The engine in my 1967 2+2 Hardtop has been rebuilt, balanced and lovingly reassembled by professional engine rebuilders. New radiator, new water pump, new oil pump, reground and balanced crank - the works! Rebuilt Rochester QuadraJet 4 barrel carb, mild cam, new long branch headers, electronic ignition - the list goes on. However, we have a problem with overheading at idle and low speed in city traffic - 230+ degrees and pumping out about a quart of coolant. No problems at 50-60mph - 180-190 degrees. Water flow looks good, first new water pump replaced with another Flowcooler water pump from Ames, divider plates are set up to spec and according to advice from Ames. We have even adapted a fan shroud from a later model Pontiac to improve the air flow, as well as replacing the fan with one with more blades to increase air movement. But we think the problem is more one of heat generation tha heat removal if that makes sense. We've doubled checked the water flow through the new radiator ( radiator guy wasn't too impressed at being asked to verify his new radiator). Does anyone have any suggestions or a Pontiac engine guru my mechanics can contact and bounce thoughts off. We have heard some suggestions that this can be an inherent problem with the 428 - correct or just well spread rumour?

Our mechanical guys are 50's-70's car enthusiasts - but they mainly see Fords and Chevs, know where to go for Mopar info, but haven't seen much in the way of Pontiac engines.

Too much effort and time has gone into this project to risk damaging the engine by overheating in a parade or at city traffic lights. Any help will be appreciated.

Bruce

Thermostat?
You mention a new fan - it it has a clutch, make sure that is functioning correctly at idle.
Also, there are problems with incorrectly rebuilt Pontiac water pumps, so check that this PY thread:
Click here for link

Good luck - probably something simple.
Let us know what you discover, please.

Thermostat - tried it with and without. Perservering with one, correctly set and opens as per specs. Actually it is on to the 4th thermostat to make sure it is dead right. Saw the link about water pumps - the last two we have used, plus divider plates, have come from Ames in the last couple of months so assume we have the correct version. At this stage we suspect their may be a coolant flow issue in one side of the engine (maybe a blockage or air lock) based on running a temperature sensor over the heads and block. Thanks for the help.