A/c works great heater doesn't

Trailblazing

Original poster
Member
Jan 1, 2013
7
So I an 2003 tb. Ls with Manuel temp controls The a/c (temp controls on cold driver and passenger ) works great out all vents and all blower speeds work good. But if I switch to heat the air is restricted out all vents on no matter passenger or driver vents. What air comes out is hot But vary little air comes out It changes from defrost to dash to floor but all are restricted. I tried to remove for few minutes and reinstall the hvac-b fuse and let the system recal. But no differance.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Welcome! Heater cores have been known to get clogged with dust, debris, and mouse nests. I'd pull the blower motor under the glove box and peer in to the left and look. It may not be easily visible, and I've never had to look, so I'm guessing.

As the saying goes, when the factory builds a vehicle, they put the heater core on a little pedestal and EVERYTHING else attaches to that.
 

Trailblazing

Original poster
Member
Jan 1, 2013
7
the roadie said:
Welcome! Heater cores have been known to get clogged with dust, debris, and mouse nests. I'd pull the blower motor under the glove box and peer in to the left and look. It may not be easily visible, and I've never had to look, so I'm guessing.

As the saying goes, when the factory builds a vehicle, they put the heater core on a little pedestal and EVERYTHING else attaches to that.


I forgot to note that I removed the access panel that were once used for the cabin filters on the older models they used the hvac system on. The only thing that is accessible from there is the a/c condenser what is pretty clean.there is not access to view the heater core. My under standing of the system air flow is all air goes threw the blower then condenser then gets blended by the temperature controls( depends on what you set the temp to. For illustration lets say mid temp) half air flows threw the heater core and other half around the heater core then to the mode door what directs the air to what vent you choose.
 

AbsoluteZero

Member
Nov 21, 2011
211
To further verify the possibility of heater air path blockage have you tried all blower speeds on heat? Can you hear a change in noise level suggesting the blower is actually running at different speeds but not delivering much airflow? You mentioned you can vary blower speed on AC mode but didn't say if that's the case on heater mode. Also HVAC system faults can set a "B" code that can give a clue if a servo is bad.
Also curious if when in the AC mode can you signicantly vary the outlet temperature. If you can this might suggest the heater air path isn't blocked
 

Trailblazing

Original poster
Member
Jan 1, 2013
7
AbsoluteZero said:
To further verify the possibility of heater air path blockage have you tried all blower speeds on heat? Can you hear a change in noise level suggesting the blower is actually running at different speeds but not delivering much airflow? You mentioned you can vary blower speed on AC mode but didn't say if that's the case on heater mode. Also HVAC system faults can set a "B" code that can give a clue if a servo is bad.
Also curious if when in the AC mode can you signicantly vary the outlet temperature. If you can this might suggest the heater air path isn't blocked

On max heat temp I can change blower speed, I can hear the blower change speed also. No faults either. If I start temp on max cold temp and have mode door on dash vents only and start bringing the temp down to hot there is a minimal change in temp until i get to max hot but at the same time I start to get restricted almost immediately when a start moving the temp.
 

AbsoluteZero

Member
Nov 21, 2011
211
Moving the temperature from cold to hot and seeing restricted airflow is suggestive of something blocking the heater side. I'll mention your typical DIYer scanner won't read B or C (chassis) codes and neither generally will display a trouble light. I've recently read some of the parts stores have scanners that can read these codes. Might give one near you a call and inquire.
 

Trailblazing

Original poster
Member
Jan 1, 2013
7
AbsoluteZero said:
Moving the temperature from cold to hot and seeing restricted airflow is suggestive of something blocking the heater side. I'll mention your typical DIYer scanner won't read B or C (chassis) codes and neither generally will display a trouble light. I've recently read some of the parts stores have scanners that can read these codes. Might give one near you a call and inquire.

