Question:  I'm a building manager for a small data center in the Phoenix area and am responsible for the maintenance of the emergency generator.  Lately, an oil/dirt residue has been building up on the inside of the radiator.  I don't see any visible oil leaks, but it has to be coming from somewhere, but where?

Answer:  This is a great question, so let's dive right in.  For oil to be present on the radiator, there has to be oil coming from somewhere.  Since you mentioned that you don't see any visible leaks, it's likely the oil is coming from the engine breather.  But you may be asking why this happens, so let me explain.

During normal operation, a small amount of combustion escapes past the piston rings and enters the crankcase.  This is referred to as "blow-by".  This "blow-by" must be vented into the atmosphere to avoid pressurizing and damaging the oil pan, gaskets and seals.  To counter this internal malady, a crankcase ventilation system must be present.  Unlike a gaseous engine which uses a PCV valve to draw fresh air in, a Diesel engine on a generator typically uses a breather tube directly off the valve cover.  This tube simply hangs down and disburses oily vapors directly under the engine.

On most generators the cooling air flows from the generator end across the engine and through the radiator.  Because of this air flow, the air picks up the oily vapor escaping from the vent tube and distributes it onto the radiator.  Once the radiator has an oily film on it, dust and dirt collects, eventually creating a partial blockage of the air flow.  This in turn will reduce the efficiency of the cooling system and cause bigger problems for you in the future.

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