That plug may not “Unscrew” in the typical sense. It may be the type that turns just a little, then it goes into a groove and opens with a quarter turn. If its been in for awhile, you may have to pull as you turn. They are designed to be installed quickly and easily at the factory, not designed to be convenient for the mechanic.
I usually use a hose in the port at the bottom and stick the end into a gallon jug, like a cleaned out mild jug or old antifreeze bottle. I have the bottle sitting in the drain pan just incase it overflows. Unfortunately the block drain is usually not so easy to drain, its always messy. Just a bolt in a hole.
I’ve had those plugs break when you try to remove them. And that one looks weathered and brittle. They can be next to impossible to find replacements, so I prefer to pull the lower radiator hose and use a big catch pan. More of a mess, but better than running around trying to replace that plug.
You don’t pull it out unless you are going to replace it. They do bugger up pretty easy but generic replacements can be found in the help section of O’Rielly’s or Advance auto Parts, but not AutoZone for some reason.
I’ve never broken a petcock either. But that’s usually because I’ll give up first and pull the lower hose rather than breaking the dumb thing. I hate those radiator drains.
I’ve never broke the threaded type petcock, but these ones that use a little pin stuck in plastic that is supposed to ride in a groove are easy to break especially if they have been in place for 10 years or more.
OK I finally got a chance to flush and replace the coolant today…
drained all the coolant through the coolant drain plug till no fluid coming out and fill with prestone extended life coolant
I put in about 5 liter of new coolant (not premixed), the mazda manual says 3.7 L v6 mazda6 coolant capacity is 9.8 liter
so I was expecting the coolant level will be low and I can keep adding water… but the coolant level is full!! after only putting in 5 liter of the concentrated coolant…
drove around the block with heater on and coolant level only went down a tiny bit…
how come?? were there a lot of coolant/water left in the system?
That is my experience with cooling systems. That is why I never buy pre-mixed fluid. Unless you remove drain bolts from the block (and who knows where they are and are the back ones are accessible?), the lower rad hose and the thermostat, you can’t get all the fluid out.
This is why I drain what I can, run plain water through the system until the system drains clear.
After it is done draining, fill with proper amount of full strength, fill the coolant jug with half and half and keep topping the rad with water until full.