Coolant Leak
 

I have a 98 F150 4.6L with 192,xxx miles. This week my heater started intermittently blowing cold air. This was happening today as my wife was driving home and then the truck started to overheat. However, it did return to the normal temperature range quickly as she was driving. I thought the thermostat might be bad. However, when I got home I saw the resevoir was bone dry. I put coolant back in and started the truck and let it run for 20 min. It never overheated, the heater worked, and I could not see leaks anywhere. Could my thermostat still be the problem or is it something else? I did just replace my heater core 2 months ago because it was leaking. Thanks for the help.:beers:

First check your oil. Look for water in the oil or overfull oil pan. If thats the problem, head gasket replacement time. That could explain why your heater core went out from the head gasket problem.

If you can't find the leak...fill the system and have it pressure tested.
If you get a pressure leak down you have a leak somewhere.

Believe it or not a lot of these problems are just bad radiator caps.

What is the best way to do this? I looked at the dipstick today and the oil is just barely over L (it's time for an oil change) and I did not see any water residue. Should I be dropping the oil pan to be able to see this? Thanks.

I'm due for an oil change so I will have that done when I am there.

Sounds like your oil is ok and maybe no head gasket problem. The pressure test like Colorado Osprey said would be the next move. Go to Auto Zone and they will loan you a coolant pressure tester and show you how to use it.