Heater Not Blowing Hot Like It Used To

[kyleholland]

Hi everyone,

I hope you’re well and staying warm this fall.

Yesterday, I noticed that the heat in my ‘99 two-door Tahoe isn’t blowing as hot as it used to. I’m sure threads like this have been posted countless times—I read some of them trying to diagnose my problem—but I’m hoping that you all can help me narrow it down and that it’ll be something that I can fix myself.

This is what I’ve noticed since yesterday:

First, the temperature gauge in my instrument cluster is reading around 155° Fahrenheit after the truck reaches operating temperature (see attached photo). Usually, it reads around 173° when it reaches normal operating temperature. I did just get my truck back from my mechanic a few weeks ago. It was there to have an oil leak fixed, but they also replaced the thermostat while they had it. I called them today to make sure that they put in the correct thermostat and they confirmed that the one they put in starts to open at 195°.

Second, when I change the temperature controls, the air does change from warm to cold and vice versa. It just doesn’t get hot like it used to. Today, I also noticed that the air is noticeably cooler when the truck is idling. When I’m accelerating, the air warms up.

Third, I checked on the coolant level in the reservoir and it was slightly below the “Hot” line on the tank when I got home this evening.

Lastly, I felt the heater hoses after driving the truck tonight. Going from the firewall to the heater control valve, one was very hot and the other one wasn’t hot or cold. Going from the heater control valve to the engine, both hoses were very hot.

I’m wondering if anyone can help me identify what is causing the air to not blow as hot as it used to when I have the heat on in my truck. I’m hoping that whatever parts I might need will be relatively inexpensive and that the issue is something I can fix myself. Let me know what you think if you have a chance.

Thanks!

 

[OR VietVet]

I have to say that the normal temp reading is still too low, on the temp gauge. My guess is failing new thermostat and attach a live data scanner to see what engine temp is.

 

[Joseph Garcia]

As @OR VietVet suggests, your low coolant temperature is not normal, and it may very well be impacting your heater air temperature. You need to verify the actual coolant temperature by a second reader.

It is possible that you may have some residual air in your cooling system, and it may be impacting your actual coolant temperature reading, or simply not allowing the proper coolant volume and velocity across your heater core inside the truck.

 

[Eman85]

Let the engine cool completely and remove the radiator cap and check the coolant level. You say it's hot at the heater control valve but not going to the core. Does the heater control valve have vacum going to it? If you remove the vacum hose does the heat work? Is the heater valve free, you can move the linkage with your finger. Valve should have no vacum when AC is off.

 

[kyleholland]

Hi everyone,

Thank you for your guidance!

I don’t have an OBD scanner so I tried going to O’Reilly and Advance Auto today to see if they could measure my coolant temperature from the coolant temp sensor, but neither of them could do it with the OBD scanners that they have.

Instead, I used an infrared thermometer to measure the temp of the radiator and heater hoses once my truck reached operating temp. I’m attaching photos of those readings below, but all of the hoses except for the lower radiator hose were between 141°–165° which seems to confirm what the temp gauge in my instrument cluster is indicating.

The heater control valve does have a vacuum line, but I’m not sure how to measure if the vacuum is working or not. I did remove the vacuum line from where it connects to the firewall and the heater seemed to be working about as well as it was when it was connected. However, it seemed to work slightly better after I reconnected it to the firewall.

I also checked on the amount of coolant in the radiator after the engine was cool and it was very close to the top (photo attached). I went ahead and tried to burp the system, but I didn’t see any air bubbles come out and the amount of coolant never lowered. However, it did rise almost instantly after turning the truck on to the point that it reached the small hole in the radiator neck and started flowing to the reservoir.

The other thing that I did today was I bought a new coolant temperature sensor and installed it to eliminate that as something that could be causing the issue.

Let me know what you guys think. I sincerely appreciate any guidance or suggestions you may have!

 

[kyleholland]

I should also say that I saw a very small amount of coolant leaking from the thermostat housing when I went to install the new coolant temperature sensor. It’s not enough to see coolant on the ground when the truck is parked in my garage, but it’s definitely leaking very slowly.

 

[Fless]

Does the mechanic's invoice show a part number for the thermostat?

I would suspect a faulty or incorrect thermostat. Since you have a leak at the thermostat housing, it might be a good time to change the stat and fix the seal.

 

[Eman85]

Any leak is no good. That thermostat uses a rubber seal around it and no gasket. Coolant sensor only reports info, can't make heat. You check for vacum by putting your finger on the hose, you can feel i. The solenoid on the firewall controls the application of vacum. They can fail open and apply vacum which closes the valve stopping water flow. I installed mine in the trash when it leaked and eliminated it, AC works fine.

