Another overheating thread :(

[lxfiveoh]

Hello!!

My car has started to exhibit some strange overheating behavior.
It has been gradually building up.

It has ALWAYS run near the bottom of the gauge, perhaps right above the first line or around the "A" in NORMAL. In the last few months, it started going up past the second line, or half way up the gauge. This is mainly afternoon (HOT weather) and evening driving.

I usually drive it only for a little while on Friday nights to meet
with the club I'm in. They are city miles, and the gauge doesn't get very high at all.

Now, when I get on the highway, this is when the gauge creeps up. This past weekend, I drove about 30 miles in the morning (not too hot, yet) and the gauge went all the way to the "R" in NORMAL. I was driving around 70-75mph.

My car has the stock radiator and stock water pump. It has a 160
thermostat & underdrive pullies. These parts have been on the car for several years of hot South Florida summers with no problems, so I'm not quick to blame them.

I partially drained my radiator and it didn't look corroded to me.
When I cranked up the car the water seemed to be flowing through it just fine. I let the car sit idling for about 30 minutes and the temp gauge never got past the bottom...maybe around the "A" in NORMAL.

Any suggestions at what this might be?

To surmise, it seems to run about 'normal' in the city, but the gauge quickly rises on the highway.

Thanks, MustangWorks!