[SOLVED] No heat at idle

[Txlady_65]

I have an issue with my vehicle and I'm hoping maybe some of you can give me some advice or lead me to what might be the problem.

I have a 2005 GMC Yukon with a 4.8 l engine. It has 192,000 miles on it and I've really not had any issues with it since I've owned it.

A few weeks ago I woke up to go to work, went out and cranked the truck to let it warm up, and usually by the time I get ready to leave it's warm. Well this morning I got in the vehicle and it wasn't nowhere near the operating temperature that it usually runs. It's running about 170 degrees and normally runs around 200. I got in to go to work and going down the road it started blowing hot air but the temperature gauge still didn't rise to where it normally runs. So I went and bought a new thermostat just figured that might be the problem. Replaced it that didn't fix anything. So the next thing we did was to flush the heater core. That still didn't fix it. So we took the thermostat out and exchanged it for a new one thinking maybe the thermostat was a bad one. Put it in the vehicle the temperature gauge got to normal operating temperature but still didn't have any heat from the heater unless you were going down the road, if you're sitting and idling it didn't get hot. The heater core hoses going into the heater core both get hot. Every other hose that has anything to do with the water and coolant are hot. Burped the lines while putting coolant back in and let it run for about 30 minutes with the radiator cap off.

So in talking to a friend of mine he said it sounded like the water pump was going bad and wasn't able to pump enough coolant through it while it was idling. So I went and bought a new water pump along with replacing a couple of pulleys that needed to be replaced too and belts. That didn't fix it either I'm still not getting hot air through my heater while the vehicle is sitting idling. The heater core doesn't leak I don't smell antifreeze or a sweet smell in the vehicle or outside the vehicle there's no leaks under the vehicle anywhere. I am stumped. We have tried everything to think of what might be the problem. The couple of mechanics shops that are in my area I don't trust them at all. So we're trying to figure this out on our own. I have a Launch scan tool but I don't know if or what to look at that might would help me figure out what's wrong with it. So I was just wondering if maybe anybody else has had this problem or have any ideas or tips or suggestions or any information they might could offer on this problem. If not I may have to look for a mechanic over in another city or something. Thank you.

Oh, we also replaced the temperature coolant sensor and no change.

 

[Fless]

Is the cooling system fully burped, with no air left in it? Coolant up to level in the surge tank, and a standard mix of coolant and water (~50/50)?

If your scan tool shows that the engine is up to operating temp (~200 deg F) then look elsewhere. Does your scanner show you the HVAC PIDs for the mode and blend doors? You might look in live data on it to see if the "actual" position is close to the commanded position.

I know that other members will have more suggestions.

 

[Joseph Garcia]

Have you tested the operation of the blend doors that regulate the flow of air through the truck's HVAC system to ensure that they are operating correctly?

@Fless - We're thinking the same thing for the next troubleshooting step.

 

[DoubleDingo]

What is the ambient temperature?

While this was my 91 Cherokee, one time I was driving home and it was bitter cold outside. The temperature gauge would go up to operating temperature and then and immediately drop to 160-170-ish, and the heater was blowing out warm air. Stopped and with the engine running so the coolant didn't spew in my face, I was going to top off the radiator, it was full. Have never experienced that before or since in any vehicle.

If it's really cold where you are that may be contributing to the issue?

In your description, does the fan stop blowing at idle? I have had that happen, not on the Tahoe, but it has happened where the fan stops until there is a change in the vacuum signal, such as giving it throttle or letting off the throttle.

 

[Scottydoggs]

you have air in the heater core, and one or both of the heater hose quick connect fittings at the firewall are leaking. top off the coolant and heat at idle should come back like normal. then replace them two fittings.

 

[Txlady_65]

Thanks for all the help. I've been down with the flu so it took a while. We did get it fixed.

Apparently, there was air in the lines that we thought we cleared out the last time. Last time we did not jack the front of the vehicle up when adding the coolant. This time we drained all the coolant out. Started completely over. We put the car on ramps raising the front end. Made sure there were no air bubbles this time. Everything is working great. The heater is actually warmer than it was before.

So thank you for all your help.