Ok, so here are the facts as I understand them. Please correct any wrong statements.
1) The car did overheat about 6 months ago. You replaced the water pump & thermostat. Soon thereafter, replaced water pump again with NAPA unit when car almost overheated.
2) With the pressure tester hooked up to the cooling system and the water pump deactivated, you get an immediate 3psi pressure when you start the car.
This would seem to be a good indicator of a combustion leak. Especially since the air in the cooling system has not had time to heat up & expand. Sources of combustion leak that I can think of are:
A) Head gasket failure.
B) Crack in the head between combustion chamber and coolant passages.
C) Exhaust leak. i.e. a crack in the cylinder head in the exhaust port lets exhaust gases into the coolant passages.
3) With engine off, you apply 15 psi pressure to cooling system. You see 1/2 psi drop in 5 minutes. This may reinforce #1 above. However, there could be many sources for this leak besides a head gasket.
4) With water pump drive installed, the car will overheat just idling in about 15 min. I am assuming that your thermostat is still removed here. Are we sure that water is flowing through the hoses? Can you feel the hoses get warm? Does the radiator get hot? Do the fan(s) come on? If it's not flowing then:
A) There's an obstruction
B) Air is blocking it??
C) The pump is not really pumping even though the input shaft is turning. Loose impeller?
5) You consistantly cannot get heat from the heater even with the engine almost overheating. From what I have read, this commonly occurs when there is a bit of trapped air in the system. Perhaps caused by air from #1?
Other CEGers, more input would be appreciated. I'm at my wits end here. I'm much more familiar with the actual engine than any accessory equipment.
