I have a truck that has been overheating, only on long runs. It is fine on the highway but when i exit it sometimes starts to get hot and i can hear the radiator fluid boiling. I have changed the thermostat and viscous fan. Any input, my friend thinks its a head crack or leak,anyay to check this?
You have adequate cooling at speed, but not when you slow down. That often points to a bad fan, but you have already checked that. We can probably also assume that you have checked to see if the radiator or AC condenser is clogged with debris. If it were just boiling and not significantly overheating, I would suggest the cap is releasing pressure too soon. Check it out anyway. For my money, even though your truck is a bit young for this, the radiator should be checked out. It might not be able to transfer enough heat. Even though the fan is working, you probably still get more flow on the highway. Another possibility is the water pump impeller might be eroded to nubs. Has the cooling system service been kept up with the correct coolant?