Hello. I am working on a 1999 F150 4.6l. Upon startup, the vehicle will blow slightly warm air but not hot by any means. After about 30 seconds the air coming out of the vents is cold though, no heat at all. I verified that there is coolant in the engine and the heater hoses going into the firewall are hot once the engine is at normal operating temperature as they should be. I removed the dash and heater box cover. The blend door appears to be in the correct position for the air to be forced through the heater core. The heater core is not leaking coolant. I took one of the hoses loose from the engine that goes into the heater core and blew into it. Coolant then flowed back out of the engine where i had disconnected it.

My question is, could it be possible that the heater core is clogged? When i blew into the hose and coolant came back out of the engine i thought that it couldnt be clogged. But then i thought that, if it was clogged, maybe the coolant was just going through the end tank on the heater core and not through the core itself and back out into the engine.

Originally I thought that the heater/water valve could have been stuck closed or was not being actuated to allow coolant into the heater core. But I cannot find a heater valve anywhere so i suppose that means this truck doesnt have one.

Any helpfull information on this would be greatly appreciated.

 

i would continue to look at the blend door. move the tmep **** from hot to cold, does it control right?? there have been stripped out ***** that rotate on the shaft and will not stroke the door. if you cn blow thru the heater core, you would think you can at least get mild warm air, not Cold.

 

Well i believe the blend door is operated by an electric motor. I dont want to hook the battery back up to test this with so many connectors and grounds disconnected from where I removed the dash. Im afraid I might damage some electrical component by doing that. The air coming out of the vent is only slightly warm to start off with, but there was a noticable difference when the dial was turned from hot to cold, so that tells me that the door is being actuated. Plus there is a change in air sound that can be heard when you turn the hot/cold dial.

Yes, i would think that too about being able to blow through the core. But is it possible that im not actually blowing through the core itself and instead just through the end tank on the core?

 

How many miles? Have you checked the water pump impeller? My 97 exhibited similar issue. Turned out the water pump impeller was worn away to just a thin wafer. After replacing the pump, the heat was better and more consistant. Still replaced the heater core this past weekend and man what a differance. Really hot heat and have to turn down the temp for the first time in years!

Start with the water pump as it is easy to replace if needed. The heater core is not hard, but takes a lot of time to remove the dash.

 

Subscribing as I'm dealing with a 99 with the exact same issues. I have changed the thermostat, checked coolant, checked dial movement to no avail. Warm air but deff. not heat.

 

Truck has about 200,000 miles on it.

 

Check your water pump first. Cheap and easy. Mine had 240k with no outward sign of a problem. No overheat or leakage. If pump is ok your only out a gasket and a little labor.

 

Try switching the temp **** and fan ****.

 

Did you blow thru easily? Mine is hard as heck to blow thru and only a trickle of fluid comes out and it hurts your jaws ( yes using my mouth) to blow it thru you have to try so hard.

 

Well I fixed it. Turns out it was a clogged heater core after all. When I took the old one off and turned it upside down, very little coolant came out. And what did, looked like mud. So, I flushed the cooling system and hooked up the new heater core. Heat works great now, very hot.
Thanks to all the replies and suggestions.
Somewhere on this forum I found instructions on how to replace the heater core, so thanks to whomever posted those.

I think i may have switched the two hoses going into the heater core from the way they were originally by accident. I suppose this doesnt affect anything since the heat works good?

 

I had a problem of no control, in summer no cooling, but now in winter it was full heat on. The **** had a hairline crack, superglue the crack with clamp,,, drying now........great Great!

 

Update on the truck I'm working on. 99 with no / low heat. Tried thermostat, water pump and flushed the core to no avail. Ended up putting in new core and blend door with immediate fix. The core flushed fine but apparently was not flowing efficiently. Everythings good now. Thanks for the tips.

 

I have the same problem after I installed a new heater core. Got heat just not as hot as it used to be. Maybe an air pocket in the system? 4.6 with 99K, water pump impeller? Emptied 2 gal of coolant when I did the heater core and replaced a bit more the that. So not sure about the air.
Any ideas?? and the blend door is working.

 

I got a '99 w/ the exact issue. You said the blend door? Would the a/c work if it was the blend door? Help?

 

the blend door is a motor actuated door that sets in the air flow stream. moving the door makes the outside air go thru the heater core or around the core. Putting the door at some middle position lets the air mix and give a variety of temps. The air also goes thru the air conditioner coils and gets cool (when it is on). You mix this with a portion of hot air to control the temp.