Overheating problems can be frustrating to diagnose, but are fairly simple.
for the truck to run at the correct temp you must have three things Air flow, water flow, and no engine problems. If you don’t have enough of each under a particular running condition the truck will overheat.
Truck overheats only at low speed: look for a bad fan clutch, problems with the fan blades (missing or trimmed), slipping fan belt, bad thermal switch for electric fan
Truck overheats only at freeway speed: look for sand, dust, dirt, dog hair etc. blocking airflow to rad. We used to have to remove rads from cars and hold them over the floor about 12" up and drop them flat on the floor. You would be amazed at how much dirt is dislodged sometimes. Also aftermarket grilles, license plates in the wrong place, missing or mis-adjusted air guides.
Truck overheats all the time: probably not an airflow problem but look for plugged fins in rad.(see freeway speed)
NOTE: a fan clutch will generally not cause overheating when driving at speeds above 20 mph or so. I only know of one case where a fan problem contributed to O/heat at speed in all the years I have been doing this. Due to an accident the fan blades on this car were about 1" too short. Car would O/heat going up a hill.
Truck overheats only at idle: low coolant, air pocket, incorrect coolant/ mixture colasping lower hose,
Truck overheats only at freeway speeds: Bad T/stat, bad water pump (corooded impeller) restriction inside engine, poor flow through rad, incorrect coolant/mixture
Truck overheats all the time: Low coolant, incorrect mixture, bad water pump, poor flow through rad due to restriction, poor flow through engine due to restriction
Overly lean conditions (overheats at idle)
Ignition timing retarded (more often O/heats at speed)
Blown head gasket (maybe not fully blown, but leaking)
Truck only O/heats when low on coolant (no visible leaks)
Slightly blowen head gasket or internal crack in engine
Bad pressure cap
Some things to keep in mind:
Fan clutches are to help the airflow at low speed, at high speed they disengage to allow for better gas mileage/quieter running. So a bad fan clutch gives low speed O/heating, and normal on freeway.
Bad pressure caps do not cause O/heating. Putting coolant under pressure does not make the engine run cooler, it allows the coolant to get hotter before it boils. When coolant boils it will boil out of the system, and when the coolant gets low, then the system will overheat. A bad pressure cap by itself will not cause O/heating.
There are a number of different coolants being used by car makers nowadays. While writting this I called a buddy of mine and asked how many, he told me that at last count he was up to 13 different types (not brands but types) of coolant. Mixing the wrong one together can cause serious issues. My suggestion is use what the car maker put in the car from the factory, the engineers that designed the cooling system know more about it than you do. Bottom line is that for a newer car (say 1990 up), a bottle of Prestone may not be a very good idea.
Getting back to the OP if this truck were to be brought into me for a diagnosis here is what I would do:
- Check for the presence of hydrocarbons in rad. If hydrocarbons are present, head gasket is blown, or head is cracked. With the right equipment a shop can do this test in just a few minutes.
- Pull rad and have it sent to rad shop for cleaning and flow check. (yeah I know it was done last year, but it is this year NOW)
- While rad is out, take a high pressure hose, and working from the engine compartment side shoot water through the A/C condenser. When done shine a light from one side and verify that the light can be seen all the way through all parts of the condenser
- Replace T-stat (cheap!)
- Remove water pump and inspect impeller. I have seen technicians go round and round on O/heats and when the finally pull the water pump the impeller is rusted away to almost nothing. If you are a belt and suspenders kind of guy you might want to go ahead and replace it while you are there.
- Fill with factory coolant mixed to factory spec.