This year my heater isn't heating like it was before. The heat isn't as hot, plus when I"m driving, it will be heating and then all of a sudden go to blowing cold air. Then the heat will come back on a few minutes later. I know it's a 97 with 172000 miles on it, but what would be making it do this? ANy ideas? Thanks.

 

If its on turns you mostly notice the change, or stops, you probably have a broken blend door. New plenum, full dash pull. Otherwise your blend door controller is "wandering" but that would be pretty rare. Of course with 172000 miles anything is possible.

 

Sure it's not low on coolant? That can cause it too, or a bad waterpump not circulating good because the impeller is slipping on the shaft.

 

Good point, often I find customers complaining of no heat and its about a gallon low of coolant.

 

how do you check to see if the waterpump is bad?