The cap comes off very easily. I did that a few years ago. I was not able to put in the same quantity that came out. I don't know what I did wrong. I freaked out and had it changed from the dealer from then on. Also, when I completely drained the radiator, the white radiator fluid bottle did not drain and there was still the SAME old fluid in it. I think the dealer uses a pressure machine that sucks/pushes fluid in and out and forces the fluid out that we are not able to take out with a simple drain...... Just my experience. May be I never knew what I was doing and never asked anybody about it.
When changing out coolant there are a few things to keep in mind...
1.Bleed the air out of the system, either via the bleed valve or by running the system with the cap off the radiator until no more bubbles appear.
2.Make sure you use a coolant that won't harm your engine. Here, color doesn't matter, regardless of what the sales drones at the parts store tell you. Coolants that are silicate, borate and phosphate free are what you want. I've found Zerex Extreme Life to fit that bill.
3.Distilled water is a must.
4.If you're draining the system, have a good check of hose conditions prior to filling it up again...might as well swap them out if there is any evidence of cracking...
When I change my coolant, I open the drain on the radiator and the drain on the engine block, and remove the white coolant overflow bottle to dump it out. I then close up the valves and reassemble the reservoir, fill up the system with a garden hose, and let the engine run until the fan turns on. Then I drain out the system again and refill with a bottle of coolant and top it off with distilled water. It works out to about a 60/40 coolant/water ratio.
I like the idea of running straight water thru the system until it heats up enough to start the fan (and presumably open the thermostat).
These days I don't want to just flush the system by letting water run thru and drain onto the ground. I'm more eco-concious now than I was in the early 1970s when I did all my own service (because I was the only "tech" I could afford).
Glad you mentioned the coolant drain plug on the engine block.
The Chilton manual shows it; I never knew such a thing existed.
Question: Is the coolant drain plug on the engine block easy to see and access?