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Unknown cause of oil odor in my '97 Nissan Sentra

Last fall I had several oil leaks on my car repaired as well as replacing the fuel pump and alternator. I also had some welding done on the tailpipe. Once the weather turned colder and I started using the heater, I noticed what I think is a oil-type odor inside the car after a few minutes or more of driving. I roll the windows down to clear out the odor and after that there is no more odor, until one of the next times I drive my car again. It doesn’t smell like exhaust fumes. I have had the exhaust system checked out by a garage who specializes in exhaust systems and they couldn’t find any problems with the exhaust system. This problem doesn’t happen every time I drive, but almost every time. Does anyone have any suggestions as to what might be going on? I haven’t noticed any oil drippings on the floor of my garage.

I you had a relatively minor seep from anything above the exhaust manifold (e.g. valve cover) that might explain it. While the car sits enough oil seeps, the exhaust manifold burns it off, the fresh air intake pulls it in, but the seep is not enough to keep soaking the manifold. The initial “coating” burns off…until it has enough time to seep and build up a little more again.

+1 to cig’s post.

Perhaps the easiest way to find low level oil seepage is by using an additive (available at any parts store) that’s sensitive to UV light. Follow the directions to add it to the oil and run it, then explore with a blacklight. The seepage will glow.

Also know that the overwhelming majority of oil leaks on older cars are totally harmless as long as the oil level is kept above the fill line. Oil leaks do not kill engines; oil depletion does. Often, too, oil leaks on old engines won’t leave drops on the garage because they’ll only weep when the engine is running and blowby is pressurizing the crankcase and pushing the oil past tired crank seals, valvecover gaskets, or whatever. When you shut the engine off, the pressure dissipates and the seepage stops.