Slowly heats up
I have an older pickup, it will maintain a good temp just driving through town, but if I let it sit at idol, or drive it on the highway it slowly heats up to overheating, I have changed the water pump and that seemed to slow the process but it still does it, it also has a new thermostat. No leaks that I can see, any ideas what my problem could be ?
Your Fan is no longer doing its job or the fan shroud is missing. Your vehicle may have had an aftermarket flex fan installed in place of the factory clutch fan. This situation may also occur if the thermostat is removed or stuck open.
Flex Fan
Another helpful accessory for vehicles using a belt-driven radiator fan is a flex fan. These fans are large diameter units designed to provide more airflow at low speeds, by using fan blades that have deeply cupped surfaces. The blades then flex, or flatten out, at high speed, when less cooling air is needed. These fans are far lighter in weight than stock fans, requiring less horsepower to drive them. Also, they are far quieter than stock fans. If you do decide to replace your stock fan with a flex fan, note that if your vehicle has a fan clutch, a spacer will be needed between the flex fan and water pump hub.