The mysterious overheat

[mercuryseven]

Strange issue.

2003 Ranger Edge - 3.0L - 96,000 miles.

Anytime the outside temp is at or below around 75 degrees, this thing never overheats. I can climb hills, do 80 up the Grapevine, sit in traffic with the A/C blowing, zero issues.

However, if the outside temp rises, maybe about 75-80....and up, it will overheat the minute the engine is put under any heavy load (climb, traffic, etc)

So basically, this truck ONLY overheats when it's warm out.

Coolant is clean and green, no leaks anywhere. Oil is free of any coolant mix issues, plugs are spotless and engine has tons of power.

Radiator was flushed this summer and done right by a reputable rad. shop. Water pump however, has never been changed.

I'm going to guess my pump impeller is rotting away but I'm curious if there might be another reasonable suggestion for why this is happening.

 

[Dune Runner]

Fan clutch working?

 

[RonD]

After engine is warmed up I would have some one raise RPM to about 2,500 and hold it there, then squeeze the upper rad hose and then the lower rad hose.
They should be the same pressure, if lower hose is easy to squeeze then water pump is pushing coolant into upper rad faster than it can be passed thru the rad to the lower hose, so lower hose has less pressure, this means rad has some blockage.

Also after shutting off engine, pull fan shroud out of the way and feel the rad fins, should have a nice even heat all the way across and top to bottom, any cooler spots are blocked tubes so no coolant is flowing.

Flushing cooling system and even rad is always a good idea, but.............you really need to pull the rad out of the vehicle to back flush it, and then do a "flip test" to see if there are any blocked tubes.
Flip test is very old but still works and is free
Place rad on its side(side flow rad), so tubes are going up and down
Put rad cap on
Put hand over bottom hose opening
Fill rad thru the upper hose opening
stop filling
remove hand and watch the flow, rad should empty instantly.
Tilt rad to get any water out the bottom
Now the test...............Flip rad upside down, if ANY water comes out it could only come from blocked tubes............simple and definitive.


Yes, fan clutch should be checked, many youtube videos on testing those.

You could also have a thermostat that is not opening all the way, or is failing to open at the correct temp.
Ford uses 192deg-195deg t-stats, this maintains the 200deg engine temp required for best economy and lubrication.

Water pump would be the last thing on the list, no real way to test these without a visual inspection and at that point you might as well change it.
With the invention of the small "snake cameras" you can drain the coolant and remove the lower rad hose from engine end, then use the camera to see the impeller.
These can be rented, but you would need a picture of new impeller to compare it to