[lust4speed]

The possibility of an air bubble is grossly overrated. Even if the passage between the pump and manifold has been blocked, the pump will quickly route any air entrapment. For that matter this passage is about an inch down in the water pump cavity so there will always be an air bubble present on initial fill. Think about this - if air were a problem, then the heads would quickly overheat since there are about a dozen places per head that will trap air on the initial coolant fill up. What quickly happens is the air is displaced by both force and temporary saturation in to the coolant. Proof of this is that the radiator level always drops after the first good run of the engine - the air ends up at the top of the radiator and the coolant has replaced the air inside the engine. It does help to keep the top radiator hose at or a little below the top of the radiator, but this is after the coolant has completed its job, and the only handicap is a very slight reduction in coolant capacity.

The thermostat's only purpose is to maintain sufficient heat in the engine. First thing to do when you have a heating problem is to remove the thermostat. The Chevy tale about running hotter without a thermostat does not apply to a Pontiac engine. I have temporarily run several dozen engines sans thermostat when chasing down problems, and in every case the engine ran as cool, or cooler than a properly functioning thermostat.

Here's the latest rub - I've found thermostats from every manufacturer that were defective right out of the box, and the problem has compounded lately. So far the record is four thermostats in a row that were so far off that they couldn't be used. Problems included opening up to 20° late to only partially opening. So remove the thermostat and see if the engine runs cooler. If it runs the same, then your thermostat is good. Otherwise get another one, and test it out on the stove with a pan of water and a known accurate thermometer.

I like to think that I've done all my reading and all my homework on cooling, and that I have tested out everything I have read, and weeded out ideas that don't work. It comes back down to not believing everything you read on the internet. There can also be a coexisting problem that the person solves and incorrectly attributes the solution to the wrong fix. I started looking for answers 25 years ago with the purchase of my second '67 GTO. This one had A/C that didn't work, and I would have to pull over and let the engine cool off if I took it out past a 10 mile drive. This car already had the longer radiator and 19.5" fan. Fast forward to today, and the car runs at 165° with the air conditioning blasting cold air on our 100°+ days running around on the flat. Going up long, steep grades running the A/C on these hot days will still only get the car up to 180°. Best step forward out of all the things I changed was the divider plate fix that I read about first on PY.