When my climate controls are in any position other than windshield defrost (hot or cool), my coolant temperatures run up to around 210F-220F and climbs quickly at low speeds or a stop. As soon as I switch to defrost (hot or cool) the temp falls to 190-196. Is this normal or an issue that could or should be addressed?

 

The direction of air flow in the HVAC system should have no effect on the operating temperature of the engine coolant. Something is not normal. Can you tell us something about your truck?

 

Truck is bone stock with 236k miles. Zero DTCs or freeze frame data stored.

Could this be a blend door or heater core issue since coolant runs through the heater core?

 

No, I do not think it is a temperature blend door issue. The coolant always circulates through the heater core regardless of what HVAC mode is being used. Whether or not you are actually transferring some of the heat in the coolant to heat air for the cab should have no noticeable effect on the overall operating temperature of the coolant. Remember, the coolant temperature you see is a calculated by the PCM temperature base on the CHT (Cylinder Head Temperature) sensor and other factors that the PCM monitors, not an actual coolant temperature from a coolant temperature sensor.

If the heater core was plugged you would not be getting any (or very little) heated air in any mode. The plugged core condition would be the same for any HVAC mode, so it shouldn't have any different effect on the operating temperature of the coolant in the different modes.

I would be looking at someting like radiator shutter operation in the different modes or the overall health of your cooling system.

I have a few questions:

1. Does your truck have EMTC (Electronic Manual Temperature Control) or DATC (Dual Automatic Temperature Control)?
2. What antifreeze are you running and how long has that been in the truck?
3. What thermostat are you running and has it ever been changed?
4. What is the ambient temperature when you experience the temperature differences?
5. When the coolant temperature is operating at 210-220°F in non-defrost mode, is that with the AC on or off?
6. When the higher coolant temperatures are present, are the radiator fans operating, and if so, on low speed or high speed?
7. When you say "Zero DTCs or freeze frame data stored", what are you using to read the DTCs? FORScan, high-end computer based diagnostic tool, or OBDII reader?
8. When requesting heated air, does the system provide the desired warm temerature in all modes?
9. When requesting cooled air, does the system provide the desired cold temerature in all modes?

 

Hope this can help. Thanks for your reply.

 

Well, I'm not sure it helps, but I know a lot more about your truck than I did before.

When in defrost mode, even though the AC light goes off, AC is on as long as the ambient temperature is above 32°F. The air, after passing through the evaporator, is then flowed through the heater core, if demanded by the HVAC settings, to reheat it to the desired level. This is to pull as much moisture from the air as possible to increase the ability of the defrost air to remove moisture and frost from the inside of the windshield. In defrost mode, air will always be fresh, not recirculated. It even turns the recirc light off.

Definitely not a blend door actuator issue.

Have you looked to see if the radiator shutters are open when the higher coolant temperatures occur?

Does the coolant level in the overflow tank very much from cold to hot? I'm thinking about trapped air still in the system. This is kind of a long shot as it has been two months since the change of fluid, but it is a common problem is a vacuum filler is not used.

You used a Motorcraft thermostat, but which one. For some engines there has been a couple of updates to the thermostat part number. Did you test the thermostat before installing it? Do you have the latest? I don't know what MY and engine you have so I can't suggest what the latest part number should be.

 

Again, thank you for your interested response. I will go back tomorrow and see if I can release any more air pressure/air pockets after a drive when it comes to temp again. If I end up feeling more uncomfortable with the temperature variation, I’ll just pony up at the stealership to do the proper vacuum flush.

 

When the AC runs in defrost modes, the radiator fans run also to cool the condensor.
This is probably why it is cooling to normal temperatures during those times.

Maybe the problem is that the fans are not being turned on in during normal operation causing the higher temps.


Edit, If you drive for sustained period of time with the defroster running does the engine temp stay normal?
If it does that seems to prove its not a coolant-trapped air-thermostat issue but an airflow issue, I.E. fans, shutters.