Why is the oil extracted from the fuel tank black?

[RobertLee]

It may be due to the injection pressure of the fuel injector being too low, lower than the cylinder pressure, causing exhaust gases to enter from the injector. Below is relevant information about fuel injectors: 1. Definition: A fuel injector is essentially a normally closed valve (a normally closed valve remains closed when there is no control signal input, whereas a normally open valve remains open without a control signal). The opening and closing of the valve are controlled by the up-and-down movement of a needle valve. 2. Reasons for low pressure: Dirty diesel fuel, impurities in the high-pressure fuel line causing poor sealing of the needle valve assembly, high-pressure gas from the combustion chamber backflowing, and burning out the needle valve assembly, among others.