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In the diesel world, "blow-by" is a term you're bound to run across at some point if your line of work involves diesel-powered vehicles of any kind. What is blow-by, what causes it, and how concerned should you be? At Ward Service Auto Repair in Monrovia, CA, our fleet maintenance and diesel technicians see it on a regular basis. Here we'll take a quick look at blow-by, correcting it, and preventing it.

What is Blow-By? - Diesel blow-by is a term that describes an event of lost compression into the crankcase. Blow-by occurs whenever a burnt or unburnt fuel and air mixture escape from the cylinder and past the piston rings and into the crankcase. In a perfect world, the piston would be sealed tightly in the cylinder, able to move up and down but keeping all the air and fuel and exhaust that enters or exits the cylinder during the combustion process out of the crankcase. As you'll see, this perfect world doesn't exist, but it is important to minimize the amount of blow-by in a diesel engine.

What Causes Blow-By? - Blow-by in diesel engines can be caused by a variety of factors. Here are some common issues that cause diesel blow-by. Worn

Piston Rings: Piston rings create the seal between the piston and the cylinder wall. These rings allow for minuscule changes that occur to the cylinder wall and piston as the engine heats up or cools down, while maintaining a tight seal between the cylinder and the crankcase. As they wear down, the gap increases and more blowby escapes into the rest of the engine.

Worn Cylinder Walls: The cylinder walls themselves also wear down, though the rings are designed to take most of the wear. As the cylinder wall wears, it becomes larger, increasing the gap between the wall and the piston and allowing blow-by into the crankcase.

Scratching or Scouring: If the cylinder walls, pistons, or rings are damaged even slightly, it creates an avenue for combustion to escape the cylinder. This is caused by dirt and debris entering the cylinder, carbon or other debris in the oil, or leaking fuel injectors that wash the cylinder walls of oil and prevent the cylinders from lubricating properly.

How Can Blow-By be Prevented? - It all comes down to two things - maintenance and wear. Eventually, blow-by will increase in any engine as it simply wears out. However, lack of proper maintenance or unresolved issues will wear one out much, much more quickly. If oil changes aren’t kept up with, leaky injectors aren’t replaced, the fuel system isn’t maintained, the engine overheats, or any other issue that causes conditions in the cylinder to operate inefficiently, blow-by will rear its ugly head at some point. It’s essential that scheduled maintenance is performed and documented, to lessen the chance of a scenario where blow-by becomes a problem. If the truck is having an issue, don’t continue driving it - misfires, particularly on startup, often lead to entire engines needing to be replaced. Bad or dirty turbos or intake systems with leaks to unfiltered air are another major concern. These problems can be corrected easily, and prevent catastrophic engine damage. It is important to note that a certain amount of blow-by is simply unavoidable.

No matter what condition the engine is in, a percentage of the compression and combustion process will be lost due to blow-by. This is particularly apparent when the engine is cold before the metal has had a chance to expand. However, there is a threshold that must be maintained, or your diesel engine will experience power loss, hard starting, and poor running conditions. Blow-by also contaminates the oil, which will ruin bearings, cylinder walls, timing gears and chains, and other moving components, resulting in total engine failure.

How is Blow-By corrected? - Unfortunately, there aren’t any easy answers. The best thing you can do is have the engine professionally maintained, to offset the possibility of blow-by happening in the first place. Once the rings, pistons, or cylinder walls are worn or damaged, there’s nothing you can do besides break the engine down and look at the damage. In some cases, new rings or other repairs might get the truck back on the road.

Often, it’s better to rebuild or just replace the engine entirely - since blowby causes other problems and is a symptom of an all-around worn-out engine, this negates the possibility of the same issues coming back to haunt you. That call should always be made by an expert diesel technician. For any and all of your diesel needs, our technicians have you covered. For the best diesel and fleet maintenance and repair shop in Monrovia, CA, call or stop by Ward Service Auto Repair today.


Thank you for visiting Ward Service Auto Repair in Monrovia, CA. Count on our automotive repair technicians to keep your car, truck, suv, or van on the roads longer and safer.

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