Moose02

I have a 2007 f150 supercab 5.4 gas 160000 miles overheats only when pulling a 4500 lb trailer. replaced water pump was a slight leak out the weephole replaced radiator, thermostate, fan clutch, radiator cap (or bottle) all new Motorcraft antifreeze. all Ford parts.

only overheats when trailering and I'm lost as to what to do

ShirBlackspots

Maybe wrong thermostat was installed? Perhaps there's some large air bubbles inside the engine, or somewhere in the cooling system? Did you rev the engine to around 3000RPM for 30 seconds to a minute to get the coolant flowing and to remove air?

dukedkt442

^ +1. Ensure the cooling system is bled properly: turn heater temp all the way up with fan off (or on low) to open the valve to the heater core, leave expansion tank cap off and run the truck to operating temperature and another 10 minutes thereafter.

Beyond that, a lean mixture or wrong timing can lead to overheats, along with bad head gaskets, head cracks, etc. KISS first.