1979 Ford F-150 • V8 4WD Automatic • 135,000 miles

I have 97 f-150 4x4 extra cab with the 4.6 ltr v-8. A couple of weeks ago I warmed it up for work and acouple miles down the road my heater went cold and I noticed the temp gage dropping. It dropped to a little above cold and stopped and a minute later shot to the top. I turned the heater off and a minute later it dropped back down to normal and I was able to turn the heater back on and it held normal all the way to work. It did the same on the way home. Thinking it was the thermostat I went to napa and bought a new one and replaced it. I had to ad about a half galon of coolant. All was well for a few days when on my way home from work it did the same thing. I found where a little coolant escaped out the pressure cap so I went and replaced it, I really didnt have to add much for coolant this time. Again all was ok for a few days and it started again. There is no white steam from the exhaust, the fan clutch appears to be doing ok, theres no water in the oil, it has a new thermostat and coolant cap, things appear to be clean, and there appears to be no leak. I would be very gratefull if you could help me out with this mystery. Thanks, wa
February 5, 2011.

Pressure test coolant system
white steam from the exhaust usually is an indication of a head gasket failure

Feb 5, 2011.