I always get a lot of questions about blowby, specifically with regards to diesel blowby (well, you know, most vehicles here in Manila are diesel powered).
So to give a definitive answer and to save me tons of time answering to future questions, I am writing this post that will serve as a guide to all blowby related queries.
Read on and learn everything about blowby!
What is Diesel Blowby?
Diesel blowby or “blowby” in general is a leakage of combustion gases (or mixture of air-fuel) between a piston and the cylinder wall into the crankcase of a vehicle engine.
Its the same with either gasoline or diesel engine, it is a common term. See the pic above.
When your piston rings are worn, compressed air and fuel or gases from the combustion chamber will escape into the crankcase resulting to an overwhelming pressure which is then mixed with oil.
When this occurs, oil and gas mixture will put tremendous stress on your engine, significantly reducing its efficiency and reliability.
Signs of Diesel Blowby?
Take note that not all blowby is a sign that your piston rings are worn or damaged. Some blowby is normal as rings are not 100% infallible, even in a new engine.
With variable-valve diesel engines which has individual pressure sensors that provides the computer with real time data that allows it to compensate any erratic cylinder pressures.
As such, not all blowby can be considered as a “damaged” engine.
Some common signs of blowby include the following:
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- Rough idling
- Misfiring
- Low engine power
- It consumes more oil than usual
- Excess exhaust smoke
- White smoke coming out of the oil-filler tube
- Oil and smoke coming out of the dipstick tube
- Diesel runaway (where unburned fuel reaches the combustion chamber)
To check for any diesel blowby signs, just open the oil filler cap or your oil dipstick while your engine idles. If tremendous gases and oil blow off, there’s definitely too much crankcase pressure.
Causes of Diesel Blowby
Blowby starts with bad piston rings. Overtime, these rings gets worn out and with the amount of pressure inside the cylinder, gases will leak past the piston rings into the crankcase.
The number one cause of blowby is the carbon buildup in the top ring grooves, allowing cylinder leak down and some oil burning (which further produces piston ring carbon deposits and more blowby).
You can’t do anything to prevent blowby as it is a natural process of engine’s wear and tear. Diesel blowby is caused by piston rings sticking to the cylinder walls, causing it to get destroyed overtime especially if the engine is abused (high mileage vehicle, lack of maintenance, etc).
Unknown to many gearheads out there, engine braking can cause piston rings to wear prematurely. Engine braking cause rings to vibrate and will eventually break their seal when pushed to the limit.
So the next time you engine brake, make sure that you are in the right gear and avoid that red line. Otherwise, just use your brake pedal instead.
How to Diagnose Diesel Blowby?
What makes blowby very difficult to inspect and diagnose is because you need to open the valve cover of the engine, meaning you have to teardown every component just to determine the status of your piston rings.
A good alternative before replacing your rings is checking your diesel engine’s compression. By performing a leakdown test is the best method of diagnosing your engine’s condition while determining the severity of blowby.
Using a compressor and a special dual-gauge testing tool, it shoots compressed air into the combustion chamber. The gauge displays the right amount of air pressure that is being injected. Any percentage of pressure that’s lost is the amount of blowby your engine emits.
How to Prevent Diesel Blowby?
As mentioned above, blowby is caused by worn piston rings which lets gaseous pressure to escape into the crankcase. If you can prevent cylinder damage or carbon accumulation in the top ring grooves, then you can significantly reduce any chances of moderate to excessive blowby.
One way to do this is to use decarbonizers to help reduce blowby fumes that shortens the life of piston rings. These fuel-add on decarbonizers help eliminate those hard to remove carbon deposits in the piston rings.
You can use decarbonizing products after you complete an oil change. These fuel additives also helps lubricate sticking piston rings which helps improve the seal between the rings and cylinder walls.
Speaking of oil change, you need to regularly maintain a fresh motor oil to ensure that your engine is spot-free when it comes to carbon and soots. So change your oil according to its scheduled maintenance period.
Another prevention method that you can do is to install an oil catch can (oil separator). Oil catch can helps trap excess oil-gas mixture (which is a normal blowby) coming from the crankcase. This mixture is designed to be re-directed back into the combustion chamber.
Without filtering this mixture, the excess oil and gas will be transformed into gunk and will accumulate as carbon wastes, clogging the intake manifold, EGR, throttle body and of course, affecting your piston ring’s efficiency as well.
So regular cleaning of the said air intake system components is also a must in order to prolong the life of your engine’s piston rings.
Doing these steps can greatly reduce diesel blowby.
How to Fix Diesel Blowby?
For diesel engines to get the best compression and overall performance, they need to have optimal piston ring seal which eliminates any chances of severe blowby.
A durable and reliable ring seals are required especially if the engine is turbo powered to help endure high boost pressures and internal high heat temperatures.
You also need to pinpoint the clearance between the piston ring to piston groove, as they are designed to work together. Once you verified the correct clearance in setting or installing piston rings based on your engine’s specification, you greatly minimize the risk of producing blowby gases for another thousands of mileage.
So this isn’t just a simple plug and play scenario where you buy a replacement rings and install them directly to your pistons. There’s more into it that needs to be considered.
All of these fixes requires engine teardown as piston rings are located inside the engine.
Summary
That’s it! Here are the various information about diesel blowby and blowby in general.
In conclusion, you can minimize blowby by regularly changing your oil, installing an oil catch can, useing decarbonizing fuel additives and avoiding excessive engine braking.
And if your engine has excessive blowby, you’ll need to teardown your engine and check the status of your piston rings. Replace if necessary.
As you learned from here, this isn’t for regular DIYers and car enthusiasts. So have your engine checked by a trusted mechanic.
Here is my Youtube video explaining more about diesel blowby. This is in Tagalog language but has English subtitle. Enjoy watching!
