Rear heat not working

I assume you have air blowing out of the rear, but it is just not hot.

If that is the case and you have one of the hoses going to the rear that is hot the other should be about the same temperature.

Basically, the way this works is the coolant is routed to the heater core and the air is blown over it which heats the air as it comes into the passenger compartment.

I suspect you have a restriction in the heater core which is preventing the air from heating because the coolant is not flowing freely through the heater core.

So, what we need to do is check the temperature of each of the lines running to the heater core and they should be pretty much the same temperature.

Let's use a laser temperature meter to find out what they are, and we can go from there.

https://www.2carpros.com/articles/how-to-use-an-infrared-temperature-meter

Once we know this we will know if we need to replace the heater core. However, the next thing we will need to check is the coolant level. If it is low, then it can cause this as well.

Here is a video that talks a little more about this:

https://youtu.be/SNRb6pSwYuU

Thanks

So, you may have two issues. The heater core may be starting to clog but the fact that you have a large difference in air temperature between the mode's points to a blend door issue on the rear unit.

Unfortunately, the manual has no info that would be helpful on this rear system. This is typical on these older vehicles as the manuals were not detailed.

The way your system works is the doors are vacuum controlled so I suspect when you are changing the position that the vacuum is leaking on the blend door motor and allowing the air to not be directed solely over the heater core and so it is not as hot.

Also, those temperature differences are more than I would like to see but we have to remember this is a rear unit so by the time it gets all the way back there and through the heater core and returned, I guess that 20 degrees is not that much.

What I would do is access the unit in the rear and make sure you have no vacuum leaks in the lines and test the diagrams in the actuators. You can do this by using a handheld vacuum pump and apply vacuum to move the door to make sure it can move to the full travel. You can do this and then check to make sure the heat is blowing at the proper temp. You then know there is an issue with the line. Otherwise, it is the motor.

Let me know if this doesn't make sense.