What Made the Mustang Burn Oil?

My V-6 powered 2000 Mustang produced a cloud of oil smoke when it was started, and about every 200 miles, the check engine light flashed, indicating that the engine was misfiring. Investigation with a scan tool always told me that the number six cylinder was misfiring. Sometimes, there would also be codes indicating that one or both banks of cylinders were burning too lean. This made no sense at first, since a misfire would cause oxygen sensors to detect a rich condition due to unburned fuel in the exhaust.

When the engine start misfiring, I replaced the number six spark plug, which was always oil fouled. It was obvious that oil was getting into the cylinder, but how was it getting there? And why was the engine running too lean?

I considered the possible causes of an internal oil leak: a problem with the PCV (Positive Crankcase Ventilation) system, worn piston rings, a worn piston or cylinder, and a leaking valve-stem seal. I ruled out the PCV system initially because it seemed that a PCV problem would affect all cylinders, not just one. The other internal engine issues were unlikely because I had recently rebuilt the engine.

An Internet search for diagnostic information found nothing that fit the symptoms. I consulted with several mechanics, but they seemed more interested in taking my money than providing useful diagnostic information. One mechanic told me that I needed a new engine; another told me that I obviously had installed the piston rings incorrectly; and a third mechanic wanted to run an emissions test, which would not have identified the cause of the oil leak.

Thinking about the problem, I reasoned that an air leak at the joint between the lower intake manifold and the cylinder head could cause the lean condition, and might allow oil to be sucked from the valley area.

I opened the hood, and started removing the upper intake manifold (plenum) to gain access to the lower intake manifold so that I could tighten its bolts, hoping that this would stop any air leak. When I removed the PCV tube from the left side of the upper intake manifold, I found that the inside of the PCV tube and part of the inside of the manifold were wet with engine oil!

Early automobile engines had simple crankcase vents, and later "draft" tubes that extended below the engine to allow crankcase fumes to escape beneath the car. Some racing vehicles still use draft tubes, but they are no longer used on production cars. For decades, the modern PCV system has removed fumes from the crankcase, which helps prolong the life of engine oil and the engine itself. The modern PCV system also routes the crankcase fumes to the engine intake stream to burn them. Crankcase fumes result from combustion gases leaking past pistons and piston rings.