[wesman4711]

When the truck warms up the heat inside the cab is very intermittent it turns on and off but there is a strange almost popping sound comming from underneath the passenger side dash. Not sure what caused it but the problem came up when it started to get cold outside but it's still only 25 degrees. Should I be looking at the heater core or something else?? Please help!!

 

[madpogue]

What do you mean by "on and off"? Does the blower itself actually run and stop intermittently? Or is it just the temperature of the air coming out that goes up and down?

 

[wesman4711]

The temp of the air comming out. The fan blows at a constant. The air gets warm (not hot) for a few seconds then goes cold and fluctuates in and out of being cold and warm. It doesn't seem like it gets as hot as it needs to either. Maybe a vacuume pump might have a bad diaphram? Should I look at that or something else

 

[Steve83]

Click this, read the caption, then fold the glove box down, & shine a flashlight in: