I picked up a retired government Blazer about 6 months ago. 2004 4DR 2WD. Normal 4.3 Vortec, No bells or whistles (not even remote keyless entry). Car mostly sat from the service records for about 3 years before they surplussed it. Only getting drove a few times a month.

After driving it for a bit I noticed the temp was only getting to the 160F range and then holding there. I also got an intermittent P0128 code. With that info and some looking around I changed the thermostat. Local parts house had one in stock. I wasn't sure on the temp to get (was kind of leaning on 195F) but the parts house only had 185F in stock. I installed that, temps now run in the 185-190 range (by my estimation on the gauge). I was hoping my gas mileage would go up from the change but seems to still be in the 18-20 mpg range. BUT I am still getting the P0128 code. I also only get a little heat from the heater, although sometimes the heat is better than others but lately it has been less heat more often than not .

Also the ac vents have quit changing. I seem to get the majority of the air flow from the dash vents with a small amount from the defrost. I haven't felt any from the floor vents.

Should I change to a 195F thermostat? Or is 185F ok and I should change the temp sending unit from the side of the block?
Am I right in thinking the vent issue is probably a rotten vacuum line?

Could it be possible both issues are from the vacuum lines?

 

Blend door actuator...
Under the dash in the truck maybe?

 

My thoughts exactly... Blend door actuator seems too be the most likely thing...

 

It should have a 195 thermostat. 4.3 v-6 are typically not very fuel efficient. if you're averaging 18-20 with a 2WD - then I wouldn't complain about that. If that's you max on the highway, there may be a little to improve.

There is a plastic vacuum reserve tank hidden in the drivers fender just to the side of the brake booster. This sometimes gets unplugged and that causes a lot of problems with the A/C controls.

 

The temperature is (mostly) controlled by the electronic "blend door actuator" whereas to where the air is directed is controlled by vacuum controlled actuators.

For the vent actuators, the vacuum supply is commonly compromised by leaks in the vacuum supply hoses in the engine compartment. Much less likely, as evidenced by posts on the forum, would be failures of the hoses within the dash or at the HVAC control modules. In my signature below I have a link to a thread in which I described the replacement of my engine compartment vacuum hoses. The failure mechanisms seem to include rotting of the hoses so that cracks form around or along the hoses and them becoming loose at the nipples. The vacuum reservoir tank hose can come loose either because of the hose being loose or it being pulled off by accident. There is also a one-way valve which keeps the vacuum system isolated when the engine is not drawing much vacuum that could fail. (It is a not expensive part so don't worry.)

For the blend door actuator, the gear through which the axle for the door passes, will crack and slip. This can be replaced by reaching in from the opening for the glove box. A search for videos about replacing the blend door actuators on Blazers will result in many hits that will include videos too. Note that there is a vacuum actuated door that controls whether or not air flows through or bypasses the heater core so even if the blend door actuator is working correctly, the amount of heat you get can be not what you expected.

Because it is much easier to check the vacuum hoses, I would recommend checking them first. You might want to change them out anyway since they seem to rot out on many Blazers when they get 15 y.o.

There is a way to check the blend door actuator. You can reach in through the glove compartment opening and using your finger and try to feel if the hollow axle of the actuator is moving when the temperature control is changed. If you have a manual HVAC system, not the computer controlled climate control system which would be the "bare bones" one, you might be able to remove the blower motor fan control resistors at the top of the air box downstream of the AC evaporator and see if the door is moving and if necessary, move it manually to the position you need for the time-being until you get the replacement part; for winter that would be for full heat.


Opening into air box (red circle) downstream of AC evaporator (green).




Blend door (magenta arrow) and axle for door (yellow). The heater core is to the right of the yellow line. This position of the door would be for full heat.

 

Thanks for the replies!!! And the pictures are a great help.

Just went out in the parking lot and peeked under the hood and I am beginning to think its going to be a combination of several failures.

#1 - the first vacuum line I looked at appears to be dry rotted and cracking. Looks like I will be replacing those soon as well.

#2 the thermostat being a 185F when it should be a 195F is probably causing the P0128 code. I get 19.5 MPG at best. I am mostly rural highway driving as well. The norm is 18-19 MPG.

#3 I suspect the blend door actuator has went bad like suggested. I do have the "bare bones" model and the "Access door" does match the photos Christine posted so I can check that as soon as I can get a bit of free time.

 

Glad to hear these posts have helped so far. Be sure to report back on what else you find and what works.

 

Ok.

I replaced the thermostat. Hopefully that cures the random P0128 codes and the not quite hot air from the heater.

I also replaced all the vacuum lines associated with the AC inaccthe engine bay. They are all terrible and fell apart when I pulled on them. That cured the air direction issue I was having with it not changing. Yay!!! I Can maybe defrost my windshield next time it's frosted over.

I checked the mix door from the engine compartment from the port in the picture above. My Blazer has some kind of sensor in it (I assume temperature). But I was able to revome it and could see the door operating in all directions as I set the temps to different settings on the controls.


Thanks for the help!!!

 

Glad the information helped. That the blend-door seems to work means it is likely ok. Hope you get nice and toasty in your Blazer now! :-)

 

You also might want too flush your heater core... I did mine not too long ago and it made a BIG difference