If I am driving my truck (1998 F-150 4.6,auto) at or around 50 to 70 mph my temp gauge starts to climb to the tip of the red (hot). If I slow down like when coming to a traffic light or a stop sign the temp drops back to the normal range. Any ideas what might be causing this? The fill tank was low the other day so I topped it off. I had to fill it today as it was empty. Thanks. Chief Mike.

 

Check for leaks. If you don't see any, have a hydrocarbon test done on your coolant, you may have a blown head gasket. Do you see any moisture in the oil?

 

faulty fan clutch?? thremsotat?? see if the fan has play back and forth by pulling it back and forth when its off of course

 

at 50 mph the ram air effect would cancel out a faulty fan. look for leaks or a faulty thermostat

 

clogged condenser could be a possibility too

 

Having very similar problem, changed fan clutch and as Raptor said made no difference, over 50

 

check the thermostat or the water pump.

 

pressure test it, see where the leak is

 

if the water pump has failed it will seep coolant out of the top hole

 

O.K. guys checked no leakage & no water in oil. I tend to believe it is the thermostat. I plan on changing it tomorrow. My question now is will installing a 170 degree thermostat help or hurt me? Thanks for your help & imput. Chief Mike.

 

It will hurt. The engine management system is set up for a 192 stat.

 

yes, it will hurt been there done that, go with OE 192-195.