General rule: before upgrading your cooling system, make sure that the stock setup is working as intended.
test 1: (only for cars with mechanical clutch fan): cold engine (morning): Give a hand spin to the clutch fan. if it turns an entire rotation or more, it's faulty. Should spin half rotation.
test 2: (only for cars with mechanical clutch fan): hot engine running. have a buddy look at the clutch fan. turn off the engine. if the fan spins more than 2 seconds, it's faulty. Should stop relatively quick.
test 3: cold engine (morning). turn on the engine, grab the hose going to the radiator. if the hose starts to get warm straight away you have a stuck open thermostat. should start getting hot with coolant above 86C (check at what temperature your thermostat is designed to open)
test 4: inspect the radiator stack for dirt and debris. wash it with a water hose (no pressure nor aggressive detergent or you damage the fins).
test 5: you need a radiator funnel for this... attach the funnel to the radiator, partially fill it with coolant or distilled water. turn on the engine and after some idling accelerate. if you get no bubbles you are OK, if you get bubbles and after some time they stop you effectively removed air, if you keep getting bubbles you may have a damaged head gasket
test 6: check AC gas pressure. if the gas is too low or too high the AC compressor may cause unnecessary load and heat
Other potential causes:
- tap water in the radiator reduce heat exchange, cause rust, and clogs the radiator
- leakage in the coolant reservoir cap (also called overflow tank) can prevent optimal coolant pressure and cause cavitation and micro-boiling. check if your car is designed to have a pressure valve on the cap of the tank or of the radiator.
- inverting the caps (radiator and coolant tank, see above) can cause issues like air not exiting the radiator, cooling system not being pressurized of building excessive pressure, dirt getting in, etc..
- damaged water pump can prevent effective circulation of water and cause micro-boiling
- cracks/leaks can cause air in the system
- mixed coolant types (i.e. red Toyota with green Nissan) can cause gunk/deposits and clog the radiator
- if the radiator fluid is too old it can turn acidic and damage the head gasket or other components
- if the PCV valve is stuck open it can cause gunk in the intake plenum and get you hotter air sucked in from the crankcase
- a thermostat that doesn't fully open (because of damage or deposits) restricts the flow of coolant circulation.
