Why Does Diesel Engine Oil Turn Black Quickly?

There are affiliate links on this article. If you make a purchase through any of the links, I may earn a small commission at no extra cost to you.

Okay, you had an oil change done yesterday and the oil in your diesel-powered engine is already looking black in just 24 hours? It’s nothing to be surprised about, diesel engines turn oils black pretty quick. But why does diesel engine oil turn black quickly?

The reason is pretty simple; diesel engines produce more carbon deposits, which of course, is very dark soot. This is what contaminates the oil and makes it turn black within the shortest timeframe you may expect.

Having black oil in your engine doesn’t necessarily mean the oil is bad and should be replaced immediately. Here’s a detailed explanation of why motor oils turn black quickly in diesel engines.

Why Does Diesel Engine Oil Turn Black Quickly?

Diesel engines are high-compression ignition engines; they typically have a higher compression ratio than gas/petrol engines. With the high compression comes high carbon production (dark soot), which could perch on various sides of the engine.

Thus, when you pour in new oil, as the oil flows into the engine, it picks on those engine sides that have the soot on them, and that’s what makes it turn black immediately – even before getting into the main engine.

Also, high cylinder compression can force soot to escape the piston rings and mix with oil in the engine. So, simply, the reason why oil turns black quickly in diesel engines is due to contaminants (high carbon deposits) caused by high compression.

To this end, motor oils meant for diesel engines are formulated with more cleaning ingredients – detergents – because they would need to be very active to keep diesel engines clean.

However, it is important to note that having black oil in the engine does not necessarily mean the oil is due for a change. The black oil could still be effective in cleaning your car’s engine and guaranteeing optimal performance.

See Also: Top 4 Bad Diesel Fuel Pump Symptoms

You should only be concerned if the black oil has lost its viscosity, or you used the wrong oil for your engine. Asides from these two scenarios, it’s not a “big deal” to have black oil in your engine.

You could drive all the way – until your next oil change time without experiencing any issues because you’ve got “black” oil in your diesel-powered engine. However, if this is very unusual to you, try draining the oil and refill another one to see if it’d turn black quickly, again.

What More?

Why does diesel engine oil turn black quickly? Asides from the fact that diesel engines typically turn oils black quickly, a dirty oil filter can cause such scenarios too. It is important to change the oil filter as you change oil, or at specific intervals that work best for you.

Oil filters can get very dirty from accumulating contaminants that try to flow with engine oils into the engine.

Change the filter first, and if the issue persists, then it’s because your engine produces much more soot simply due to high compression, which is typical of an engine that runs with diesel. Hopefully, this article answered your question.

Scroll to Top