The car was clean before the intake went on. Now, under load, there is a grey-black haze in the mirror, most obvious in the middle of the rev range and worse in a higher gear. It clears at a cruise. Black smoke is carbon that did not burn, which means the mixture in the cylinder was richer than it could consume.
On a diesel that richness shows as soot at a low threshold, so a small error becomes immediately visible. On a petrol turbo the same error is less obvious but still real. Either way the cause sits on one side of the air-fuel ratio, and the intake work just completed is the obvious place to start looking.
The control unit fuels for the air it believes it has, learned from a mass air flow sensor or from manifold pressure and temperature depending on the strategy. If air is counted at the meter and then escapes through a joint before the cylinder, fuel is still delivered for it and the mixture runs rich.
That is the most common mechanism after intake work: a coupler not fully seated, a vacuum line left off, a breather hose pushed back on at an angle, or a clamp tight enough to look right and not tight enough to seal. The leak sits on the pressure side of the meter and opens only under load, which is why idle and cruise stay clean.
A mass air flow sensor is calibrated to a specific housing bore and to a particular flow profile through it. Fit the sensor into a different tube, or into the same tube with the sensor rotated, and the reading changes for the same actual airflow. The engine then fuels to a number that no longer matches the air arriving.
This is a calibration error rather than a leak, and the two behave differently. A leak shows as a fuel trim that is wrong at idle and improves as load rises, or the reverse. A housing error shows as a trim wrong across the whole range in proportion to flow. Log the trims at idle and at a steady cruise and the shape of the error says which one you have.
An oiled filter is often blamed for black smoke and usually should not be. Over-oiling puts oil on the sensor element, and a contaminated element typically under-reads, which makes the engine fuel for less air than it receives. That produces a lean condition and often a hesitation rather than soot.
Check it anyway, because it is quick, but do not stop there. A heavily oiled filter and a contaminated sensor show as trims that have drifted and a car that feels flat rather than smoking. If the symptom is black smoke, the lean direction of that fault points you back to a leak or a housing error.
Black is carbon. Blue-grey is oil, and it smells different, hangs longer, and often appears at a hot idle or on the overrun rather than only under load. White is coolant, or on a diesel unburnt fuel in a different failure mode. Colour and timing together separate these better than colour alone.
Soot leaves the tailpipe dry and black inside, while oil leaves it oily and sticky. Wipe the inside of the tip with a light cloth. That single check rules out the whole oil-burning family before you start taking the intake apart, and it costs nothing to do first.
Pressurise the tract and check every joint you disturbed with soapy water. Then confirm that every line which came off went back onto the right spigot, including the ones behind the engine you cannot see. Check that the sensor sits in the housing it was calibrated for and that it is oriented the way the original was.
If the smoke appeared with a filter element change only, refit the old element as a test; that isolates the element from the pipework in a single step. Where the intake itself changed, compare bore and sensor position against what came off. The air intake has to present the same signal to the meter as the original did, and a washable prefilter ahead of the element keeps debris off it without touching that calibration.
