Hi. I have a major or maybe minor problem. I have changed the actuator doir, checked heater core and still have a problem. I have heat for about 1 minute then it turns really cold. I can not find any help on this. It is a 1999 F150. It has vacuum lines and is nondigital. We have tried everything. At first it would only give a little heat on defrost then it finally gives some heat on the face. But it ONLY gives heat for about 1 minute then nothing but cold. I can cut it off wait a while then i have heat again but only for a minute. Help please. ANY ideas????? It is a 5.4L engine. Touched both hoses in engine compartment. They are hot. Don't have a regular radiator cap so I am a little confused with the set up. We changed the actuator door. Have no special tools like temp gages

 

How did you check your heater core? Have you bled your cooling system?

 

I would guess you have a big air pocket. You get heat up until the thermostat opens and the coolant chooses the path of least resistance (the radiator vs the heater core). Just to narrow things down a bit, after the heat cuts off, with the engine running use a IR thermometer on the lines to get the temp differential. If you don't have one you can (VERY carefully) raise the hood and feel the lines going to the firewall and compare the temp to the top line going to the radiator. If the lines going to the heater core are cool or just not very hot you know your problem is not on the blend door/dash side but in the cooling system.

 

They are hot.

 

How do I bleed it since it has no radiator cap but a pressurized reservoir cap instead. Don't know much on how to do this kind of system.

 

It has to be a flow issue. The blend door wouldn't just decide to move after a minute especially on a non digital setup. Did you compare both heater hoses that go to the firewall. Is one hot and the other warm but not even close to hot?

 

Both are warm but maybe not as hot as they should be.

 

Well you can give the garden hose trick a shot. Disconnect both heater hoses at the motor. Rig one end up to a garden hose and crank that baby open. Let it blow out whatever's left in the heater core and make sure your getting a good strong stream of clean water coming out the other end. Then blow the tap water out with a air compressor. I try to start the filling process by pouring coolant into the higher end of the two hoses until it starts to come out the lower hose so its easier to purge air the whole system. Connect both ends back up and finish filling the system.

Only other thought I have; do you know if your truck has a heater core valve? Its a mushroom shaped piece of plastic in line with the feed hose that goes to the core and has a vacuum going to it.

 

Don't know yet. Will look and see. I will check back on here after while. Got to run my son to parts store. Thank you

 

I agree with flow, - either the impeller in the pump or a plugged or partially plugged core. Partially would explain it.

Did you know you started two identical threads RavenS ? Guess I'll let others know so they don't waste their time posting in the other for yuh. I'll redirect them.