Overheats only when going uphills!

[strap21061]

99 expedition. While climbing steep hills, the vehicle runs hot and the antifreeze boils over out the expansion tank bottle from the cap. Any ideas as to what my cause this?
Thanks

[Schaff68]

water pump impellar may be missing acouple blades or your radiator is plugged up enough that the hill climbing is making it get to hot. try flushing the system, put in new antifreeze and a small bottle of water wetter.

[strap21061]

Thanks for the help. I'll try it tonight.

[danielsatur]

1) Are you towing anything?
2) Is your electric coolant fans working proper?

[strap21061]

Not towing anything and it's a clutch fan.

[danielsatur]

I had a thermo/mech fan clutch go bad!
Over 65MPH it would overheat, I replaced it with a new mechanical fan clutch. ''ok''

1) Clean road litter like bugs, leaves, and other derbis in front of the air cond condenser, and radiator.
2) Are you ready for a coolant flush?
3) A plugged catalytic converter can make her run hot.

[strap21061]

I replaced the thermostat, the expansion tank cap, flushed the system and blew away all debris from the radiator and the truck is operating good again.
Thanks for all the help.