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1998 Toyota 4Runner - intermittent burn smell

I have a 1998 4Runner and sometimes I smell something burning and sometimes I dont. The smell is like exhaust but when I take it to a mechanic, the smell does not appear. Sometimes I get the small when I first start the vehicle and sometimes I get the smell in the afternoon or thru out the day.

First guess-oil leaking onto an exhaust manifold. Doesn’t take much.

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Agree about an oil leak being most probable. To be on the safe side check the whole length of the exhaust system for leaky gaskets, broken studs, holes, etc. To be on the extra safe side ride with a CO2 detector for a while, look for one that displays concentration.

Oil leaking on a hot exhaust manifold or exhaust pipe is a common reason for this symptom. The oil evaporates and forms a vapor, which you notice as an odor. But if you notice the odor immediately after starting a cold engine, that’s not a likely cause b/c the exhaust system isn’t heated up yet. Oil dripping on a cold exhaust pipe won’t create much odor. So if that’s the case — you notice the odor immediately after starting a cold engine – then my guess is an exhaust leak. Something like that could go away as the parts warm up, b/c metal expands with temperature and the expanding metal could seal the leak. And exhaust leaks tend to be more noticeable on starting a cold engine b/c the fuel mixture is much richer then.

One common way shops will diagnose a non-obvious exhaust leak is a helper tech will partially obstruct the tailpipe with a rag or piece of wood with the engine idling, while the other tech will go underneath & check for pulses of exhaust gas coming from a leak along the entire length of the tailpipe.