I drive for a while, turn the heat on and it's barely warm.
It remains like that no matter how long I drive.

The last time I changed the Thermostat was in 2013. A 195.

How can I tell why this is so? Should I install a new thermostat?
Should I do a reverse heater core flush?

thanks

 

Either the core is clogged or you are low on coolant. Check your coolant level first. If it's full, get the engine up to temp and check the heater core hoses. They should be hot. If either one is not hot, your core may be blocked. At that point, take the hoses off and flush out the core with a garden hose both ways (inlet side, then outlet side).
If your coolant level was low, begin diagnostics as to why. A pressure test would be in order.
Keep us posted and gl.

 

Thanks. I'll check the heater hoses today. . . I may add. Earlier I had checked the coolant level, noticed that it was low and filled it up. That had no effect on my heat issue.

 

Same thing was happening to me. Turned out it was the water pump leaking as I was driving. Check your coolant level again after a day of driving (and once it cools down completely, of course). If it's low again, time for a pressure check to find your leak. 👌👍

 

Have you checked the blend-door actuator? Mine failed (with the internal gear slipping) making it impossible to change the temperature. In my case it got stuck in the hot position.

Definitely do the easy checks first like feeling the temperature of the heater core hoses. They both should be warm/hot.

 

I checked the heater core hoses. They were both hot. The coolant level had been low before and I added close to a gallon till it was full. It is still full. And it is still barely barely getting warm. It seems, even though not 100% sure, that since i added more coolant it is worse now.

 

It is worth checking if the blend-door actuator is working correctly or not. It is not an uncommon problem. It "blends" the air coming into your heater system by directing none, some or all of it through the heater core.

There is a way to manually set it by gaining access to this door from the engine compartment. You remove an access panel and within you will see the door on a vertical axis. If the actuator is bad, I've heard of people manually moving it.

The pictures below are from when I was diagnosing if my actuator was bad. In the pictures it shows it in the full heat position. When I tried to change the temperature I could hear the actuator motor working but the door did not.

There are quite a few videos that show how to replace the actuator (if that turns out to be your problem).


Access panel to heater core



view down through access panel. blend door is in center and heater core is on the upper right.

 

Thank you for the detailed blend door actuator write up. Am just a noob but the write up is very understandable. Mine is manual temp control. Is there a way to identify if in fact the actuator is faulty without taking off? Or should i just replace it? Ebay $26.

 

You are very welcome. I prefer to diagnose problems as much as possible before replacing parts but in this case I think there is a good chance that the blend door actuator is bad. However, as long as the weather allows it, I think it is not a bad exercise to do the diagnosing if for no other reason than to build your confidence in working on your Blazer.

With the manual heat control, I would think it would be easy to test if the door is moving or not. I have the auto temp control system so what I've written below is an extrapolation to your manual control system of what I did to diagnosis my bad actuator.

Open up the access panel to the heater core from the engine compartment so you can get a look at the blend door and note its position. On a manual temperature control system this would be the place where the blower motor resistor is mounted. (See video below) Then with the engine running and the blower motor on (just to make sure everything is working) change the temperature setting for your heater and then observe if the door changes position. If it does move, the actuator is fine. Full hot should have the door situated so that all of the air coming in must pass through the heater core. Full cold would have the door situation so that the heater core path for the air is bypassed.

If the door does not move, there is either no electrical command going to to the actuator or the actuator has had its main gear slip or disengage from the shaft for the door. (There are plenty of videos online of people repairing these if you are interested in how they fail.)

If the gear is stripped in the actuator, you should be able to tell if the motor is working in the actuator, which would mean it is getting signals from the temperature control. You can tell if it is getting a signal by putting your hand on the actuator by reaching in from the opening for the glove compartment, and changing the temperature control. If you feel a small electric motor being turned on and off, then the mechanical bits of the actuator are likely the problem.

If you don't feel or sense any electric motor motion in the actuator, check the electrical connection to the actuator. It is unlikely that the connector is bad but it is always worth checking easy stuff like that!

(The glove compartment door can be swung down to hang vertically by releasing a spring loaded catch in the upper part of the opening for the glove compartment door.)

Hope this helps you figure out what it going on!

 

BTW, when replacing the blower motor, and I'm guessing the blower motor resistor, when/if you seal the rubber part, use non-corrosive RTV sealant. My GM manual said explicitly NOT to use RTV (but that was back in 1999). Now they have non-corrosive RTV. I suspect the out-gassing of the RTV as it cures creates a corrosive gas that can mess with electrical circuitry.