Q.  What is “Blow-by”?

A.  Blow-by is the compression gas that goes past the piston rings and into the crankcase.

Q.  What causes Blow-by?

A.  Blow-by is caused by the tiny gap between the piston rings and the cylinder walls.  There must be some gap, or the pistons would not move up and down.  The trick is to have the smallest gap possible and the pistons still move freely.

Q.  What happens if the gap between the piston rings and cylinder walls is to large?

A.  To much compression gas goes into the crankcase.  Hot oil vapor will be pushed out of the crankcase ventilation system.  Emission laws require the oil vapor to be recycled back into the engine via the EGR valve and the PCV valve.  Mercedes calls the PCV valve an “Oil Separator”.  As the hot oil vapor travels through these systems, it leaves behind a hard carbon sludge on everything it touches.  Ultimately, the engine chokes on this sludge and the Check Engine Light switch on.
Crankcase oil also goes past the rings and out the exhaust.  The engine burns oil.
When the engine has excessive crankcase pressure, it also causes oil leaks.  The higher the crankcase pressure, the more the engine leaks oil.

Q.  How do you prevent the oil sludge from clogging the EGR and air intake system?

A.  Keep the Blow-by at the piston rings as low as possible.  You can start when the engine is new.  Drain out the engine oil the factory used, and refill the engine with a special Diesel Break-in oil.  This type of oil will help the piston rings form a near perfect seal with the cylinder walls.  Unfortunately, Mercedes doesn’t use Break-in oil in a new engine.  You have to do it yourself.  After 1000 miles, the oil is changed to a better oil that doesn’t breakdown on the piston rings or vaporize when it gets hot.  A better oil withstands higher temperatures and won’t let combustion gases enter the crankcase.
Special fuel system cleaners help prevent the oil sludge from accumulating in the EGR and the intake system.
Blow-by can actually be measured.  It’s called “leakage”.  If a mechanic pressurizes the cylinder with compressed air, he can measure the percentage of air that escapes past the piston rings.  As the engine wears, the leakage increases.  Preventive maintenance minimizes the Blow-by at the piston rings.  The more dedicated the preventive maintenance, the longer the engine will perform as it did when new.  Frequent oil changes with the highest quality engine oil is the first step to preventing Blow-by.

Q.  Is there any way to detour the Blow-by and prevent it from going back into the engine?

A.  Yes.  There are two ways to reroute the blow-by and prevent it from reentering the engine.  Installing a Catch Can is one way.  By splicing into the oil vapor hose going to the Turbocharger, oil vapor is sent to a auxiliary tank where it’s prevented from entering the EGR.  When the Catch Can is full of oil, it is drained off.  Oil vapor can no longer cause sludge to build up inside the EGR and air intake.
The second way of preventing the oil vapor from entering the intake system, is to route the vapor into the exhaust system.  Instead of the oil vapor going into a Catch Can, it is sent to a special port in the exhaust system.  The oil enters the exhaust after the DPF and Catalytic Converter.   These two methods are not legal.  They violate the emission control laws.

Q.  Why does a BlueTec diesel require more maintenance than a Gas engine?

A.  Both engines start with roughly the same amount of Blow-by.  But a BlueTec diesel has much higher compression than a Gas engine.  The BlueTec emission system is much more complicated than a Gas engine.  Preventive maintenance is much more critical on the BlueTec.   It is inherently harder to minimize Blow-by in a BlueTec diesel.  A BlueTec diesel runs substantially higher combustion temperatures than a Gas engine.  Extreme heat puts the engine oil under much higher thermal loads in a BlueTec diesel.  The maintenance on a Gas engine is simple by comparison.  Mercedes misleads diesel owners into thinking they can operate a BlueTec like a Gas engine.  BlueTec diesels are not meant for short trips or extremely cold climates.  BlueTec diesels were designed for long distances at Freeway speeds.

Q.  How does Blow-by effect the engine oil?

A.  Blow-by is primarily the light weight properties of the motor oil.  Over time, the remaining oil becomes thicker as the light weight additives are boiled away.  After 20000 miles, the remaining oil is a heavy thick sludge.  Obviously changing the oil before its had time to boil away most of its additives will improve the Blow-by.

Q.  What effect does fuel dilution have on Blow-by?

A.  Regenerating the DPF causes fuel dilution.  The tighter the piston rings seal against the cylinder walls, the harder it is for diesel fuel to get past the rings.  Diesel fuel is a lousy lubricant.  The more diesel fuel that’s in the oil, the more Blow-by there is.  If the regeneration of the DPF is frequently interrupted with sort trips, more diesel fuel is washed into the engine oil.  The more diesel fuel there is in the oil, the more Blow-by occurs.   The more Blow-by there is, the more oil is burnt and clogs the DPF.  The more oil that’s in the DPF, the more frequently the DPF needs to regenerate.  As you can see, it’s a vicious cycle that feeds on itself.

Q.  Will freezing temperatures increase Blow-by?

A.  Yes.  Condensation quickly accumulates in engine oil.  Especially in cold weather.  It’s a natural process.  When condensation in the motor oil freezes, the ice crystals act like sand.  Down to about 12F, a BlueTec engine will typically start without a Block Heater.   Below 12F, the AdBlue can freeze.  When the AdBlue freezes, the DEF system tells the Engine ECU that AdBlue is inop.  Below 12F, starting becomes more unpredictable.  Mercedes offers a optional Block Heater.  It splices in one of the radiator hoses.  It is very important to prevent ice crystals in the engine oil.  However, the Block Heater does not prevent the DEF system from freezing at temperatures below 12F.

Q.  What is “coking”, and how does it effect Blow-by?

A.  Coking is when oil gets so hot, the molecules break break apart and the oil reverts to tar.  When this happens at the piston rings, the ring groves become packed with tar.  The piston rings become stuck in the ring groves.   The piston rings can no longer form a tight seal against the cylinder walls.   Blow-by goes extremely high and the engine burns so much oil that it is virtually useless.