i turn off the car, the overflow tube pours out about 100mL of coolant.

what causes this?

would a new overflow bottle solve the issue?

Could be one of three things:

1) Blown head gasket pressurising the system.

2) Faulty radiator tank cap (spring wont hold pressure)

3) Air block in cooling system

2 and 3 are easy fixes, number 1 sucks.

Generally when the engine stops the heat rises as the water is a heat sink for the engine. it is not passing through a cooling radiator so the temp rises. This can cause an over pressure and the tank cap is releasing this. I would go an buy a new cap (not one that is rated higher, get the right one). Start engine with cap off and turn the heater on full. Let car idle for 10 mins then slowly rev the car. This should remove all bubbles. Put cap back on and hopefully all problems are gone. If not go to local radiator shop and get a free pressure test.

cheers
Scott

Replace thermostat after a full cooling system flush (wise to have radiator professionally cleaned/flushed). Make sure the internal coil in the lower radialtor hose has not collapsed/rusted as this keeps the hose from sucking in and restricting coolant flow. Check seal on reservoir cap. If its hard or cracked spend the $10 and replace it. As stated above the engine temp does raise when stopped causing more pressure and if seal is poor coolant will leak via a poor seal. Ensure thermo fans are operating (turn on AC. Fans should come on) I hope for your sake it isnt a head gasket but check the simpler and cheaper things first as above.

im actually in the process of taking out the motor to replace the piston rings... ill repace the hoses, thermostat (for cooler one), and radiator cap

I vote radiator cap.

havent tested the radiator cap yet (due to car rebuilt engine not being in yet)

but the water pump was screwed (bearings f**ked) ... so that probably contributed to increased temperature.