Heat problems!

JCedar · December 12, 2006

I am having a problem with the heat in my 99 Sierra NBS w/ 153,000 miles. There was a night about two weeks ago where it got down to about 5 degrees F. I started my truck up and turned the dial all the way to the right so it was all heat and the dial to the far right to defrost and lower vent. I let the truck warm up for about ten minutes and when I got inside it was still freezing, also it was only comming out of the dash and not the floor vents. I adjusted the dial so it was for the vents in your face and the floor vents and nothing happened, it stayed in defrost and it was still very cool temperature wise.

I said screw it and took off! That was the only time that the selector did NOT work properly, to this day the selectors work, BUT...... The heat just isn't as strong as far as temperature goes. It seems like it's blowing in the low 60's at the most. Before it would roast you right out of the cab, and now it seems I have to drive an hour just to get it so I can take my jacket off.

So my question is.... Could it be the Climate Control Module (Where you select the fan speed, temperature and which vents) or does it sound like it is more likely a thermostat or a heater core issue??? Like I said earlier the other problem does not exist anymore it's just the fact that it doesn't get as hot as it used to.

Any thoughts or suggestions would be appreciated!

asilverblazer · December 12, 2006

I am having a problem with the heat in my 99 Sierra NBS w/ 153,000 miles. There was a night about two weeks ago where it got down to about 5 degrees F. I started my truck up and turned the dial all the way to the right so it was all heat and the dial to the far right to defrost and lower vent. I let the truck warm up for about ten minutes and when I got inside it was still freezing, also it was only comming out of the dash and not the floor vents. I adjusted the dial so it was for the vents in your face and the floor vents and nothing happened, it stayed in defrost and it was still very cool temperature wise.

I said screw it and took off! That was the only time that the selector did NOT work properly, to this day the selectors work, BUT...... The heat just isn't as strong as far as temperature goes. It seems like it's blowing in the low 60's at the most. Before it would roast you right out of the cab, and now it seems I have to drive an hour just to get it so I can take my jacket off.

So my question is.... Could it be the Climate Control Module (Where you select the fan speed, temperature and which vents) or does it sound like it is more likely a thermostat or a heater core issue??? Like I said earlier the other problem does not exist anymore it's just the fact that it doesn't get as hot as it used to.

Any thoughts or suggestions would be appreciated!

<{POST_SNAPBACK}>

The two combined issues would seem to point more towards the controller.

First does the engine come up to normal operating temperature?

If so I doubt it is the thermostat.

Second, once the engine is warm are both the hoses coming out of the heater core warm/hot? If only one is the heater core is completely blocked, if both are warm hot it could be partially blocked.

Third can you hear the air mix door moving in the dash when you cycle the temperature control? It might be pretty quiet so just be cause you can't hear it doesn't mean it isn't moving. The motor that controls it migh have become disconnected from the door istself as well.

I would first try flushing the heater core, then replaceing the thermostat. Simply because those are the two cheapest possibilities. Less than 10$ The controller is over a 100$

You could also take it to a dealer and they can check all the electronics too. Making sure the controller is "adjusting the temperature as indicated on the switch, and should be able to veridfy air mix door position, according to switch...

I think the three likely culprits are the clogged heater core, or controller.

JCedar · December 12, 2006

Thanks man, I appreciate the quick reply!

I'll have to look into that. I won't be able to check it over until I get home this weekend.

The truck does come up to temp and I just had my coolant flushed around 5,000 miles ago. But I'm really curious to check the inlet and outlet hoses on the heater core to see how hot they are... Say that one is hot and the other is cool (So it's completely blocked), does any lube place do this and you say it's cheap the get the heater core flushed??

Just out of curiousity, how do I know if the heater core is crap?

pm26 · December 13, 2006

Also, check the heater control valve first. If the engine comes up to normal operating temperature, and the heater valve does not work, then the hot coolant will not flow into the heater core. So no or weak heat. If the heater valve is good, and heater core is good, then the temperature controller is your problem.

If you had a thermostat sticking open due to some debgris in the cooling system, then the temperature gauge would indicate low temperature at all times and heater would not put out sufficient heat.

asilverblazer · December 13, 2006

does any lube place do this and you say it's cheap the get the heater core flushed??

<{POST_SNAPBACK}>

It's not the thermostat so...

To flush the core, pull the two hoses going though the firewall to the core loose, so that two tubes stick out of the fire wall. Use a water hose and run as much water as you so desire in each tube going in to the fire wall. That will flush any debris out of the core.

The two signs of heater core failure is coolant smell in the cab and or foggy windows. Means a leaking heater core. Heat on only one side, weak heat, no heat. Can mean a clogged up heater core.