I have a 2001 F-150 with the 4.6L. It blows luke warm air at all fan speeds regardless of rpm range. Both of the heater hoses get hot, I replaced the thermostat and flushed the radiator. I noticed that the lower radiator hose does not get hot and was wondering what could cause this besides a plugged radiator and if this could be a problem even though the heater hoses heat up. The truck reaches operating temp fine and never overheats.

 

Sounds like a plugged radiator or weak/inoperative water pump. When you added water to the radiator inlet to flush it, did the water come out of the radiator outlet?

 

Yes, the radiator drained fine.

 

The radiator could still be partially plugged. This might sound dumb, but are you sure you put the new thermostat in correctly when you replaced it?

 

Not a dumb question, but yes I am sure it is installed correctly.

 

That's my 2 cents then, I'll wait for someone else to chime in. I'm curious as to why it heats fine for a few minutes then stops. I was going to suggest checking the blend door for proper operation, but if the lower hose isn't hot after the engine is heated up, then I doubt it's the blend door.

 

I forgot to mention that I'm taking this truck on a 1000 mile trip this Saturday.

 

Anyone, I need some direction.

I'm planning on replacing the water pump tomorrow to see if that fixes the lower radiator hose problem, but I don't understand the heater problem.

Both hoses get hot which signifies to me proper flow through the heater core, which should give me heat, I've considered the blend door but is there a way to hone in on that being the problem without tearing out the dash?

 

I've seen this before. The heater core can become plugged and the coolant will enter and exit the core without actually passing through the section where the fan blows across. In which case, both heater hoses will be hot, but the core itself is only luke warm.
Dont replace that water pump just yet. That's probly not it. Get some cooling system cleaner from the parts store, drain the coolant, put cleaner in, top off with water, and drive it for a day. Then drain the system and refill with correct antifreeze mixture. If you think it's necessary, i've also heard of disconnecting the hoses going to the heater core and flushing the core with CLR (calcium lime and rust).

 

Ok I did some research on what I said about CLR. Dont use that. It's not meant for aluminum or copper surfaces and could damage them. Use the cleaner from the store or apparently white vinegar works really well.
Ive got to do this to my truck to, as it is only blowing luke warm air. I worked on a truck last winter with the identical problem that you are describing, so I hope that helps.

 

 

Are you sure the thermostat had opened when you felt of the lower radiator hose and it was cold? Seems like it would be overheating or at least running hotter than normal if you had a problem like that.

 

Yes, the truck had been running for more than an hour at the time. I replaced the water pump today and I'll have a new radiator in hand tomorrow to be replaced. I figured I'd replace them both while I had it apart.

 

I'm surprised no one else has chimed in with more thoughts. I hate seeing people throw parts at a truck without knowing for sure if it'll fix the problem....but it seems like I do it all the time! I would be inclined to at least replace the radiator. And you have a good point, if the system is going to be drained and the radiator is out, might as well replace the pump while it's easy to get to.

Let us know how it turns out.

 

Will do. I was originally planning to check just the water pump, and it did seem bad to me; It had a rough but no resistance when I turned it. I wouldn't have replaced the radiator but I was able to get one at a discount of $120 and figure I might as well.