Whether the air coming through the heater core is hot or cold it wouldn't cause the blend door to move. The blend door mixes hot air with cooler air. If there was an automatic temperature control, it would try to keep the blend door in position to move air through the heater core even if the air coming through isn't hot like it would be at initial startup.

Without knowing exactly how the system is designed, it seems to me that the blend door position should be determined by where the temperature control dial is set. If it moves without turning the dial, there is something wrong. There could be a potentiometer in the motor to tell the computer what position it is in. The temperature control is probably a potentiometer too and the computer might be set up to keep the two values the same and runs the actuator forward or back until the values match.

If the system keeps moving the actuator, it could be in the actuator, the temperature dial, whatever module controls the actuator or a wiring problem.

If the air coming through the heater core is mixed with either cold outside air or slightly warmer inside air, the air will be lukewarm. The easiest way to check is to put the blend door in the full hot position either by disconnecting the mechanical or electrical connection.

The original post states the heat comes and goes while driving, which sounds more like a coolant delivery/flow problem, unless he is just turning the dial from hot to cold and vice versa while driving.

If the blend door cannot hold position, the temperature of the incoming air would fluctuate, as db2006 said.

I'm with donbrew, the HVAC headunit. The actuator has already been replaced, right?

Hey Guys!

Yes believe it or not, I still have not found the issue.......I’m concerned as to what to try next and not be a waste of money. I certainly don’t want to bring it into the dealer. Yes the coolant level has been staying full.

Let's not make this magic. The heater core is always hot. The blend door controls how much cool air is allowed to flow over the heater core. If you turn the temp knob to cold the blend door does not let any air flow over the heater core. Air is always flowing over the evap core so you can have heated conditioned air. When the A/C is on the temp control knob allows some conditioned air over the heater core.

There is no thermostatic temperature control. The blend door stays where you set it.

If you have a malfunctioning cooling system t-stat it is possible that the coolant flowing through the heater core is not hot enough at times. A proper t-stat keeps the coolant temp at a fairly constant 180-190F. A broken t-stat can let the temp get down under 170F.

The A/C is turned on with no indicator when in defrost mode or recirc mode.

If the system is blowing cold air when the temp knob is set to hot
1. the coolant temp is too low
2. the A/C is on
3. the blend door is malfunctioning
4. there is some sort of variable obstruction in the heater core circuit
5. the mode door is malfunctioning


https://cimg5.ibsrv.net/gimg/www.che...a91d8717e3.png

Diagram aside, that is the clearest, easiest to understand description of the A/C and Heating system I have ever read. :bow:

Are we skipping over the low coolant level? Possibly the thermostat stuck open?
Oh, are there just two many cars running cold.

Post #84 states coolant full.

I address t-stat in post #85.

I had a Ford with a stuck t-stat, nearly froze while driving in 24F weather. The heater was putting out 28F air. But every once in a while it got hot.

I’ve replaced the thermostat and dexcool already. I’ve kept an eye on the coolant level. I’ve replaced the temperature actuator.

This morning I removed the cabin air filter and examined the blend door for recirculating and fresh air. That is opening and closing as should. I warmed up the car and the heater was working correctly, let the car sit and idle and while and couldn’t get it to fail again, although it hasn’t been working the last couple of days.