Ive replaced all heating componants truck runs at good temp. but i can not get any heat to the cab if i shut the heat off on the switch and leave it off for a few minutes and then turn on it will blow heat for about 2 min then blows cold air again any ideas

You have to make sure the thermostat is working properly and is the correct one. Idle the vehicle for about 20 minutes and then feel the upper radiator hose. It should be hot to the touch. Drive the vehicle and when the temp gauge falls to 'cold' stop and feel it again. If it is much cooler to the touch than before the thermostat is defective or wrong.

I am very doubtful you have air in your hoses causing the problem. Feel the upper and lower heater core hoses at running temp and verify they are both hot, and should be same temp.(make sure heat is on high in car) Some newer cars lose some heat to the cab just because they are low on fluid. More likely though, you may have a bad radiator cap. If your cap is good, watch your hoses and make sure they aren't collapsing( the hoses will look like they are breathing in and out with car idling). You could even have a bad thermostat, or your heater core could be plugged. You can flush your heater core and see if that don't fix it, but since you have heat then it decreases, your core is probably ok.

That loud pop is the vacuum door moving closed. These trucks were well known for issues with not having enough vacuum to operate the heat doors (especially while driving up a grade). An after-market vacuum pump is the way to go. Disconnect the vacuum hose from the engine and install an electric vacuum pump to run all the controls. While you are at it, hook the vacuum hose, that goes down to the transfer case (4X4) to the vacuum pump, also.

disconnect the heat hoses going into fire wall , put hose into one with pressure and flush as it sounds like it could be bloked in the heater core,the other thing that could be the problem be the hot ,cold switch as it restricts the flow to makes the heater hoter or colder , it might need tlc or replacing as it is manaul not elec

Thanks for choosing FixYa and welcome. That would be my next guess. Maybe a bad coolant sensor. You might have a bad water pump as well. Make sure the fans are kicking on properly also. Check your radiator.

It's a definite possibility. Make sure your coolant level is good. Is the engine temperature coming up on your temperature gauge? If the engine temp is low and there is no heat in the cab, it's most likely a thermostat stuck open. If your engine temperature is good, but no heat in the cab, then it's probably something else like a plugged heater core, kinked hose to the heater core, or even your heat selector switch not working properly.

if one hose is too hot to hold onto,then your thermostat is good,but if both hoses are not the same temp you should replace the heater core,you can get maybe one or two of the heater core fins to open but there is a majority of them not letting coolant thru.

opps clicked clarification .... -- run your engine up to operating temp . shut off engine and check your radiator by placing your hand on top - middle - and bottom, they should be the same temp , if not your radiator is plugged... (be careful not to burn yourself) .... also grab the heater hoses going through the firewall, if they are not the same temp feel you may also have a blocked heater core.

if you are referring to the coolant temp sensor and not the coolant temp switch,then yes,this sensor tells the ecm what temp the engine is at and the ecm delivers the correct amount of fuel to the vehicle.The switch in most cases just sends the signal to the guage or light.

Some models have a bleeder valve located on or near the thermostat. Not all manuals will show this vital component. If yours has one( if your coolant recovery bottle is above highest point of radiator you do) you need to bleed the air out of your cooling system. Do this by opening the valve until coolant comes out. Do this a couple of times as your car is getting to operating temp. should solve your problem.