Chevrolet Repair: no heat, vortec engine, gravel road

Question
Hi Mark

I have a 1994 1500 chev pickup.

It has a 4.3 L vortec engine,automatic, no air conditioning.

recently changed engines with one that has less mileage.

Problem: Pickup runs at normal temp but blows cold air
when I had the motor swapped I had the heater core replaced with a new one. The old one leaked.  
bought it back to the auto mech. that did the job..  He said something about a broken box...that is why its blowing cold air.

it was supposedly fixed,  mind you it did work for 2 days, then it started blowing cold air again. Drove down gravel road, stopped for a time and started driving again,  that is when I noticed that I was getting no heat.

can you tell me the number of reasons this would happen.

I hope I gave you enuff background.

Please answer asap. or direct me to someone who can give me an answer.

thank you

garry

Answer
Hi Garry, Well this could be many things but lets look at that broken box thing. I wonder if they broke the heater box when they installed the new heater core. If so then maybe the temp door isn't moving from cold to hot. If you can listen to the door when you move the temp lever from hot to cold, you should hear it stop when you go back and forth. I would feel the heater hoses coming from the heater core to make sure that they are both hot. If only one or neither is hot then you don't have any flow through the heater core. Then you need to check for restrictions in the system.

                Mark