1997 F150 4.2 V6

I'm not sure why but the heater on my 1997 f150 4.2 will only blow hot air out after about 5-10 seconds of pushing on the gas. The coolant levels are correct and just put in a new water pump (although I've had this problem before the water pump was installed). Also it wont blow hot air when on floor at all even when I am pushing on the gas so I'm thinking the blend door. Why would it work sometimes through the vents? Why would the blend door not move full travel? The temp needle doesnt have any significant movement once the engine is warm. Please help

Good old Bill

Not trying to hide my old age, just try to help you out with this problem. If you are talking about air temperature going from hot to cold, that would be related to the blend door not holding its position or even air in the cooling system. If you are saying the air flow changes from floor to vent or even to defroster then its the function door. Most common fault is the vacuum line feeding the system in the engine compartment. There is a hole or break in it. My routine on my 02 4.2, vacuum line (very small hard plastic) comes off of engine up against firewall near where heater hoses are located and passes under the battery relay to a check valve, then to a tee, one line tee's off to go thru the firewall to actuators, other line off of tee goes under battery to vacuum reservoir. Its usually this line to the reservoir that gets eaten up and is open or broken. Not remembering what year and engine you have, the system is basically the same and breaks are under the battery and tray.

1997 F150 4.2 V6

It is the temp of the air going from hot to cold. But it's only hot through the vents. it never gets hot on the floor. What do I do?

Good old Bill

Sounds like what I call the function door isn't selecting where to let the air come out. Its stuck in one position. Need to remove the fancy covering under the dash to expose the actuators and go from there. If you see any plastic lines follow them to find the actuator or vacuum motors.

jhelrey

My GFS car did that... it was low on coolant...

Soby

Check both of the heater hoses at an idle.If one hose is colder then the other the core could be starting to plug up and not passing enough coolant through at idle.

1997 F150 4.2 V6

I checked my coolant level and it is correct. I felt the heater hoses and (from sitting in the drivers seat) the left hose (proximal to the intake manifold) is hotter than the outer hose, but not by much. Both are pretty hot but one is hotter than the other - return hose. I'll check the door but is there any way to check to see if the relay to open the heater lines is bad?