[TheSilverBuick]

I'll admit I didn't read all the thread (lots of stuff here! Probably good stuff too!), but my observations on my Skylark, with EFI and a live dash cluster showing most the data the EFI system is seeing and commanding. My observations of influences are generally at idle because always running lean at cruise and my radiator is large and well shrouded (with 7-blade clutch fan) so never have noticed a temperature issue at speed.

AFR matters, particularly at idle. I see a 5ºF coolant temp change with 1 point of AFR change. Same timing and idle speed. If I am idling at 14.0:1 it runs 5ºF warmer than idling at 15.0:1. There are times when lean conditions can cause overheating, but that is typically under load and bad news for exhaust valves, etc. I've never experienced a lean condition at idle that cause my coolant temps to go up.

Timing matters, particularly if you are idling rich. I have active idle control that involves retarding or advancing the ignition advance to meet the intended idle speed so it waivers a bit, but before that I did notice in general if I was idling above ~23º, the engine ran warmer. Pulling the timing back to 15º-18º, it ran cooler. I've never had to little timing at idle cause an overheating condition, nor too much at cruising speed cause a temperature issue that I've noticed (At light cruise I run around 45º advance at 2,400rpm).

If I was running mid-20's timing and below 14.0:1 idling it would easily run 10ºF hotter than normal, perfect storm'ish (and actually what it intentionally does to speed up engine warm up). It normally ran right at 185ºF, so would poke at 200ºF sitting at idle in this scenario, but with leaner AFR's and reduced timing it would stay right at 185ºF. With the proper timing and tune, I've idled in stop and go Las Vegas traffic in ~120ºF weather, where I was dying in the car, but the car was idling fine at ~185º-190ºF. In 2013, when I had the A/C system working, I was in surface street rush hour in Denver and it was 103ºF according to a bank clock, and I was sitting cool in the car with the windows up, A/C on and coolant temps still in the 185º-190ºF range. I changed thermostat's last fall and now it runs right at 180ºF, so shows the new thermostat is calibrated better than the old. I still don't expect any great temperature swings going into this summer.

As for water pump efficiency, I don't have any real experience with the Pontiac engines, but the OE timing cover I originally had on my Skylark had massive pitting and general ugliness behind the water pump that caused me to over heat at idle, or at least run far warmer than I'd like it too. Then I installed a new aftermarket timing cover, no other changes, and the coolant temps were brought under control and have been reliable ever since. So does show water pump impeller tolerances are pretty important, especially for slow speed efficiency.