I have a snap on scanner , and still no codes, next thing to do is remove the hvac system and inspect it. I was hoping someone else with the same issue
 

dacoach

Member
Jan 4, 2013
25
I have the same problem with my 05 envoy. The blower speeds work fine in heat and a/c. When you have the heat on their is very restricted air flow. When you move the manual temp slide slowly down to cool then the air flow speeds up to almost perfect when you get to the bottom on the cool slide. I have not looked at it nor have I fixed it. I am not smart like the others on this site. Hope we can figure out what the problem is. It is cold in Nebraska.
Dacoach
 

Trailblazing

Original poster
Member
Jan 1, 2013
7
dacoach said:
I have the same problem with my 05 envoy. The blower speeds work fine in heat and a/c. When you have the heat on their is very restricted air flow. When you move the manual temp slide slowly down to cool then the air flow speeds up to almost perfect when you get to the bottom on the cool slide. I have not looked at it nor have I fixed it. I am not smart like the others on this site. Hope we can figure out what the problem is. It is cold in Nebraska.
Dacoach

I am going to use a scope camera to go down the dash vents to check the mode door and the temp door tonight ill post some pictures if I can see anything odd in there.
 

Denali n DOO

Member
May 22, 2012
5,596
Trailblazing said:
I am going to use a scope camera to go down the dash vents to check the mode door and the temp door tonight ill post some pictures if I can see anything odd in there.

Now there's an interesting way to diagnose! I can hardly wait to see the pics. Great idea :thumbsup:
 

Trailblazing

Original poster
Member
Jan 1, 2013
7
View attachment 25751 passenger temp blend door. I can't get the videos uploads but I was able to see only down to the blend door. I was trying to get to the heater core, I will try again tomorrow. So the mode door is working the way it should and the blend door is working the way it should. So the next step would be to open it up and inspect the hvac system
 

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dacoach

Member
Jan 4, 2013
25
Did you ever figure out the problem with loss of heat Trailblazing? What was blocking it.
thanks, Dacoach
 

Daniel417

Member
Oct 14, 2015
50
Joplin
Thread hijack. I have a problem with my heat barely blowing. Everything functions as it should besides low circulation when heaters on. Has anyone had to replace heater core or clean it? I just want to know steps to access it, to clean it.
 

Ziggy

Member
Feb 8, 2015
207
Necropost! Daniel, I think you probably want to start a new thread for this. I don't think what you're describing is the same issue as the OP, and this thread is almost 3 years old.
 

gpking

Member
Dec 27, 2013
534
Berkeley Springs, WV
I've got a related issue. I replaced both my passenger and driver heater blend door actuators about a month ago, but they still do not work.
The original issue was that they were stuck shut (cold) and they would not budge.
After installing the new actuators, I pulled the HVAC B fuse on the BCM fuse block multiple times to reset the system, but I could still not see/feel the actuators actuate as I changed the temperature.
I read online that either a scanner or multiple drive cycles can reset the system, but it has been a month with no change and I have no diagnostics scanner.

The air blows lukewarm (because the new actuators were indexed 50/50) and I cannot change the temp to cold or hot.

I'm guessing either the system hasn't reset or something is catastrophically wrong.
 

Mooseman

Moderator
Dec 4, 2011
25,483
Ottawa, ON
Possible you have a blocked heater core. Tried flushing it out like in this video?

 

gpking

Member
Dec 27, 2013
534
Berkeley Springs, WV
Possible you have a blocked heater core. Tried flushing it out like in this video?
I flushed the system when I replaced the upper radiator hose around June 2014.
Even if it was just partially clogged, it wouldn't explain why the system doesn't blow cold either.
It seems like an electronic issue to me.
I can't change the temperature, period. The actuators are stuck exactly where they were out of the box.

The old actuators were stuck at 0% heat and I got no warmth out of the vents.
The new actuators were indexed at 50% heat, and the air blows slightly warm (enough to warm up the cab after a few minutes of driving), but nowhere near as hot as it should be.

I guess I'll take a peek at the service manual and see if there aren't any fuses I missed or relays along the line I could swap out.
 

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