I know that generally when your heater core gives way - you start to get a combination of symptoms including a sweet smell inside the cabin, windows fogging up, no heat blowing out, etc.
Recently, I started to noticed almost like a wet burning smell occasionally while driving with the heat on. I figured it was just some dust or something from lack of use for almost a year (the heating that is, not the car).
Today leaving work, I noticed the interior was just slightly fogged up - but while driving home on the highway my car started to smoke up (the heat was only slightly on - the fan speed at 1, and the temperature knob in the 12 o'clock position. I pulled over, popped the hood and couldn't find anything. Went back to the car and it smelled like burnt something inside. The smoke seemed to be coming from inside.

I pulled off the highway, let the car sit for 20 minutes and then attempted to continue home with the heat off. I had no issues besides freezing my butt off. I believe it is a bad heater core, but before going about tearing apart my car to get to it I was hoping for some opinions. I never got, and still haven't got a sweet smell, and when it is on, I do get hot air.

Any thoughts?

Ok, so I think this problem is a little more complicated. Was going to change the heater core this weekend, but I wanted to troubleshoot a little more before throwing some money at it. I drove it around for well over an hour on the highway and city with the heat and blower at max. No burning smell, no smoke no problems nice hot air.

HOWEVER! while playing around with a few things I noticed that the blower doesn't turn off when the car is turned off. I know this is a well documented problem but mine is a little different. Let me explain a little better.

The car in question is a 2004 Sunfire 2.2 ecotec. It has 4 speeds on the blower and all 4 speed work, so I have low speeds and high speeds. It WILL turn off it I turn the knob to off - but if I leave the blower on and turn the car off the blower continues. I have not noticed this before and I know it didn't do it last winter (I have a car starter and I would have noticed it then). I thought maybe it would turn off after the 5 minutes that the accessory power stays on, like the radio. The radio turned off and the blower continued. I could still switch the speeds.

I decided with the car off, and the blower going, to check the relay under the hood. Obviously when I pull the relay it stops. I put the cooling fan relay in its place to test, and the blower started again, so the relay isn't the problem, and it doesn't seem like a blower resistor from the fact that I still have all speeds.

Where do I check next?

Hi Jeff,

The heater blower is controlled in the following way:
The blower motor gets +12V through that underhood Heater Blower relay.
One end of the relay coil goes to the ground, it is terminal A2. The other end of the the relay coil goes to "Hot In Run", through the HVAC fuse in the fuse block inside. That would be the switched +12V, it should be on when the ignition is on. It is terminal B4 on the relay. Check in the relay socket if that power is indeed switched with the ignition.
One of the relay contact gets the +12V from the "Hot At All Times", through the blower fuse underhood. That contact name on the relay is A4. The other contact of the relay switch, that gives the +12V to the blower motor, it is B2 on the relay.
With the relay out, do check if the contacts are fused, if B2-A4 shows a short.
The blower switch gives the ground to the motor, through resistors in the lower speeds.
My bet would be a bad relay.
Hope I did not mangle it too much!
Note that this is from a '05 factory service manual.

The heater core leakage would be independent from this.

Good luck, Peter

I don't think the heater core is leaking at all tbh. I'm just unsure as to what all the smoke was that one time IN the car. I'm thinking something shorted. I don' t think it is a bad relay - because I swapped it out, but it may be a wire that shorted. I'll have a look at it this weekend. Thanks for the info, it helps a lot!

Ok so long delay I know. I've been really busy as of late. This weekend I had a chance to trouble shoot and I believe all the problem is coming from the heat control unit (is that the right name? the thing on the dash you control the heat from). I pulled most of it out and the whole thing in the back is black, smouldered, and melted. I also noticed that one of the wires was marrette-ed together. Its a nightmare back there, don't know who had the car before me. So I'm going to start by fixing the wiring and changing the heater control unit.

My question here is I have a 04 Sunfire with rear defog but no ac. What year/model units can I use. I have found a few from different years and they seem to be some differences. Also, assuming I find one that is of the same model/year as mine but has the AC button, can I use it even though I don't have AC? Will the wire harnesses in the back match up?

You need the entire dash harness unless your splicing it in. Usually the reason for the hvac wires melting is because of a short at the 12v accessory port/ cigarette lighter port. This usually starts melting all the dash wires from the cluster to the hvac controls. I have seen this happen a few times and its a pain to fix because the wires melt fast to the plastic dash. Its not hard to replace the entire dash harness as it can be done in a few hours.




On the inside my car looks like a fighter jet.

I will splice it in. It seems as if there is a short with the accessory, but im hoping it is actually at the HVAC controls. If I leave my fan speed on anything other than off, when I turn the car off it continues to run. But the fan is the only thing, radio turns off like usual, remote starter performs as usual etc. So, I will cut the old hvac out, as i can't remove it because it is melted, and before installing the new one, once all the wires are cleaned, I will test the accessory wire there. I'd rather not change the whole harness - this is a winter beater. But with quebec winters I really need the heat.

Sorry for double post. So back to my original question, would a heater control with the AC button, function in a sunfire without the actual AC? I'm assuming when I splice it in I'll just have an extra wire or 2 that I can leave unconnected.