Why Does Diesel Oil Turn Black?

Posted by: Jeremy Patterson on 11/17/2021

Have a seat and partake in this morning’s diesel knowledge.

If you’ve ever driven or been around a diesel truck while the oil was being changed, you know it comes from the engine looking very dark, almost completely black, even if the oil has been recently changed. Many don’t understand why this is, so YouTube host Adept Ape has the lowdown on why the oil in diesel engines turns black so quickly.

Quite simply, it’s soot, which diesel engines produce in much higher levels than their gas-burning counterparts. The soot enters the oil pan from the combustion chamber due to the gap in the piston rings, and it takes very little soot to turn the oil the inky black diesel mechanics are all to familiar with. Adept Ape even discusses how the oil remaining in the engine during the oil change – which can be as much as a gallon – will immediately contaminate the fresh oil and turn it black within minutes.

The black color doesn’t mean the oil is ruined though, as the soot particles are minuscule and don’t hinder the lubricating properties of the oil until the concentrations build to very high levels, at which time you may start to see decreased oil pressure. Now you know why diesel oil turns so black so quickly and that it doesn’t necessarily mean the oil is ruined.