To make a not that long story shorter, wifey ran over a plastic bag today (quite proud of it to, I was there, she wasn’t aiming, she really didn’t have a choice). Anyways, when we got home we noticed a burning smell. After a couple of minutes laying on my chest with a flashlight, I found it has melted onto the exhaust system. It’s not even recognizably just looking at it, the only reason I figured out what it was is becuase I scrapped some of it off with a stick and notcied that it was a dark blue (color of the bag), otherwise it’s just dark brown looking. Anyways, I told her not to worry about it, it should burn off in a couple of days. Is that right? A few minutes later, I starting wondering if it could catch fire. I don’t think it could, and even if it did, anything around it is designed to handle high heat situations and there’s not much there to burn. So am I right, just drive it around for a couple of days and let it burn off?
I had a plastic bag get stuck to my catalytic converter. After a week or so, it was completely gone.
If you’re worried, you can try and scrape as much off as you can. I seriously doubt it’s a fire hazzard.
YMMV, IANAM(echanic), etc…
Yeah, that’s what I thought too. Many months have passed and I can still smell that damn bag!
There was this old thread about the same problem. Not very helpful though 
I agree with Blue and Blown, in practice it should slowly burn/devolatize off in a few days.
How quickly it disappears on its own depends upon what type of plastic, the thickness of the melted-on blob, and just where on the exhaust system it is. It’s very unlikely to generate a flame, but I wouldn’t say it’s impossible. I wouldn’t count on there being no problem if there were a flame; that’s a different situation from the normal heat around the exhaust system.
So, it’s probably safe, but that’s not a sure bet. It may go away unaided, or it may not. If it were on my car, I’d scrape it off.
Low car (toyota matrix), no pit to work in, and it’s melted to the top of part of the exhaust system in the dead center of the car. Anytime it’s hot enough to be scrapped off there’s no way I’m reaching in there. I could just barely see it, I’m not blinding sticking my arm near it. So far it seems to be fine, I think it’ll be okay.
If one were to scrape it off, it would be done when things are cool. Rather like with a blob of dripped candle wax – you want to work with a solid chunk, not with a gooey mess.
I’m not a firefighter, but I was once told that plastic does not go on fire. It just burns as in melts, not flames. But don’t mark my words, you should ask a firefighter.
That may be true of some plastics, but I have seen a plastic trash bag burn with flames.
Consider it rustproofing!
When I was younger, a couple of friends and myself used to LOVE setting things on fire with magnifying glasses. We actually got some pretty big fires going. I still remember one time we tried a plastic bag and yes, it caught fire. The reason I remember it so well is becuase I was probably about 10 or 12 years old when it happened, and I tried to stomp it out, so now I a 12 year old pyromaniac that just got in a little over my head and I’m running around with a flaming plastic bag stuck to the bottom of my shoe. :eek:
Una:
Admit it, you posted just so you could write that sentence.
Well, I phrased it that way on purpose, but it didn’t drive my reply… 
Not to worry you, but I flaged down a pick-up truck that had some thing similar happen. Turns out it was a news paper. It caught on fire and in turn started his spare tire on fire.
Chip at it, perhaps scrape it with a wire brush. Start the car and let it idle for half an hour or so and keep a very close eye on it. Keep a fire extinguisher handy.
It does not sound that serious. But I would keep an eye on it.
Well it’s been a few days and everything seems to be okay, so I’m not too worried anymore. I think all will be fine.
woohoo post number 1700 for me