What Is Blowby in a Diesel Engine? Causes & Prevention

Blowby can affect diesel engines of all kinds, so spotting the signs of blowby and knowing how to address – or even avoid – this common diesel engine problem could be the making of your fleet and business profits.

In this blog, we’ll detail what blowby in a diesel engine is, what causes it, and how you can fix it. Let’s get stuck in!

Blowby on a diesel truck, bus, or other vehicle occurs when combustion gases leak between your engine’s cylinder wall and pistons into the crankcase during the engine’s power stroke. This is a result of the combustion chamber’s high pressure, which forces the exhaust gases past the piston rings and into your diesel engine’s lower section.

While a small amount of blowby is nothing unusual, a lot of blowby could be telling of some more serious problems.

Blowby in a diesel engine is considered bad, as it can cause numerous issues for your vehicle, from oil contamination and decreased engine performance to increased emissions. It also indicates wear and tear of vital engine components like piston rings and could result in major, costly repairs if neglected.

Knowing what causes blowby in a diesel engine can help you learn to prevent it in the first place. Blowby can be caused by:

Buildup of carbon:

Your engine may experience a buildup of carbon if you do not conduct regular maintenance checkups. A buildup of carbon can trigger the piston rings to stick to the cylinder wall, stopping them from being able to seal the chamber.

Worn piston rings:

Over time, piston rings can be subjected to wear and tear, causing them to lose their ability to contain the combustion pressure.

Damaged cylinders:

Damaged cylinders lose their ability to create a tight seal with the piston rings. This can enable combustion gases to get through the rings and into the crankcase, contributing to blowby.

Engine brakes:

Engine brakes can increase the cylinder pressure. This can add stress to the piston rings, possibly causing them to wear out faster and allow more combustion gases to escape into the crankcase.

High mileage:

High mileage causes components to wear faster, potentially increasing the chance of blowby.

Know that you know what diesel blowby is, it’s worth clueing up on a few factors which can help you identify it. Here are some common symptoms to look out for:

One of the biggest characteristics of blowby is white smoke. This emits when excessive pressure builds up in the engine’s crankcase and pushes unburned fuel and oil mist through the system.

Your diesel engine can misfire or idly roughly when combustion gases escape early from the combustion chamber.

If your engine is making loud noises and emitting fumes, these may signify blowby.

Are you using more oil and fuel than you normally do? This may be telling that they are leaking and causing blowby.

Blowby results in unburned fuel contaminating oil and leaking into the crankcase, causing oil film to form around the tube.

Because blowby causes the fuel to escape past the piston rings instead of helping power the engine, your diesel engine has to operate harder to deliver the same results, meaning it demands more fuel. Learn more about minimising environmental harm by keeping your fleet clean.

If the symptoms above sound familiar, you’ll want to learn how to prevent the causes of blowby on a diesel engine. We’ve gathered a few regular maintenance tips for you here so you can address them head-on.

1. Check your piston rings

Piston rings are often the major culprit when it comes to blowby, so make sure you check them and clean or replace them if necessary. Tackling any piston ring issues and checking for air intake system leaks should lead to better results. Find out more about identifying piston damage.

2. Keep an eye on carbon buildup

Frequent maintenance checks can help you monitor carbon buildup and other potential problems in the long run. Carbon buildup can stop the piston rings from sealing, so it’s worth keeping an eye on it and changing the oil or cleaning the filters as necessary.

3. Check the cylinder head valve guides

If the cylinder head valve guides are excessively worn, they may cause blowby. You may need a specialist to replace the cylinder head if necessary.

4. Use an additive

Sometimes, the cause of blowby is inadequate lubrication. Therefore, you can boost performance and stop your piston rings from sticking with an appropriate diesel fuel additive.

5. Check for a tight seal

The piston rings should create a tight seal between the piston and the cylinder wall. Ensuring the seal is optimal can help your diesel engine operate better at high temperatures and pressures.

6. Survey the air compressor

An excessively worn or damaged air compressor can contribute to blowby. If you discover this is the case, allow us to replace or rebuild this component.

7. Maximise ventilation

Vent tubes can help handle high crankcase pressure, while oil separators can separate combustion gases from oil. Check and clean the crankcase breather and vent tube to ensure the crankcase ventilation system isn’t plugged up.

8. Check the engine angularity

If your engine angularity during operation exceeds the specification, you may have trouble with blowby. Ensure you refer to the Engine Specification Data Sheet for angularity specifications for the specific engine being serviced.

9. Use the right oil

Always use oil as stated by your manufacturer. Check your oil level, verify the dipstick calibration and oil pan capacity, and fill the system to the specified level. Having oil with the right viscosity can ensure your engine components receive the lubrication they require to work on top of their game.

10. Purchase the right fuel

Look out for quality when it comes to fuel. High-quality fuel has detergents that can help stop erosion and buildup. In turn, this can prevent blowby.

11. Talk to Foxwood Diesel

If you’re uncomfortable tackling blowby yourself, contact Foxwood Diesel to attend to your every maintenance need. Maintenance in our machine shop can be a fantastic way to fix blowby, but it can also be a great way to prevent blowby from occurring in the first place. By conducting regular inspections and making regular oil changes, you can reduce the likelihood of wear and tear, therefore decreasing the chance of blowby.

If you have just purchased a new or remanufactured engine and you have started to experience blowby issues, you should make the following final checks before getting in touch with one of our professionals:

If you’re having trouble with diesel engine blowby and need extra assistance, don’t hesitate to contact us; a team member will be happy to address your issues. To keep your engine in good condition, we can clean parts, inspect them for wear and tear, and replace damaged components as required.

Our team can also help with anything from identifying the causes of diesel engine oil leaks to talking you through engine air compressors reconditioning.

We can also source specific parts to cater to your engine’s needs.