Diesel engines are known for their reliability and efficiency. However, after years of use, these workhorse engines can develop issues like blow by, where gases escape past piston rings into the crankcase.
Blow by may seem innocuous at first. But over time, it can reduce engine performance and damage components. So what causes this pesky problem in the first place? As a professional diesel engine parts supplier, I will break it down step-by-step.
What Causes Blow by on a Diesel Engine?
Worn Piston Rings
A diesel engine’s piston rings create a tight seal between the combustion chamber and crankcase. This seal ensures that hot gases expand properly to drive the piston down during combustion.
Over hundreds of thousands of miles, the constant scraping motion wears the rings thin. Gaps form, allowing gases to leak through into the crankcase during the combustion and exhaust strokes.
This wears out oil faster as contaminants get churned in. Plus, engine compression drops every time gases sneak past the worn piston rings through these gaps.
Fix: Catch worn rings early and replace them to prevent blow by issues. Using high quality oil also reduces wear.
Glazed or Scratched Cylinder Walls
The cylinder wall houses the piston rings and bears the brunt of their scraping force. So it’s no surprise that – just like the rings – the cylinder wall surface also wears over time.
Tiny scratches form on the surface, which glaze over from friction and heat. This glazed texture erodes the ring’s ability to seal properly during combustion events. Just 0.001 inches of cylinder glazing can raise oil consumption by 50%!
Fix: A machine shop can re-hone cylinder walls to restore a smooth surface for better ring sealing.
Stuck Piston Rings
Carbon deposits are par for the course with diesel engines. But when carbon glues piston rings stuck in their grooves, blow by can result.
The stuck ring moves up and down with the piston. But combustion gases flow freely around the stationary ring instead of driving it downward.
This carbon buildup also inhibits heat transfer from the piston to the cylinder wall. As heat bakes the stuck ring over time, it loses elasticity and sealing performance.
Fix: Regular maintenance cleaning helps prevent carbon buildup on rings. Using quality oils that resist thickening can also free stuck rings.
Clogged Turbocharger
Turbochargers help pack more air into the cylinder during intake. But when stuck in the open position due to carbon deposits, they can lead to blow by.
Excessive turbo boost rams combustion pressures past what the rings can handle during the power stroke. This overwhelms the sealing capacity of even healthy piston rings.
Fix: Regular turbo maintenance cleaning prevents clogged vanes. Tuning down the turbo boost pressure can also prevent forces from exceeding the piston ring’s sealing capacity.
Weak Valve Springs
While piston rings form the primary seal, intake and exhaust valves also play a role. Weak or broken valve springs prevent complete valve closure. This allows hot gases to jet directly into the intake or exhaust system instead.
Leaking exhaust gases eventually pressurize the crankcase and force oil mist out. Weak valve springs also hamper proper intake air flow for good combustion.
Fix: Inspecting valve springs and replacing broken ones reduces leaks through the cylinder head. Adjusting valve gaps to spec also optimizes closure.
Crankcase Ventilation Faults
Blow by gases have to leave the crankcase somehow. So diesel engines have a crankcase ventilation path to recirculate these gases back into the intake.
When crankcase tubing leaks or ruptures, gases escape directly into the atmosphere instead. Blocked ventilation tubes also raise crankcase pressure.
Both scenarios reduce the crankcase’s ability to handle increasing blow by. Higher pressures then force gases past seals and gaskets.
Fix: Inspect the ventilation system for leaks or blockage. Replace worn tubes and clean out obstructions that hinder flow.
Take Action to Prevent Blow By
While some blow by is inevitable, staying on top of engine health keeps it in check. Stick to manufacturer service intervals for parts like piston rings, valves and cylinder surfaces.
High quality oils and filters also give a margin against wear. And promptly investigating any new rough running, smoking or performance loss nips problems in the bud.
With proactive maintenance and parts replacement, a diesel engine can log over 400,000 miles before blow by even becomes an issue!
So be vigilant, and your diesel will keep on motoring for years to come. Monitor those engines, and happy driving!