It started a couple of weeks ago. My 94 4runner takes maybe 4 min. of driving to warm up in the mornings and usually as soon as the thermostat is indicating that it is warm, the heater blows warm air no problem. Lately, I've had to crank the heat on when the truck warms up and for a brief period of time the air is cold, then suddenly as though a switch gets thrown, it blows hot all of a sudden and works fine until I start to decelerate, at a full stop the air turns cold again. I have never seen anything like this and have no clue what is going on, ANY suggestions would be greatly appreciated. THANKS!!!

 

My '93 pickup (3.0L XtraCab) started doing the same thing maybe 2 or 3 years ago. It seemed to heat up just fine, and hot air was generated only so long as I stayed in motion. If I stopped, the air turned cold...

There was no leaking, broken duct, etc. Neither my mechanics nor I ever figured out precisely what was wrong. The effect disappeared about the time a new water pump was installed. We never figured out how that could have explained it.

Bottom Line: I've experienced it, I'm past it, and I still don't know why....

 

In most automobile (and i believe as well in toy's) your heater is run off of you engine's coolong system. At some point the antifreeze/water is routed through a small radiator like core in your heating/ac system, this heat up air blows though it making the air hot out of your vents. All of this is driven by your waterpump. If you have a backup in the heating system it may be hard for the coolant to push through until the engine rpm's are raised as this speeds up the pump and pressure as well. If you replaced your waterpump and it went away chances are your waterpump was not pumping 100%, when you were driving your rpm's were reving speeding up the pump. if it was only operating at say 70% you would have to raise your idle to compensate for the inefficiency.

 

Check your coolant level. Flow to the heater seems to be the first thing to quit when it gets low.

My wifes water pump (on her Taurus) only had 1 impeller left (out of 8). It cools the engine much better now and the heater works better.

 

I had a similiar problem. I turned out that one of the link bars that connect the inside valve to the temp control lever (under the dash) had broke off. Look at post #10 in this thread to see my temporary fix.

 

I vote for a water pump.......when new it moved the water fine at low rpms..now that it has gotten older it has to turn faster to move the same amout of fluid around....you can try to flush your entire system and see if that helps, could just be gunk clogging up the works....but I think in the end you will still have to replace the water pump....the gunk had to come from some where...