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.