How to Assess Strange Automobile Odors

By Deanna Sclar from Auto Repair For Dummies, 2nd Edition

Car smells mean trouble, but you can use them to diagnose problems. The only odors you should smell inside your vehicle should come from smelly things that you’ve put in it. If you smell any of the items in the following list, take immediate action to correct it:

You smell rubber burning under the hood: One of your hoses may have come loose and landed on a hot part of the engine. Rescue it before it melts through.
You smell something burning with the hood closed: Feel your wheels. If one is hot, a brake shoe or pad may be dragging, or you may have left the parking brake on. If neither of these checks out, an overheated clutch of a manual transmission car may be the cause.
You smell oil burning (a thick, acrid odor): First, check the oil dipstick. You may be running out of oil or your engine may be overheating, and your temperature gauge may be broken. If neither is the case, look around the engine for oil leaking onto the engine block or exhaust manifold. If the oil situation seems to be okay, check the transmission fluid dipstick. Sometimes a faulty vacuum modulator can siphon the fluid out of the transmission and feed it to the engine, where it’s burned. Also, if the transmission fluid is very low, it can be burned in the transmission because the gears aren’t lubricated enough and are getting very hot.
You smell oil or exhaust fumes in the passenger compartment: The cause could be burned oil from the engine area, but it also could be a faulty exhaust pipe under the car that lets exhaust gases into the vehicle through the floorboards.

Exhaust fumes contain carbon monoxide, so if you smell oil or exhaust inside the car, be sure to keep your windows open at all times and have the problem checked out as quickly as you can.

You smell something sweet and steamy: Check the temperature gauge or light to see whether your engine is overheating.
You smell rotten eggs: The smell is probably coming from the catalytic converter, which is part of the exhaust system. The converter may be malfunctioning, or you may have a problem with your engine.
You smell burned toast (a light, sharp odor): It may be an electrical short circuit, or the insulation on a wire may be burning. Check around under the hood. Driving is a bit risky, so get to the nearest service station or have roadside service come to you.
You smell gasoline: If you just had trouble starting the car, the engine may be flooded. Wait a few minutes and try again. If the smell comes from under the hood, check your fuel injection system or carburetor to make sure that it isn’t leaking fuel. Also check your fuel pump (if it isn’t hidden inside your fuel tank). Leaking gasoline will wash a clean streak across it, which can be seen with the naked eye. Then check all visible fuel lines and hoses that lead to the fuel tank. If they’ve rotted or are disconnected, you’ll smell fuel vapors without seeing any leaks. Taking a look under the vehicle after it has been parked overnight may help, but remember that fuel evaporates quickly, so the clues may be stains rather than wet spots.

Don’t smoke while you check for a fuel leak! Gasoline ignites easily, and gasoline vapors can explode. If you smell gasoline — and you didn’t just fill your tank — find the source of the leak and have it repaired immediately.