Blowby on a diesel engine is a common issue that can negatively impact performance if left unchecked. In this complete guide, as a professional diesel engine parts supplier, I’ll cover everything you need to know: what blowby is, what causes it, how to detect it, prevention tips, and potential fixes.
What Exactly is Diesel Engine Blowby?
Blowby occurs when combustion gases leak past piston rings into the crankcase instead of exiting through the exhaust. Some blowby is normal, but too much indicates an underlying issue.
On diesel engines specifically, blowby happens when pressure inside the combustion chamber – which is higher than gas engines due to the higher compression ratio – exceeds what the piston rings can contain. Then the air-fuel mixture and gases blow past the compromised seal into the crankcase.
What is blowby on a diesel engine? It’s the leakage of gases from the combustion chamber into the crankcase due to worn piston rings, pistons, cylinders, carbon buildup, manufacturing defects, or extreme engine settings.
What Causes Excessive Diesel Blowby?
Several issues can lead to excessive diesel blowby:
Worn Piston Rings
The continual motion of the rings against cylinder walls wears them down over time. As the seal between the ring and wall fails, blowby increases. Fuel residue and carbon buildup accelerates ring wear.
Damaged Pistons and Cylinders
Hot combustion temperatures expand pistons, while long-term scraping wears down cylinder walls. This combination widens the gap between components, allowing gases through.
Carbon Buildup
Carbon stuck in piston ring grooves prevents them from sealing properly against cylinder walls. Fuel injector problems also leave unburned fuel that turns into carbon.
Manufacturing Defects
Improper piston-to-cylinder clearances from factory defects can lead to blowby. Quality control issues may also result in poorly fitted components.
Extreme Engine Settings
High-performance engines with multiple turbos, high boost pressure, and extreme heat and vibration are more prone to blowby. The additional force exerted pushes past worn seals.
How Do You Know if Your Diesel Engine Has Blowby?
Watch for these common diesel blowby symptoms:
- White smoke from oil fill tube or valve cover
- Loss of power during acceleration
- Increased oil consumption
- Rough idle, misfiring
- Rattling noise from crankcase
- Fouled spark plugs
- Higher fuel consumption
- Oil leakage around cylinder head gasket
You can also conduct tests to check for excess blowby:
Upside Down Oil Cap
Run engine and place oil cap over fill tube upside down. If it blows off, you have high crankcase pressure.
Compression Leak Down Test
Measures % of pressure lost past rings with a special gauge. 15% loss is acceptable; over 20% indicates issue.
Crankcase Pressure Test
Measures actual pressure in crankcase. Over 3 psi at idle indicates extreme blowby.
Catching symptoms early prevents further engine damage down the road.
How to Prevent Diesel Blowby
While some blowby is inevitable, you can take preventative maintenance steps to minimize it:
Use High-Quality Fuels and Oils
Premium diesel fuel with lubricity agents reduces carbon buildup and cylinder/ring wear. And high-end oils like Valvoline Premium Blue Extreme offer advanced anti-wear additives.
Regular Maintenance
Follow factory recommendations for oil and air filter changes. Clean intake valves, turbo ducting, EGR and crankcase ventilation components. Replace worn parts like piston rings and valve seals early.
Drive Gently
Aggressive driving with high RPMs, temperatures and loads accelerates component wear. Gentle acceleration and avoiding high speeds/loads makes blowby less likely.
Upgrade Parts Proactively
Install reinforced piston rings, multi-layer steel head gaskets, improved cylinder wall coatings and bearing shells rated for high pressures.
How to Fix Diesel Blowby
If you already have excessive diesel blowby, repairs may be needed. A mechanic will:
- Run diagnostics to pinpoint extent of issue
- Inspect pistons, rings, cylinders for damage
- Replace any damaged gaskets or seals
- Clean crankcase, oil passages, valve covers
- Install updated piston rings or low-friction bands
- Hone or resurface damaged cylinder walls
- Adjust valve lash if needed
Parts upgrades during repair include:
Piston Rings
Gapless or Napier rings prevent gas leakage through the ring end gap. Moly or DYNATHERM rings reduce friction and withstand higher temps.
Pistons
Coated and reinforced pistons resist damage from blowby contamination and heat.
Cylinder Walls
Plasma transfer wire arc spray coats cylinder walls with a low-friction, wear resistant alloy layer.
Diesel Blowby – The Bottom Line
Blowby is an inevitable diesel engine issue, but attentive maintenance and operation goes a long way in minimizing it. Watch for telltale symptoms, test regularly, and repair any underlying issues early. With some TLC, your diesel can give you hundreds of thousands of miles of reliable service.
For more automotive and diesel engine care tips, or if you have any engine repair questions, post them in the comments below!