What is Diesel Blow By? – Causes, Signs, & Fixes

Engine blow by is a common term with all internal combustion engines – diesel and gasoline, including those on commercial generators and tractors. However, diesel engine manufacturers design their engines to last long and offer efficiency and high-performance capabilities. But these engines still experience blow by. Of course, I know you’ll be wondering what is diesel blow by, anyway?

Here, I’ll outline what causes blow by in diesel engines, what it means, the signs, how to fix diesel blow by, and explain other related issues. Grab a seat and have a 5-minute read.

how to tell if diesel engine has blow by

What is diesel blow by?

Auto manufacturers design diesel engines in a way the fuel burns in the combustion chamber and exits the engine as exhaust gases. The fuel should not get into the crankcase or oil pan.

Diesel blow by occurs when internal combustion pushes contaminants like air, fuel, and moisture from the combustion chamber into the crankcase through the space between the piston rings and the cylinder walls. If the piston rings don’t properly seal the space between the piston and the cylinder walls, it won’t withstand pressure during internal combustion.

If a large sum of fuel gets into the oil pan, it’ll disrupt the oil’s lubricating properties, resulting in oil contamination and pressure buildup in the oil pan. If you do not address it on time, it’ll reduce engine performance and may cause engine breakdown in the long run.

When the engine oil losses its lubricating properties, it’ll affect reciprocating engine parts like the crankshafts, camshafts, and other internal engine parts like the valve train, main engine bearings, and cylinder walls. So, if you’re asking, is blow by bad in a diesel engine? You now know the answer. Let’s see what causes blow by in a diesel engine.

What causes diesel blow by?

Several factors can cause engine blow by in your diesel engines. I’ll only outline the most probable causes.

Normal blow by

All engines, including new ones, from the factory, have minor levels of blow by because piston rings are not watertight. So, your diesel engine can experience a minor degree of blow by even when everything is working as it should.

However, you’ll hardly notice this type of blow by. But how much blow by is normal in a diesel engine? In a 12-liter diesel engine, there are about 1.5 cubic feet per minute of blow by under normal operating conditions.

Worn piston rings

According to researchers at the Helsinki University of Technology, worn piston rings are one of the significant causes of blow by in diesel and gasoline engines. The truth is, the back-and-forth movement of the piston as the engine is running scrapes the piston rings against the cylinder walls. The continuous scrapping wears down the rings, and their sealing capacity fails.

Aside from the blow by, worn piston rings will cause high oil consumption, excessive gray or white exhaust smoke, poor engine acceleration, and loss of engine power.

Worn cylinder walls

I mentioned earlier that the back-and-forth movement of the piston forces the rings to be scrapping on the cylinder walls. As this normal occurrence continues, the rings wear off, and the cylinder walls expand. As the engine ages, the gap between the pistons and the cylinder walls increases, causing a blow by in the engine.

If you are wondering what causes blow by in a gas engine, a worn cylinder wall is one of the probable causes, especially in older engines.

Worn pistons

Engine pistons are one of the essential internal engine components that an engine cannot do without. The explosion in the combustion chamber takes place on top of the piston. As the piston rings and the cylinder walls wear over time, the pistons can slightly become smaller.

This is true because the pistons are usually manufactured with aluminum material. And aluminium is a soft metal. Grit accumulation in the combustion chamber will carve grooves on the piston, allowing the fuel to enter the crankcase.

Mechanic error

Blow by can happen in gasoline and diesel engines due to a mechanic error. If you recently overhauled your engine, it could be the mechanic did not maintain proper cylinder-to-piston clearance or there’s a poorly machined surface. It can also mean the mechanic did not file the piston rings properly. Thus, allowing the fuel to enter the crankcase.

This explains why overhauling engines is not for amateur mechanics. You can have any mechanic do minor jobs on your car, but contact certified mechanics when you have engine problems. This will help you extend your engine lifespan and save you money in the long run.

What are the signs of a blow by on a diesel?

I have explained what’s a blow by on a diesel engine and outlined the possible causes. Now, let’s look at the signs that show you have a diesel engine blow by.

what causes blow by in a diesel

Knocking engine sound

A common sign of a diesel blow by is a knocking sound from the valve cover. The knocking or ticking noise occurs when the fuel in the combustion chamber is pushed into the crankcase. It can also happen when the engine oil loses its lubricating properties due to excessive gasoline in the crankcase.

Let’s be clear here; several factors like low motor oil pressure, defective rocker arms, bent push rods, worn-out engine bearings, and other issues can be the culprit of a knocking noise. So, you don’t have to conclude you have a blow by when you hear a knocking engine noise without a proper diagnosis.

Blue exhaust smoke

Another prevalent sign of a diesel blow by is blue smoke from the exhaust pipe. Blue smoke from the exhaust means oil is mixed with the gasoline in the combustion chamber. However, if the fuel enters the crankcase, it’ll lead to pressure buildup and invariably cause a blow by.

Like every other car issue, several factors like a blown head gasket, burning oil, blown turbocharger, lousy glow plugs, valve guide issues, and faulty transmission modulators can cause blue exhaust smoke.

Black soot on the cylinder head

Black soot on the cylinder head means the engine burns oil in the combustion chamber. This usually occurs when the engine has excessive blow by. What you may not know is that once there’s a diesel blow by, some amount of engine oil in the crankcase may be entering and burning in the combustion chamber. If this is the case, there will be black soot on the head cylinder.

Milky fumes

Typically, if you see a milky substance when you open the oil filler cap, it means there’s water or coolant in the oil. As the fuel vapor in the crankcase causes pressure build-up in the crankcase, it creates condensation. If a diesel blow by occurs, it creates an emulsion. And when heated, it turns to the milky substances you see in the combustion chamber.

