I am having a problem with my truck putting out heat all the time. I can drive down the high way and it will put out good heat but as soon as you let off the gas it starts blowing cold air. I just changed the water pump and the thermastate but it didn't help. Any suggestions?

i am having a problem with my truck putting out heat all the time. i can drive down the high way and it will put out good heat but as soon as you let off the gas it starts blowing cold air. i just changed the water pump and the thermastate but it didn't help. any suggestions?
You will need to check for a vacuum leak that feeds the vacuum reservoir and the mode doors. Look for pinched or deteriorated vacuum tubing.

i am having a problem with my truck putting out heat all the time. i can drive down the high way and it will put out good heat but as soon as you let off the gas it starts blowing cold air. i just changed the water pump and the thermastate but it didn't help. any suggestions?
So where would I look to find the vacuum reservoir and the mode doors? And what's the easiest way to check for a vacuum leak?