What Is Blowby on a Diesel? Causes and Fixes

Understanding Diesel Blowby: Causes & Solutions

Blowby occurs when a mixture of air and fuel or combustion gases leak past the engine’s piston rings, down the cylinder wall and into the crankcase. While some blowby is normal, too much can indicate a more serious issue that will negatively impact engine performance.

When armed with the right knowledge, it's possible to tackle blowby head-on and prevent it from happening. Stick around because we’re about to dive into diesel blowby causes, symptoms and solutions.  

What Is Blowby on a Diesel Engine? 

Like gas engines, diesel engines generate power through four strokes. This process takes place in the engine’s combustion chamber:

1. Intake stroke: Inlet valves open and air enters the chamber, forcing the piston down.
2. Compression stroke: The piston travels back up and the inlet valves close. The air compresses, increasing the temperature and pressure inside the chamber. The fuel injector sprays an atomized version of fuel into the cylinder that mixes with the pressurized air and causes spontaneous combustion.   
3. Power stroke: As the fuel burns, resulting pressure forces the piston down, transferring the energy to the crankshaft.  
4. Exhaust stroke: Inertia forces the piston back up and the exhaust valves open to expel combustion gases.  

The difference between gas and diesel engines is that diesel engines don’t need spark plugs to ignite the fuel mixture and initiate combustion because they have higher compression ratios. However, higher compression ratios make diesel engines more susceptible to blowby.

In a diesel engine, blowby occurs during the compression or power stroke, when pressure is maximized. Worn piston rings or cylinders can’t contain the combustion pressure within the chamber, allowing the air-fuel mixture and gases to sneak into the crankcase and increase pressure there.

Causes of Diesel Blowby

High-mileage vehicles that have not had routine maintenance are prone to blowby. A few common causes include:

Worn Piston Rings 

Piston rings sit in the ring grooves on the piston head. Their primary function is to create an airtight seal between the piston and cylinder wall so fuel doesn’t escape during combustion. A lack of lubrication, an overheated engine, contaminants like dirt and debris, or improper maintenance can wear down the rings and jeopardize their seal.   

Eroded Pistons or Cylinder Wall

A diesel engine's pistons and cylinders erode when not routinely maintained. This widens the gap between the rings and cylinder wall, leaving room for the air-fuel mixture or combustion gases to leak into the crankcase.    

Carbon Buildup 

A lack of maintenance also causes carbon buildup on essential engine components. For instance, carbon buildup can cause the piston rings to stick to the cylinder wall, preventing them from sealing the chamber.

Symptoms of Diesel Blowby 

Now that we’ve answered “What is diesel blowby?” let’s talk about common tell-tale signs your diesel engine is experiencing blowby and how it's harmful to engine efficiency. 

• White smoke from the oil fill tube or valve cover: Pop the hood and locate the oil tube (the tube the oil dipstick rests in) and the valve cover (a plastic or metal cover bolted to the top of the cylinder head). If there’s white smoke billowing out of either one, you have blowby. 

• Loss of power: If fuel sneaks out of the cylinders, the engine doesn’t have all it needs, resulting in incomplete combustion. This may cause rough idling or misfiring.  

• Increased oil consumption: Worn piston rings also leave room for motor oil to enter the combustion chamber where it’s burned away. Combustion gases that leak into the crankcase can also dilute the oil, making it less effective. In both cases, engine components won’t have proper lubrication, increasing carbon buildup and chances of damage. 

• Decreased fuel efficiency: Blowby means fuel from the combustion chamber is sneaking out past the piston rings instead of helping drive the piston down to power the engine. This means your engine must work harder to deliver the same output. And when your engine works harder, it needs more fuel.

If you’re experiencing any of these symptoms, visit a diesel repair shop as soon as possible. When ignored, diesel blowby can be fatal to the engine.

Diagnosing and Fixing Diesel Blowby 

A trained diesel technician can help diagnose and fix blowby. To confirm excess blowby they may:   

1. Place the oil filler cap upside down on the opening: You can try this at home. With the vehicle running, remove the oil filler cap and place it upside down on the opening. If it wobbles uncontrollably or blows off, your engine is experiencing excess blowby.

2. Conduct a leak-down test: During a leak-down test, a technician will force air into the combustion chamber with a compressor. A dual gauge will tell how much pressure was injected versus how much was lost to confirm blowby.

3. Blowby flow meter: Blowby meters attach to the crankcase tube to measure how much pressure loss there is in real time. 

Once the technician confirms the level of blowby, whether the issue is worth repairing can be determined. If it is, the technician might start by examining the piston rings, pistons or cylinders.  

How to Prevent Diesel Blowby  

• Routine maintenance can significantly reduce the chances of excess blowby. Regular oil changes and inspections of essential engine components ensure everything is operating at optimal condition, preventing the wear and tear that causes issues like blowby. 

• Use high-quality fuel: High-quality fuel has refining detergents that prevent carbon buildup and erosion on components like piston rings and cylinder walls to prevent blowby. 

• Use manufacturer-specified oil: Using oil that meets the manufacturer’s viscosity standards can help ensure all your engine’s most important parts are receiving the lubrication they need to operate at peak conditions for an extended period. 

FAQs

What are the potential consequences of ignoring diesel blowby? 

If ignored, blowby can damage multiple engine components, resulting in such a costly fix that your diesel technician might recommend a new engine altogether. In extreme cases, blowby can cause a runaway engine. Runaway occurs when the fuel-oil mixture in the crankcase travels back to the combustion chamber. The mixture burns and causes the RPMs to exceed standard ratings, resulting in mechanical failure.  

Are there any aftermarket treatments available to reduce diesel blowby in diesel engines? 

An air oil separator can help reduce diesel blowby. It works by catching and separating motor oil from blowby gases before being recirculated into the intake system.

Can diesel blowby be prevented or minimized?

All diesel engines experience some level of blowby. Normal measurements are 1.5 cubic feet per minute (CFM) when the engine is hot to 3 CFM when the engine is cold. Prevent blowby from exceeding these levels through routine maintenance, high-quality fuel and manufacturer-specified motor oil.  

Learn About Other Diesel Engine Issues at UTI 

Diesel blowby limits your engine’s power output and if left unaddressed, causes issues as serious as a runaway engine. Regularly maintaining your vehicle can keep blowby at bay and prevent costly repairs.  

If you want to learn more about fixing diesel engines, consider diesel mechanic school! Universal Technical Institute (UTI) offers a 45-week Diesel Technology program that prepares students for careers.^1,7 Request information or find a campus near you to get started:

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