For the last few days my car has reeked of rotten meat. I am sure I haven’t spilled anything. I am not totally sure, but I think it is worse when I have the heater fan on. I thought maybe I was leaking engine coolant, and my mechanic said he thought that if that was dripping onto something and burning, it might give off a rotten meat smell. But there are apparently no leaks. It has me a bit spooked. Am I breathing in some kind of death-fumes?
A dead animal in your engine would cause that smell. 
Dont want to spook you unnecessarily but recently on our local TV there was a bit about a car that had a funny smell about it. Somehow the pest control people were sent for and they found
A dead RAT -electrocuted on the HT wires.You could see the way its evil teeth still hung on even in death
Might be your heater core. Antifreeze kind has a sickly sweet smell, but I don’t know if I would call it rotton meat.
Are your windows getting foggy when it’s cold out? If your heater core has a leak, your defroster will likely make your windows WORSE.
Are you sure it’s not a rotting egg smell? Sulfer smells from a cold catalyictic converter is pretty common.
Bad gas (high in sulfer) can be the problem.
What kind of car do you have?
My first thought was a cat in the fanbelt.
Cats are notorious for crawling up into the warm parts of the engine to sleep, especially in cold weather like this. If I was smelling meat, the first thing I would look for would be… meat.
Mad cow?
If you got stuck in someone’s engine compartment you’d be mad too… 
It’s definitely a rotten meat smell. The windows actually are getting foggy and the defroster doesn’t seem to work. In fact, it might actually be making them a bit foggier. Hmmm. Why would that cause a rotten meat smell?
I would guess based on your last post that a rat got into your heater box (Not uncommon) and chewed into either the heater core, or a heater hose. The antifreeze poisoned said rat who then died in place.
Leak = steamy windows, rotten smell = dead animal.
::: Rick looks up and thanks OG that he isn’t fixing cars in Toronto:::
Squirrels, rats, cats… all like warm engine blocks and wires.
I don’t think it smells like it, but I have met people who claimed ethylene glycol (antifreeze) smelled like rotting fruit. If you also are getting a moisture problem in the car and the defroster doesn’t work, I wonder if there might not be a leaking heater hose…?
Of course, animals do die in cars, and around them. The worst I ever saw was a skunk caught in the front spring of a car. The customer kept raising and raising the amount they would pay for removal until someone volunteered to get it out. At $100 I volunteered, but then they backed out and did it themselves anyways. It probably was for the best.
With a leak, you could be getting a fungus buildup in your heater system. Some of those smell like bad meat when you cook them.
My first thought is that you have a leaky heater core, and mold is growing inside the vent system. Ethylene glycol has kind of a sweet smell, and if it’s blowing through your vents will leave a film on the windows, in addition to making them fog up worse when the defroster is on. Mold smells, well, like mold, musty and disgusting (probably a bit like rotten meat, maybe).
Have you checked your anti-freeze level lately? Check to make sure the radiator is mostly full, while the engine is cold so you don’t burn yourself. Sometimes the overflow tank won’t get sucked back into the system, so reading full there isn’t necessarily accurate.
Yeah, I’ve been checking the anti-freeze level. The smell is, to me anyway, a true rotten-meat smell. Not just close. Another weird thing is that it started more than 10 days ago. I figured if it had been real dead meat, it would have stopped smelling by now. Its intensity has somewhat abated overall, it still comes back strong sometimes. Well, it is in the garage now. Maybe they will find something. (fingers crossed)
Please follow up and let us know, I’m really curious.
I had a rotten meat smell in my car. It wasn’t horrendous, so after fruitless search I was guessing it couldn’t be an animal. Even a mouse would have been more pungent than that.
Finally discovered the cause. A squirrel had been trying to stash snacks under my hood, up near the windshield in the crevass between the hood and the wipers. A couple of chestnuts and a well-gnawed chicken wing dug out of the trash. It was that little bit of chicken that was causing just enough of an odor for me to be disgusted.
Iv’e had mice get into the heater box. Try taking the fan out and looking down in there.
I just got word from the garage. The heater was leaking coolant and it got under the carpet on the passenger side. I don’t know if that was the smell: I thought ethylene glycol was supposed to be odorless.
But now I am a bit concerned that I have been breathing these fumes for a week and a half. I checked out some sites that said that ethylene glycol is not likely to exist in large amounts in air, and that it is really only very dangerous if large amounts are swallowed. But it was in an enclosed space, and I am the paranoid type when it comes to such things. I guess I am not exhibiting signs of shaking…
My heater core blew out last winter (more correctly one hose connecting to it busted) and antifreeze spilled all over my floor - lucky I didn’t have carpet. My problem was like yours only it all happened at once. My cab was flooded with smokey mist within 2 minutes, and I had to roll the window down to see… looked just like a Cheech & Chong car moment.
There is still over a year later the occasional spray from the deforst air blowers that coats the inside of my windshield, but I have never felt sick. The fumes won’t hurt you :).
Don’t know how to explain the rotting meat smell. To me antifreeze smells kinda fruity-nice. Real rotten meat smell will make you sick and feel like throwing up, then again my observations come from hovering over a big plastic tote filled with ~ 1 metric ton of rotting salmon, so YMMV.