Heater blowing cold after 15 mintues...
 

(2010 40th Anniversary) Changed spark plugs and cleaned throttle bodies and now heat works for ~15 minutes then goes cold. Will blow "hot" if I crank the temp dial up, but temperature not consistent at all (gets warm then cold then warm...). Fan speed and selector controls work fine. Oil temp and water temp in correct range when it happens and not an issue. Coolant is filled to reservoir max line.

• Thought it might be the thermostat so changed that...not fixed.
• Thought it might be air in the heater coolant lines so drained and flushed three times with super slow fill and front end jacked up, revved out bubbles...not fixed.
• Though I might have knocked a connection loose behind controls (new radio install) so disassembled and triple-checked all...not fixed.

I'm out of ideas and about ready to head to the dealership. Does anyone have suggestions on what else I could do first?

My guesses:
1) You didn't bleed the cooling system properly. Procedure is in FSM.
2) One (or more) of the HVAC doors is not moving properly. Troubleshooting guide in FSM.

I bet your problem is inside the car. Whatever regulates the heat is going wacky. Whether it is a coolant flow valve on the heater core or a door that opens and closes. Not familiar with the Z hvac system but that is where you need to troubleshoot.

There's a climate control sensor under the dash between the steering column and center console. If it came unplugged that could cause issues.


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My guess:
Low coolant level in the radiator.

When the thermostat opens, the coolant in the block flows into the empty radiator and the level then drops below what is necessary for the heater core to be supplied with coolant.

This is what happened in a 4 liter V-6 Nissan pickup that we used at work. Refilling the radiator solved the problem.