Replaced thermostat, radiator, fan blade, fan clutch, flushed system, new coolant. No leak in coolant system. But for some reason my engine is still overheating when I idle with air conditioning on. Steamed n sprayed out from overflow tank after 40 minutes of idling with AC on. Temperature gauge fluctuates while driving, system is burped doesn’t seem to have any air stuck. Only thing I noticed is inside the radiator seems like it isn’t circulating as much as it should (maybe?). Not sure what to do for diagnosis or solution. Also note did a motor swap (old motor had knock n possible head crack) n the other motor had same heating issue. Please help!!

 

Reverse the heater hoses at the firewall, i.e. reverse flow thru the core, but you may need to change the core
OR
by-pass the heater core as a test

1994 4.0l cooling system uses the heater core as the water pump by-pass, if the heater core gets clogged up then flow is reduced and temp goes up, especially at idle/low RPMs
Reversing the hoses changes flow direction and can allow better flow, but core is most likely already clogged up at the bottom, so temporary respite
Removing both hoses at the firewall and connecting them together, by-pass the core, is another way to test if clogged core is the issue
Heater core is easy to change in a 1994 and not expensive

Fan shroud is IMPORTANT, especially at idle
Check the fan belt routing, if water pump has a smooth pulley surface then the BACK side of fan belt should be on it, if a ribbed pulley then the FRONT side of belt should be on it
4.0l OHV has a smooth pulley on water pump
This matters because water pump would be spinning BACKWARDS, and so would the fan, if belt routing was wrong, reversed


Head gasket or cracked head
Either causes the same symptom, overheating, and 4.0l OHV heads are prone to cracking if EVER overheated, so you may not have had a cracked head when you got the new engine, but may have one now because of the overheating

Cold engine, remove rad cap, top up radiator to the top
Start engine
There will be an initial "burp" of coolant out the cap opening, that's normal
Let engine idle for 2 min, and watch for any overflow of coolant from rad cap opening, if it starts to overflow almost immediately then you have a blown head gasket or cracked head, 100%

Cooling system has no internal pressure until coolant gets above 100degF or so, that takes 4 minutes or so
Pressure comes from coolant Expanding as it is heated, like any liquid does
So any internal pressure that would push coolant OUT of rad cap opening can only come from a cylinder leak into cooling system passage, each cylinder has minimum 150psi to maximum 1,100psi(when it fires)
You can do Glove Test or Block Test to confirm