My 2005 Gmc Jimmy has had no heat for 3 years. Every winter I try something different.
2 Mechanics have wasted my money. One did the garden hose heater core fix. They fixed it for most of the winter, same problems the next year.

Since then I have:
Changed the blower motor.
Changed the blower motor resistor.
Took dash apart:
Replaced the Heater core this week.
Replaced the blend door actuator. (The old one worked, new is crap and slow, will be changing that again)
Changed the thermostat.
Changed the heater control buttons (swapped in used assembly from the junk yard)
This past summer, did the garden hose hether core clean out and replaced all the Dex coolant.


Whats left to do:
Check for cracked vacuum lines in engine bay. This forum found that as one of the problems.
Replaced Radiator cap. Did notice some improvement from it. (noted below).
Temp Sensor to replace. (but I do get temp readings on my dash.)
I really thought the heater core would fix it. Did not hurt to change a 14 year old core that could have been a problem. Both hoses are warm going in an out of it now.

I had just replaced the heater core this week. Prefilled the core with coolant to minimize air in system. Ran it for an hour, took it for a drive, burped upper radiator hose as best I could and the hoses in/out of the core. There might still be air in it. I get slightly warm heat, would take an hour to barely clear half of a frosted up windshield in winter. I estimate the air temperature coming out on max heat is only 43°F (6°C).

Is there something basic I am missing?
What should I try next?

Thanks for any help!

 

After everything you've done, the only thing I could think of is the blend door that sits in front of the heater core that opens and closes is not opening properly. If both of the heater hoses are warm and you have a new heater core, it has to be the door that is stuck shut.

I just replaced the heater core in my 2001 ZR2 blazer, and it cranks out heat that is hotter than ever.

There are a couple vacuum hoses under the hood that sit by the power brake booster that control the heater controls, just maybe they are cracked/bad or leaking?

If that's not it, I'm not qualified to help you any further. I have replaced everything on mine just like you have and everything works perfect.

The only thing else could be the used heater control panel from the junkyard, it could be faulty after 15+ years, as these units have rubber gaskets internally that go bad which could be an issue.

 

What are the symptoms? Full air flow but cold air? No airflow? Airflow to wrong places?

Have you checked that there is no debris blocking the airflow through the AC evaporator?

Have you checked if the under-hood vacuum lines are in good shape?

 

Full air flow, but cold air.
Can't get the air to divert to windshield, floor etc. But that had worked before I put in a new blend door actuator from china. Something might be in the wrong position?
Vehicle only has 60,000 miles (100,000km).
Junk yard heater control: Had the original in and the junk yard and both give same results during my test drive.

Have not checked AC evaporator. Can read up on how to check that. AC works great in summer though, so it might be fine?

Will find and check under-hood vacuum lines as you both suggested.

 

Cracked vacuum lines in the engine bay.


Thanks for the help! I will replace those cracked lines, I also have the unconnected line that I think needs to go to the connector on the fender. Will fix all those issues, test and post if I still have no heat.

 

i have a link in my signature about the undethood vacuum hoses that you might check. if there is no vacuum, the actuators that direct the airflow will not work.

You can check the blend door actuator by reaching in through the glove box opening and either by ferl or visually seeing if it is moving. It can also be checked from the engine compartment by removing a small access panel on the heater box near the firewall.

Do you have a Computer Climate Control or manual HVAC control system? If the former, perhaps the interior passenger compartment temperature sensor is malfunctioning by indicating to the controller is already hot.

 

Fixed the vacuum hoses, now the heat can be direct to windshield defrost, floor etc. They respond in a few seconds like normal.
Thanks Christine_208 For the detailed hose size post! The guy at the auto parts store had not had anyone ask for Vacuum line in his extensive 1 year of experience.... I think they just sell Duct tape or WD-40 to fix every question they get...

Still have the Low heat issue. Checked with a thermometer. Now getting warmer 80°F (27°C) at the vents.At least the windows will defrost slowly now.
If you run the vehicle for 20+ min it will get to 80°F, if you run it an extra 20+ minutes it goes no higher and is at the same temp blowing out of the vents.
My understanding is that I should get somewhere from 110 to 120°F when working properly?

I squeezed the hoses by the heater core, which made them hotter, so the dexcool is not circulating quickly?
The temp gauge responded normally. Showed cold on start and warmed up to normal temps after 20 min or so.

I will drive it more and see if it gets warmer. Have ran/driven it for over 2.5 hours of testing.
This vehicle has manual AC/Heat. I saw the Computer ver at the junk yard and know it is not that system.

 

Yeah, progress!

I had to go to several places to get all of the hoses I needed. If you want a chuckle, checkout this tongue-in-cheek review of our Blazers in which they make fun of the vacuum hoses.
As for proper temperatures; here is what it says in my 1999 S/T Service Manual:



Is it possible you have air in the cooling system? I've heard of this being an issue. I've not had it. After filling up the coolant, I'll go for a drive and get the temperature to full hot (~190F) and then when it cools down it sucks any coolant it needs back into the radiator from the overflow reservoir. I wonder if your radiator cap could be bad? Radiator caps can be tested but they are also not too expensive. My driveway has a bit of an incline that might help with getting air out of the heater core.

Is the level in the overflow tank going up and down as would be expected; at the Full mark when hot and a bit lower when cold (close to the "Add" line)?

Below are some pictures of the debris I removed from the inlet side of the evaporator a few years ago.


The Computer Climate Control blower motor control module is removed exposing the larch needles and ash tree leaves. The green line shows the location of the AC evaporator. The large opening on the left is where the blower motor goes. I have the Computer Climate Control so there is a solid-state motor control module that can be removed. The manual HVAC systems might not have this and the only way to clean out the debris is to remove the blower motor which also means removing the computers and coolant overflow tank.



Some of the debris I removed from the blower motor side of the AC evaporator. My 1 gal shop-vac was full too.

 

Low heat? Maybe that used temp control unit you put in is not working properly? Again, those units have rubber gaskets that dry rot and leak over time. Perhaps the cold/hot dial portion of it has seen its better days? I bought a brand new AC delco unit for around $80 on Amazon.

What brand heater core did you install? If not ac delco, could it be inferior as far as flow restriction?

Are you using the standard 195 degree thermostat?

Lot's of variables, but glad to hear you're getting closer to getting it where it should be.

 

What are the actual temperatures of the heater core hoses going thru the firewall? Is there a large variance? (if this hasn't been asked previously)...Jerry