My son has a beautiful 97 ranger 4 x 2 with the 4.0 - about 105 thousand miles. We’ve owned the truck for about six months and have fought cooling issues since we got it - he leaves for school in two days and we’re trying to get to the bottom of the most recent issue. Here is what we’ve done so far:

First issue: the the temperature gauge that goes up and down and loss of coolant - thermostat issue - replaced with OEM Ford thermostat - while in there we replace the heater core diverter that runs off a vacuum under the hood and got all air out of the system. Replaced radiator cap as well since the radiator in the truck appeared only to be a year old / had been replaced before we got it. Problem seemed to be fixed. Son drove truck back-and-forth about an hour back-and-forth to work, still noticed a little coolant level dropping, but not as bad as before.

2nd/ current issue: First, water pump seems to be fine, no leaking around water pump, and when engine is revved, plenty of coolant circulating with cap off. When driving, doesn’t seem to be an issue - son was sitting in parking lot. Idling for 10 minutes, all the sudden massive coolant backflow into the reservoir when truck powered off after idling (we insured this hose between the radiator cap and reservoir was clean and not leaking as well when we addressed first issue). Kind of confusing as this could seem that it was from a thermostat that was closed, but it’s only about two months old and OEM. Temp gauge in truck did not reflect overheating condition, this would lead me to believe the one wire sender is probably bad and needs to be replaced (but not cause of overheating condition). Second thought was a fan clutch, since this only happened at idle - fan seems to run just fine, but does seem to run constantly when truck powers on. Once truck is turned off, after about five minutes, there appears to be resistance when manually turning the fan blades. Last thought, is this a blown head gasket? I do not detect any sweet smelling exhaust, and oil is clean as a whistle. With a brand new thermostat, new hoses, new two wire sensor, Significant coolant backflow into the reservoir after idling 10 minutes any suggestions? My next step is to rent and do the do the head gasket color fluid test - is there another way I can test this fan clutch? How do I know if the fan is not spinning fast enough at idle?