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What Is Diesel Blow-By?

Exploring reasons why it happens, how to diagnose, and things you can do to keep blow-by away from your rig’s engine.

Apr 24, 2020
"Blow-by" is a fairly common term across all types of engines—diesel, gas, etc. For diesels, it's when compressed air and fuel in the cylinder bore is greater than pressure in the oil pan, and gas leaks past piston rings and down into the crankcase. When this pressure in the combustion chamber becomes too great—typically during the engine's power stroke first, and then the compression event—it has to escape.
It's important to note that some blow by is normal, as rings aren't 100 percent infallible—not even in a new engine. However, today's late-model, variable-valve diesels feature individual pressure sensors, which provide an ECM with data that allows it to compensate for erratic cylinder pressures. For the most part, though, blow-by in a diesel powerplant typically means rings are sticking to the cylinder walls, they're severely worn, or totally destroyed; those conditions come about through use (mileage), and/or lack of engine maintenance. Damaged pistons or cylinders are also possible causes that can only be determined through teardown and inspection. Believe it or not, engine brakes can play a role in blow-by, as those systems cause rings to move/vibrate, and break their seal when they're activated.
With performance-built diesel engines—compound or triple-turbocharged, etc.—or oil burners with high mileage, there's a greater probability that blow-by can be more excessive, especially if a proper service routine isn't adhered to.
The videos that accompany this report provide examples of a high-mileage engine with moderate blow-by, and some with extreme cases of the condition. It's important to periodically check the state of ring seal in your rig's engine by at least performing a simple test with the oil-filler cap.
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How to Identify and Test for Blow-By

First, rough idling and misfiring can indicate there is a problem. However, one of the tell-tale signs of excessive blow-by is white smoke billowing from the oil-fill tube or opening on a valve cover. To check this, set the oil-filler cap upside down on the tube or opening. If it immediately blows off, there definitely is too much crankcase pressure. Another easy identifier—before the smoke, actually—is residual oil film around the tube. Blow-by also results in contaminated (by unburned fuel), diluted oil in the engine's crankcase. In some instances where blow-by is excessive, the mixture can cause a diesel to run away if it reaches the combustion chamber.
Photo 3/6   |   What Is Diesel Blow By 3
Checking a diesel engine's compression by performing a leakdown test is a more detailed method of diagnosing the powerplant's condition and determining the amount (percentage) of blow-by. Using a compressor and the specialized dual-gauge testing tool, and with the engine at TDC, shoot air into the combustion chamber. The gauge on the right displays the total amount of air pressure that is being injected. The percentage of pressure that's lost (from blow-by) is displayed in the other gauge.
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Battling Blow-By

First and foremost, ensuring a diesel engine has optimal ring seal is the best way to ward off blow-by, especially for severe-duty powerplants that endure high boost pressures and internal temperatures. High-performance piston-ring developer, Total Seal, has applied its "Gapless" technology into the second ring for diesel engines. The company's Lake Speed Jr. says this design increases seal by eliminating the second ring's end gap (the gapless/solid ring actually sits just beneath the second ring), which reduces blow by, and helps an engine run at its full potential.
Photo 5/6   |   What Is Diesel Blow By 5
Ventilating extreme crankcase pressure—using a vent tube (some older engines have this) or oil separator (for late-model and high-performance engines)—is also recommended, as they take oil out of combustion gasses. Using additives such as Hot Shot's Secret "Stiction Eliminator" and "Diesel Extreme" is also a good practice as these lubricants can help loosen up sticking rings and improve the seal between the rings and cylinder walls.
Photo 6/6   |   What Is Diesel Blow By 6

Sources

Hot Shot's Secret
800-341-6516
http://www.hotshotsecret.com
Power Stroke Enginuities
832-688-8702
http://www.psehouston.com
Total Seal High Performance Piston Rings
800-874-2753
http://www.totalseal.com
 

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