I'd like to flush the cooling system for 3 reasons:

1. I have no idea if the previous owner ever did it
2. I have really feeble heat coming from the heater core
3. I am replacing the hoses, thermostat, and water pump as part of an engine improvement plan (SC pulley, harmonic balancer)/ leak proofing exercise

Question...how do add a coolant system flush additive? Do I just pour it into the expansion tank and drive around or do I need to drain the system and refill with water and the additive? I've never considered if the expansion tank fluid ever circulates back into the engine.

 

You may want to contact Way Motor Works or Mini Mania before you add anything to flush your system.

Since you are going to replace hoses and replace all the coolant in the system, I may suggest that you
remove the expansion tank and clean it out by hand.
It seems to collect dirt and if it has never been replaced, this would be a good time to replace it
as its subjected to engine compartment heat and can crack and develop leaks.

 

I used the prestone flush on my car. I just followed the instructions. Added, it, drove it for a few days, drained it, and did my usual coolant change routine. It worked like an absolute treat. I would replace the thermostat and give it a shot, you can always replace the heater core and hoses later, but this might save you the time and money

 

If you're getting little heat from the heater core it may be partially blocked. My 2003 completely failed even though I had the coolant changed every three years. The shop cut the core in half and showed me how badly it was blocked and mentioned that the passages were much smaller that most cars they serviced. Good luck with the flush; maybe the core can be saved...

 

+1 on replacing the expansion tank. A metal replacement is highly recommended.
Take it over Gentle Annie, that should shake loose any sticky sediment.