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Hello,
I'm a newbie here, a little old lady who is a longtime owner of a 2003 RSX. It has about 195,000 miles and has been mechanically well-maintained over the years.

Just before Thanksgiving, the heater began blowing cold air. I took it to the trusted mechanic who has been taking care of the car for the past 5 years or so. He first said it was clogged, but he couldn't unclog it, and said the heater core would need to be replaced. Okay. Then he said the temperature switch on the dash was "clicking." I drive this car every day and never heard a click, but okay.

Long story short, they pulled the dash to find the "click" and also replaced the heater core. Still no heat. Then he told me the heater air mix lever assembly was worn out (I think that's what it's called...I am literally an old lady with no real mechanical skill). I asked if that was the source of the "click," and he said, yes. I asked why he didn't fix it when he was pulling the dash and the heater core, and he backtracked, saying it didn't look worn then. So, it wore out to the point of failure within a day???

Anyway, it was going to take a while for parts to arrive, so he "fixed" the air mix lever so that the heat from the replaced heater core would stay on continuously and I could drive in the cold. But, after he did that, there was a strong smell of exhaust coming out of the vents. I was annoyed that they hadn't fixed the air mix lever issue when the heater core was pulled, so they went to great lengths to try to fix the smell. It's not as bad as it was initially, but it's still there to the extent that my clothes and hair smell faintly like exhaust when I get out of the car. I'm having to drive with the sunroof open and windows cracked so that I don't get asphyxiated.

Anyone have any ideas as to what might be causing the smell of exhaust to blow through the vents when the heater is on? Could the heater core replacement be defective? This problem didn't exist before the heater core was replaced.

TLDR: Strong smell of exhaust from vents when heat is on after heater core replacement. Why?
 

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Sometimes you notice things that have always been there because somebody worked on your car. Not saying its the case here, but it does happen.

There are two hoses for the heater core that pass through the firewall. If the gromets weren't sealed or placed correctly they could let air from the engine compartment into the cabin. But, there sholdn't be any exhaust fumes under the hood anyway unless you had an exhaust leak, and replacing the heater core deosn't involve the exhaust. Is your engine oily from a leaking valve cover and thats what you smell?

Maybe the new heater core has some production oils on it or other contaminates that are gassing off when it is hot.
 

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02 Rsx Base converted to manual k24a2
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I am having this issue as well after replacing my heater core. The smell is more of a burnt rubber smell though, not exhaust-like. However, it happens after about 10 minutes of driving with the heat on. I have to pull the dash apart again in order to inspect it. I'm not sure when I'd be able to get around to it but I will definitely report back.

I thought maybe it was the evaporator, which sits right next to the heater core. So my plan was to change that out, especially since it's 20 years old now.

Is the heater core your mechanic put in OEM from Acura? I bought mine on eBay and it was a tight fit. I'm thinking that the core's rubber edges are touching metal perhaps, to the point that it burns and gives off a smell. This did not occur until after changing the heater core.

I would love to hear feedback from others who have replaced their heater core because this should NOT be happening.

I sort of feel bad for your mechanic but at the same time, there is not much information about heater core related issues. It seems like people have yet to start replacing them. Maybe more issues like this will pop up as time goes on.

Either way, please keep us posted and I feel your struggle. Staying warm is one thing, but staying warm while smelling something that makes you nausious is not good.
 

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Ok, so I did a little inspecting. I don't think I'll have to redu my heater core afterall. I found the culprit to my exhaust rubber smell. It was definitely my exhaust manifold gasket, it's leaking because I bought a cheap one. I'll be going oem.

As for your issue, it does make me wonder, has anyone inspected the exhaust manifold gasket? If it's not original, like mine, it's probably one of those cheap ones that leaks. Or the original one could be rusted up a bit. Is your car loud or a bit louder by any chance?

This is what I did, I drove around the block to get my engine warm, gave it some revs. With mine, you can clearly hear the leak. Sometimes it's not as loud. Then after doing that, pull up to your driveway, pop the hood and take strategic sniffs of areas behind your exhaust manifold and also inspect for anything touching it, like a rubber hose or line. Anything that burns off that exhaust manifold will leak into your cabin when driving, especially when coming to a stop.

I hope this helps.
 
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