My father had an 87 F150 that had a small crack at the very top of the radiator. It would only leak once the once the coolant was hot/pressurized. Never did leak in the driveway. We had the radiator repaired and no more coolant loss.

Can you smell coolant when the heater/defroster is on?

 

Nope no hot smell either

 

sounds like were driving the same truck, doesn't run hot, no smell inside truck, no sign of leaks anywhere

 

Do you ever have a whitish smoke from your exahust? It is possible that you have a very small leak in head gasket after a rebuild... if heads were not torqued correctly, or if bolts were reused, or if the surfaces were not clean. You can check for hydrocarbons in your antifreeze with a test kit from auto parts store.

 

I do have white smoke every once in a while

 

It might be the heater core. do your windows fog up on cold rainy days?

 

Nope have new heater core

 

White smoke indicates coolant burning in cylinders, leaky head gasket, unless it's only at cold startup when you get the normal fog, if conditions are right. My guess is a head gasket. Test for hydrocarbons in the antifreeze.

 

Yes only happens on cold start

 

I'm still leaning to HG, but second guess would be water pump weeping. Do you ever smell a sweeter smell from the exhaust?

 

Nope just a too rich gas smell

 

@Alwaysfishin: If I caught all of your posts, you rebuilt the motor, new water pump, new radiator, no leaks, pressure tested, not the heater core....... Certainly points towards an internal issue. I had a problem on my other vehicle (GMC Yukon XL) (yes I know.....) and it turns out that Ford, GMC and Chrysler have been putting a specially made Bars Leaks product in the radiator at the factory for several years. The reason is to seal internal leaks. Also because anti-freeze is hard on aluminum, especially that stupid Dexcool that GM runs. In order to be allowed by the auto manufacturers association to do this, the grain size had to meet a specification which is very tiny. Bars Leaks built the product to the spec and they all use it. It comes in a pressed pellet form, usually on a blister card. Pellets are about 1.5 inches diameter and about 3/4 inch thick. Break them up before putting them in the radiator. It sealed a very small internal leak that my GMC had. I no longer add fluid. These can be bought in the pellet form at O'Reilley/Checker Auto Parts and the GM dealers sell them at their parts counter too. I didn't look at the Ford dealership yet....... It worked and solved my mistery problem. Very cheap too. About $3.00 for the blister card of 6 pellets. Don't let the parts guys talk you into the other larger grain or thicker products that Bars sells. You can find these pellets on the Bars Leaks website as well. They list it as an OEM product. Good luck! By the way, do yo have a coolant recovery tank on that truck???

 

Yes and it doesn"t run hot either