When I start my car it is taking 7-8 minutes of driving for the heat to kick in. If I start it early and leave it idling to warm up nothing happens. I've left it for 20-30 minutes trying to diagnose, but it doesn't get warm. For the first 11 years I've had the car it would warm up normally where the heat would gradually get hotter and hotter.

I've replaced the thermostat, but getting the exact same result.

I uploaded a video since I'm really not sure how to explain it. Basically no heat blows out until 1:08 in the video when the gauge drops back down to between the N and O of normal where it has always operated. This is taking 7 minutes of driving and all the heat comes at once. It used to gradually put out more heat until it was up to operating temp even at idle.

Thanks for your help. What do I need to fix or check next?

 

Try replacing the coolant temperature sensor, it can be the sensor is faulty or maybe there is air in trapped ur cooling system where the sensor is reading trapped air bubble instead of Coolant.

 

I too suspect an air pocket, especially since the thermostat has been replaced. Don't replace the sensor without testing it first. Its resistance changes with temperature (there should be a chart somewhere around here...).

EDIT: ECT Testing

 

Also agree with the air pocket scenario. That did not read normal, temp came up to quick.