Drained old stuff out via petcock on lower corner of radiator of the 2000 Ford Ranger. Didn't seem like all that much came out; book says it holds 13 quarts. Put little over 1 1/2 gal of 50-50 Green in radiator and overflow tank. Went for 10 mile drive and temp gauge topped out, but fluctuated up and down some. Never got into the red but darn close to it. Never had any issues with it before; in fact the gauge normally is down below the half mark. No sludge or unusual nastiness in the drained fluid. Does this sound like an air block? If so, what can I do about it? Have drained many vehicles this way before and never had an issue.