Black exhaust smoke: causes, seriousness and affected cars

Black exhaust smoke points to over-fuelling or a turbo problem: the engine is not burning all the diesel or petrol it receives (GetCarwise). On a diesel, the three most cited causes are the injectors, a clogged EGR valve and the turbo. It often comes with a loss of power and jerking.

The most common causes

Leaking or sticking injectors

A worn injector delivers too much fuel or atomises it badly: hard starting, black smoke, vibrations and higher consumption. On the Renault Laguna III 2.0 dCi, injector wear typically appears between 70,000 and 110,000 km, for a bill of €600 to €900 (CarVerif). The piezo injectors of 2000s-2010s Mercedes CDI engines gradually stick, causing misfires, hard cold starts and black smoke (CarVerif). An injector fault code (P0201 for cylinder 1, for example) often comes with these symptoms (CarVerif, BMW 118d).

A clogged EGR valve

The EGR valve sends part of the exhaust gas back to the intake. Clogged with soot, it sticks: loss of power, black smoke, sometimes limp mode. Caradisiac rates the risk as high on the 1.9 CDTI of the Opel Zafira B (Caradisiac); on the Audi A6 3.0 TDI, failure occurs between 100,000 and 140,000 km (CarVerif). See our guide to a clogged EGR valve.

A failing turbo

A turbo that no longer delivers enough boost starves the engine of air: gradual loss of power, whistling and black smoke (CarVerif, Nissan Primastar). On the Nissan X-Trail 2.0 dCi, these failures often come after 150,000 km (CarVerif). Turbo petrol engines are not spared: FixMyCar flags it on some 2010-2014 Audi A1 1.4 TFSI (FixMyCar).

Is it serious?

Rarely right away, but do not leave it: the engine uses more fuel, clogs its particulate filter and can go into limp mode. A leaking injector or a dying turbo costs more if it fails completely. Have the fault codes read before replacing anything: an injector code calls for a check of the return flow, not necessarily four new injectors (CarVerif, Fiat Doblo).

Before buying a used car

Heavy black smoke on the test drive, a turbo whistle, a lack of power or an injection warning light are reasons to walk away (CarVerif, Renault Clio IV; CarVerif, Renault Mégane III; CarVerif, Fiat Tipo II).

• Accelerate hard during the test drive and look in the mirror.
• Ask for a fault code reading.
• Be wary of a diesel that has only been driven in town: the EGR valve and particulate filter clog up (CarVerif, Dacia Sandero II).