To clarrify. A stuck SHUT thremo you should still retain your heat untill all the coolant becomes so heated it blows a heater hose etc. But your engine will be knocking and rattling so bad that you'd better have turned it off by then anyway..The heater core is on the engine side of the cooling system and heats as the engine warms up.

bc56845...I also have a 98 F150 doing much the same thing. I recently bought the truck and the coolant was badly rusted. I flushed my system and was amazed at what came out. Almost like rust colored pellet sized wet sand. The first time I flushed it I drained from the petcock on the radiator and did not notice anything. NOT till I pulled the lower radiator hose did all the gunk come out. I have to flush it again as I am seeing rust colored things floating in the degas tank..This is what I beleive is causing the coolant not to drain back to the waterpump quick enough once the thermo opens. This is why the degas tank is filling up

 

I read in a manual and there also could be a resrtiction in the radiator,and check the flow you water pump is providing ,you know how by pinching the top hose.

 

Well an update on my friends truck. It does have extremely rusty coolant, but it turns out that its a bad head gasket. Its got milky sludge on the oil fill cap now, theres coolant running out of the exhaust, and the cooling system is building up excessive pressure. Also we checked for codes and there is a #2 cylinder miss. By the way, we checked prices on head gaskets for that motor last night. NAPA wants 130$ apeice. Summit sells them for 56$ apeice and 30$ for upper and lower intake gasket set

 

Update - Ok this was probably all self-inflicted by not checking my water level in a long time (and I should have done that before getting all worried and posting here - but I was working night shift and couldnt get it off my mind so I posted before I had a chance to cjheck anything myself). There was no water in the fill tank, and no cap on the radiator so I am not sure if or how much was in there. I found some gallon jugs at work and just put straight water in for the trip home. It took just under a gallon and a half to hit the cold fill line. No problems on the way home. Then today I took it to valvoline instant oil cahnge for oil change and radiator flush. I would have done it myself but its cold out, and I didnt have any free time. So thanks for all the suggestions, it did seem that since the level was so low that it was probably sucking air through the system or something like that.

 

its is better to find that than other major problems,your a ford buddy and you found the problem and saved a ford before the reaper could grasp it!!!

 

What I dont know still is how the water level got so slow. I just hope its not leaking anywere. I did a quick check today and didnt find any leaks, and the valvoline team didnt mention anything out of the norm either.

But since I am on the subject of problems, there is one more possible problem that has my concern. I'll start a new thread, titled 'worried: noise comming from motor'

 

78..If your cap was missing, that is prolly why your fluid was low. My problem ended up being a partially plugged radiator. I actually pulled the radiator out and flushed it till all the water was clear. Fixed and happy.. GLTY