First off, how many miles have you driven the truck since it was rebuilt? new engines always run how till the cylinders wear in,
How much coolant is in the radiator? to the top or just above the core? How much hits the ground? Is it a continuous flow?
30 minutes of idle is really hard on a fresh engine, so quit doing that! this is not a good indicator of how the cooling system is working on a fresh motor,
When driving, if the coolant temps keep rising, this is normally a sign of a weak radiator or lack of air flow into the radiator, a slow rise is telling you the system can't keep up.
Adding an electric fan probably blocks more air than it can produce while driving. A quick rise and boil over is a total lack of cooling or engine component fail.
Amazon sells digital thermocouple thermometers for cheap, they work really good for checking temps in hoses ( you can slip the thermocouple under a hose clamp or even under the hose into the coolant).
What is the water temp entering and exiting the radiator? A multi lead thermocouple thermometer will let you see this in real time driving.
A zero pressure radiator cap will let the coolant out the fastest, just one pound of pressure raises the boiling point 3 degrees. If the radiator can hold pressure, add a low pressure cap.
A mechanical temp gauge will always keep rising when the engine is shut down, the heat just doesn't go away when you shut off the engine. Mine goes up.
Does coolant run out of the block drain towards the rear of the engine? YES, block is probably clean. NO, you need more cleaning. A power washer with the RED pinpoint tip will break up the crude and blow it through the motor, so have the radiator hoses off and water pump removed, then blast though as many holes as you can. A garden hose is for rinsing it out, a power washer is for cleaning! It should have been cleaned out when it was rebuilt!
If the temp probe is in the head between two cylinders, it will read hotter than if it's under the thermostat. My 250 showed 20* hotter in the head before moving it to the thermostat housing.
Whats the idle rpm?
Can you feel air flow from the fan?
At high idle, can you feel air being pulled through the grill?
Is the pulley diameter correct for the water pump?
Is the fan blade correct, and does it have a decent amount of pitch to the blades?
Joe
Last edited by Joe H; 09/05/2026 2:00 PM.
