Do the Glove test to take blown head gasket or cracked head off the table.
Free and easy test
You will need a latex glove and rubber band, or a balloon, or even a condom
Cold engine
Remove rad cap
Drain some coolant out so it is at least an inch or so below the rad cap opening
Remove overflow hose from rad and Block that port, gum, putty, vacuum cap, ???
Put latex glove over rad cap opening and seal it with rubber band(or put balloon or condom over rad cap opening)
Cooling system is now sealed and there should be no pressure in it or any way pressure can get in it
Unplug Coil Pack's 4 wire connector, you want a No Start/No spark
Crank engine and watch the Glove
If it starts to bounce/inflate you DO have a head gasket or cracked head issue, 100% accurate
Remove 1 spark plug at a time, and crank engine, when bouncing stops last spark plug removed was from the leaking cylinder, replace it to confirm.
If Glove does NOT bounce then move on to water pump
Since you have replaced fan clutch then it is a water pump problem.
You will find the impeller blades are worn down so it can't pump coolant fast enough at lower RPMs
Do NOT replace water pump without Glove Test, waste of money if you do
Each cylinder in the engine has 150+ PSI compression pressure when cranking, thats what makes the glove bounce it there is a leak
Each cylinder is surrounded by cooling chambers and passages to carry heat away, and the coolant passages pass thru the head gasket next to the metal ring that seals the head and block cylinders.
Blown head gasket means that metal ring has failed, and when it does the pressure INSIDE the cylinder will force its way out and usually into a cooling passage.
If engine is running this high pressure "air" is pushed into the head, "air" rises in coolant, this displaces the coolant, so forces some into OverFlow tank, it also does not "cool" like coolant does, so engine starts to heat up.
At higher RPMs the coolant flow can push the "air" out of the heads so cools the cylinders and "air' ends up at top of rad or heater hoses, no heat from heater at that point
When you slow down, lower RPMs, the "air" starts to collect in the heads and engine heats up
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