 

[east302]

Going from the firewall to the heater control valve, one was very hot and the other one wasn’t hot or cold. Going from the heater control valve to the engine, both hoses were very hot.

By this, it sounds like the valve is partially closed and bypassing if both hoses before the valve were hot with one after it not. It closes when the A/C temp dial is set to the MAX cold detent. When open, the metal stem at the bottom of the valve is down.

 

[kyleholland]

Hi again everyone,

I have some updates, but sadly, I’m still having the same issue where my heat blows lukewarm at idle and gets slightly hotter when I’m accelerating, but not nearly as hot as it used to.

I went back and reviewed the mechanic’s invoice and it doesn’t have a part number for the thermostat. Since the last thermostat that they put in was under warranty, I took my truck back to them to have them address the leaky thermostat housing and they went ahead and replaced the thermostat again. Neither of those things fixed the heater’s poor performance and they recommended testing the solenoid (like Eman85 suggested) and flushing the heater core as next steps. Since this shop takes weeks to get work done, I decided to test the solenoid myself.

When I removed the vacuum line from the heater control valve with the AC on, I was able to feel vacuum suction, and when the heat was turned on, there was no vacuum felt coming from the vacuum line, so it seems like the solenoid is working. I put it back on the heater control valve and switched back and forth between max AC and heat and the metal stem at the bottom of the valve did move up and down.

Since the solenoid is working and the control valve *appears* to be working, I took it to a different mechanic that is faster and they flushed the heater core. Then, they also suspected that the thermostat that the other mechanic put in either opened at the wrong temp or was faulty, so they replaced it again. After that, they said that the heat was working on the driver’s side, but not the passenger side, and suspected that the blend door actuator needed to be replaced. Well, they were able to fix this issue without replacing the blend door actuator. They said that they got the heat working and that the reason it blows cooler at idle is because my heater core is plugged.

I will say that the temp in the instrument cluster is closer to what it used to be when the truck reaches operating temp now, after this mechanic replaced the thermostat for the third time. But, I’m having trouble believing that the heater core could be plugged up because, before I originally posted about this on November 12th, the heater worked fine until I had the first mechanic fix an oil leak in my truck. It did take that mechanic about two months to fix the oil leak, but it seems to me that a heater core shouldn’t plug up in that amount of time?

What do you guys think? Could it be the heater core, the water pump, or could the heater control valve appear to be working on the outside, but not internally? Or, something else?

Any help or guidance on this would be greatly appreciated!

 

[OR VietVet]

I did not read the whole thread. Too lazy. What is the temp going in to the heater core and coming out.

 

[Eman85]

Heater core on these trucks is pretty simple to change, especially compared to other heater core jobs. I don't think there is a way for there to be a difference in left to right heat on these trucks. What I have seen is the defroster screen of the dash get brittle and break off and fall down into the HVAC case and jam things.

 

[kyleholland]

I did not read the whole thread. Too lazy. What is the temp going in to the heater core and coming out.

I just took a reading with my infrared thermometer after I got to work and it looks like the inlet hose is around 290° and the outlet hose is around 140°.

 

[kyleholland]

Heater core on these trucks is pretty simple to change, especially compared to other heater core jobs. I don't think there is a way for there to be a difference in left to right heat on these trucks. What I have seen is the defroster screen of the dash get brittle and break off and fall down into the HVAC case and jam things.

I’ve seen some YouTube videos of guys who have been able to replace the heater core without removing the dash. Do you think that’s realistically possible?

 

[OR VietVet]

I just took a reading with my infrared thermometer after I got to work and it looks like the inlet hose is around 290° and the outlet hose is around 140°.

I think you meant 190 degrees and 140 degrees. 290 degrees is bad bad bad. The 50 degree difference indicates a clogged heater core. A difference of a few degrees is expected because of the blower pulling some heat and the coolant traveling thru another small radiator of sorts. But, 50 degrees is too much.

 

[Eman85]

I’ve seen some YouTube videos of guys who have been able to replace the heater core without removing the dash. Do you think that’s realistically possible?

Yes, there's a panel on the bottom of the box that comes off and you're looking at the heater core.

 

[east302]

What are the temperatures of the two hoses before and after the valve? Hoses number 1, 10, 16 and 18 in the image below…

10 and 18 are the outlet, so they would be the same temperature if the valve is not obstructed. Same thing for 1 and 16.