Sporadic burning-plastic smell in 1999 Chevy Prizm (vehicles, auto, gasket, idle)

[HonuMan]

This is a problem that has stumped two auto mechanics, and that I've just learned to live with, but I figured I'd post it here and see what happens... In March of 2007, I was driving my 1999 Chevy Prizm (a Toyota Corolla manufactured in the US) on a hot day through slow rush-hour traffic due to a bad auto accident. All of a sudden, I started smelling burning plastic, and assumed that something in the engine was about to fail and leave me stranded in said rush-hour traffic. But the engine didn't overheat, and I made it home fine.

My next thought was that a plastic bag had burned onto the bottom of the chassis, something that had happened to me in another car years before. I couldn't find anything. The next day, the smell briefly reappeared, as it did on subsequent days. Usually it seemed to happen after the car was warmed up, and after I pulled away from a stop light -- a whiff of it would drift through the cabin. When I drove at a steady pace or was idling, I didn't smell it. It was extremely sporadic. I took the car to my mechanic, who couldn't duplicate the problem or find anything wrong.

When the cool fall weather came, the problem suddenly stopped. Then it started back up in the summer of 2008, and has been with me ever since. I had another mechanic check it out, and he, too, couldn't duplicate the problem or find anything wrong. No fluids were leaking, and no engine parts were in need of replacement.

A few times when I've gotten a big blast of the smell, I've pulled over and checked out my car. I've lifted the hood, and haven't smelled it there. There is no smell at the brakes (and I know burning-brake smell, which is different), and nothing coming from the under-carriage. It's only in the cabin. It's different from exhaust smell.

The most plausible theory has come from a coworker, who thinks that I have an intermittent short in the wiring that's burning a tiny amount of insulation each time it happens. I blew two rear brake-light bulbs within a month of each other, which lends creedence to this theory. But I'd think that, after all this time, either there wouldn't be any more wire insulation to burn, or something would have failed electrically. Any ideas, including how I'd go about troubleshooting the problem?

[M3 Mitch]

Since the smell seems to come with conditions that would run your electric radiator fan, take a good hard look at the wiring for it. If this car has a 2-speed fan, take a hard look at the low-speed resistor.

You could try letting the car idle with the hood up, wait till the fan comes on, see what you can see/smell.

[HonuMan]

Originally Posted by M3 Mitch

Since the smell seems to come with conditions that would run your electric radiator fan, take a good hard look at the wiring for it. If this car has a 2-speed fan, take a hard look at the low-speed resistor.

You could try letting the car idle with the hood up, wait till the fan comes on, see what you can see/smell.

Thanks, Mitch! I'll check it out.

[usafracer]

The resistor for the blower motor for your heat and a/c is getting hot. More than likely it is melting the plastic connector.

[Tennesseestorm]

Could be, but electrical issues are rare on Toyota made vehicles.

I agree it could be something with the fan though.

My Lexus occasionally had a funny smell that came from under the hood that smelled like diesel fumes.. strange but it turned out being a leaky valve cover gasket and was only occasionally dripping on the manifold.

[Stevendrake456]

I have encountered this same exact problem over the last week, 2001 Chevy prism, i found that my resister for the a/c blower motor is pretty fried and the foam that coats the air intake togle is pretty gunked up, i vacuumed the piece of foam and pealed off most of the burnt shelack/plastic from the resister and the problem seems to have subsided. I smell it faintly here and there but it's barely noticeable anymore.