Poor engine performance

Another prevalent sign of diesel blow by is loss of engine performance while driving. As I explained above, a diesel blow by happens when a small amount of fuel vapor is forced into the oil pan. This vapor is full of hydrocarbon and other impurities that can affect the overall engine performance. Plus, this can leave deposits on the internal engine parts, causing engine sludge.

Engine misfire

Another prevalent sign of a diesel blow by is an engine misfire. If the glow plugs malfunction or are unable to operate correctly due to the diesel blow by, the engine will not run as it should. This will invariably lead to an engine misfire. Plus, the blow by can even damage the piston rings, making them unable to seal the combustion chamber.

Increased fuel consumption

The last diesel blow by symptom you should watch out for is high fuel consumption. The ignition and the fuel burnt in the combustion chamber create energy to turn the crankshaft in the engine. However, some of this energy is forced into the crankcase when a blow by occurs.

If this happens, it’ll cause pressure to build up in the crankcase and force the engine to consume more fuel to generate the energy needed to turn the crankshaft.

Now that I have explained what is blow by on a diesel tractor and other diesel engines, the probable causes, signs, and what is normal blow by on a diesel engine, let’s see how to address the issue.

How do you fix a diesel blow by?

There are several ways of addressing diesel blow-by issues since several factors can be the root cause. When customers drive into my garage with a diesel blow-by, I first check the severity of the blow-by. After that, I’ll recommend what the driver should do or address the issues myself. Here are what I usually recommend or do to fix the underlying issue;

Add FTC Decarbonizer to the gas tank

I often only recommend customers to add FTC decarbonizer when they visit gas stations. This cleaning agent typically de-cokes valves and cylinder heads, DPF filters, and turbos. Most times, this is all you need to fix the underlying issues.

Use flushing oil concentrate

FTC decarbonizer is an excellent decarbonizing fluid. It does a great job. But if you need a more excellent job, especially to remove stubborn build-ups like engine sludges, use the flushing oil concentrate when replacing the motor oil.

The FTC decarbonizer deals with carbon deposits on the top surfaces like the valves and the cylinder heads. But the flushing oil concentrate tackles the lower ends, such as the piston rings and every nook and cranny part of the internal combustion engine where the motor oil passes through.

However, it is essential to note that using flushing oil concentrate and FTC decarbonizer will only remove black soots, carbon build-ups, and sludges in the engine. Of course, this enhances engine performance. But it won’t fix worn piston rings, pistons, or cylinder walls.

Contact your mechanic

Whenever I diagnose a diesel blow-by, I can tell when there’s a worn cylinder wall, pistons, or piston rings. So, if the above solutions do not fix the problem, contact a certified technician to diagnose and rectify the problem. A good mechanic should be able to tell if the issue is from a worn engine part after performing a leak-down test.

However, he may not directly say precisely what’s wrong with the engine until he drops and disassembles it. After disassembling and inspecting the engine parts, he should track and resolve the leading cause of the problem. In most cases, it could be you have worn piston rings. Other times, it could be a worn piston or the cylinder walls needs re-sleeving.

How do you know if your diesel has blow by? YouTube

Frequently Asked Questions – FAQs

Q: Is it OK to breathe in diesel?

Breathing in diesel particulate matter will cause several health issues like irritation of the eyes, nose, throat, coughing, and respiratory tract. Also, breathing in diesel can cause wheezing and difficulty breathing (commonly called asthma), worsening existing asthma or lung irritation. Other issues include nausea, headache, or lightheaded.

Q: Will diesel Ungel by itself?

While gelled fuel isn’t good for any diesel engine, the good news is the fuel will ungel itself. It’s a matter of time. As soon as the temperature goes above the gelling point, the diesel fuel will return to its normal state within an hour. With that in mind, all you need to fix gelled diesel fuel could be to push it into a garage and let it stay for around 1 hour.

Q: Can injectors cause blow-by?

Yes, fuel injectors can cause blow by. Injector blow-by happens when the injectors are not sitting properly on the injector seats. You’ll likely hear a chuffing sound or see black tar around the injector areas. Other signs of diesel injector blow by include hard starting, lumpy running, smoke from the tailpipe when accelerating, hesitation on acceleration, and erratic idling.

Q: What happens if you don’t fix blow by?

Failure to fix the diesel blow by will cause several issues to the overall engine performance. For instance, if you ignore diesel blow-by for an extended period, the engine will experience bad fuel mileage, high oil consumption, lower engine compression, reduced engine performance, engine misfire, and other engine problems.

Q: Is blow by worse when engine is cold?

If you have been reading to this point, you’ll no longer ask is blow by bad in a diesel engine. However, it’s okay if you’re wondering if the blow by will worsen when the engine is hot or cold. Engine blow-by is worse when cold compared to when hot. From my experience, engines have lower power output and increased blow-by during cold starts.

Q: Can blow by cause turbo failure?

Turbos are crucial engine components that enhance engine power. However, they typically fail due to oil-related issues. And excessive blow-by from the piston rings or too much pressure from the crankcase can cause oil leakage from the turbocharger turbine seals or the compressor. Of course, this will cause the turbo to fail.

Final Words

At this point, you’ll no longer ask what is diesel blow by. Diesel blow-by is a situation where some amount of the fuel in the combustion chamber is forced into the crankcase through the space between the piston rings and the cylinder walls.

Diesel blow-by is an issue that can cause severe problems if not addressed on time. It’s essential to watch out for the signs explained above and treat blow-by issues early on before they escalate to severe issues and drop expensive repair bills on the table.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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