My heater has started taking longer to blow warm air.

2003, 250k miles 4th gen.
After idling for 15 minutes it will still be blowing cold air. Then once I start revving the rpms it will start blowing warm air. And once I’m driving it around, it blows warm like normal.

Any idea to troubleshoot on my own? Maybe the thermostat needs to be replaced?

 

Original radiator...they're know to fail at the lower corners. I'd also be looking at:

Water pump
Thermostat
Blend door(s)
Heater core

 

^^^^ is a good list of the normal suspects. If your heat is delayed coming up and the HVAC air temp is consistent with the engine temp gauge, I would start by replacing the thermostat. Particularly if it has been 3-4 years since it's been done.

 

Check your coolant level first. You could have a low level which could cause air bubbles in the system.

 

Are you noticing the coolant temp gauge rising slower than before?

Vehicle being 21 years old, thermostat should have been replaced twice already, but it's good to have confirmatory evidence.

 

This, park the 4runner with the front tires on a ramp or curb. Pop the top on the radiator, start it up, let it run for a minute or so. If you see many little soapy like bubbles you got a leak in the coolant line somewhere and would like be low on fluid, if its just a few bubbles and good level then you many have just resolved the problem. Shut er down, button it up, go for a drive to determine if more troubleshooting measures are required...t-stat next.

 

New thermostat and topped off coolant did the trick! Thanks everyone! Easiest job ever.