ORIGINAL: jeffie8696
The problem with plumbing it back into the intake is that the gasses that pass the piston ring are most oftem burnt and no longer capable of supporting combustion. If a pocket of these gasses surround the glow plug element it could cause a misfire. This is a common phenomenon with EGR equipped gasoline engines.
Is is less prevalent at high speed where the occasional misfire would not be noticed but at idle it may cause problems.
Regardless of that I see it being done on new production engines to promote a cleaner airframe. I would not choose to do it myself.
Not correct as a EGR function (Exhaust into the intake manifold to reduce combustion temperature hence nitrogen oxide (NOx) emissions reduction). The returned unburned fuel gas/oil fog who is entered into the intake manifold is socalled as PVC (Positive crankcase ventilation).