Black smoke from exhaust and the faults behind it
Black smoke from exhaust pipes is soot, made when the engine burns more fuel than its air supply can handle. On diesels it's often a boost or sensor fault, and the fix can be much cheaper than people expect.
The short answer
Black or dark smoke means the engine is running rich, with too much fuel, too little air, or both. Unburnt fuel leaves the exhaust as soot.
It's rarely an emergency, so gentle use while you arrange a check is usually fine. Running rich for weeks wastes fuel, can foul a DPF and may wash oil off the cylinder walls, so don't leave it too long.
Common causes of black smoke
Boost leak
A split hose or cracked intercooler lets air escape, so the engine gets the fuel it asked for but not the air. A smoke test finds these quickly.
Turbo underboost
Sticking vanes or a failing actuator mean less air is pushed in, often with P0299 stored. See our turbo problems guide.
Dirty air filter or intake
A clogged filter or heavy carbon in the intake restricts how much air reaches the cylinders.
Faulty MAF sensor
If the air flow sensor reads wrong, the ECU calculates the fuelling wrong. P0101 is a common code.
Leaking or worn injectors
Injectors that drip or spray poorly leave fuel unburnt, often with a rough idle and a strong diesel smell.
EGR valve stuck open
Exhaust gas replaces fresh air in the intake, so the engine runs rich. Our EGR problems guide explains the signs.
Black smoke from the exhaust when accelerating
Under hard acceleration the ECU adds the most fuel, so any shortage of air shows up as a dark cloud at that moment. On a diesel it's usually the first sign of a boost leak or a turbo that isn't spooling properly.
A small, brief puff on an older diesel without a DPF can be normal. Heavy smoke every time you accelerate, or any smoke from a car that should have a DPF, needs looking at.
Black smoke from a petrol engine
Petrol engines run much cleaner, so visible black smoke usually means a fault with the fuelling. These are the usual suspects.
- A leaking injector or one stuck partly open
- A faulty oxygen or coolant temperature sensor telling the ECU to add fuel
- Fuel pressure that's too high
- A badly blocked air filter
- Misfires leaving fuel unburnt in one cylinder
Black smoke after a remap
If black smoke appeared straight after a remap, the map may be adding more fuel than the air supply can match. That's a sign of a poorly set up map, and it can block a DPF much faster.
A remap should be checked against live data so the fuel and boost stay in balance. If you're worried about a map that's already on the car, we can read it and check how the engine is running.
Quick checks before you book
Look at the air filter
If it's black and clogged, replace it and see if the smoke reduces.
Listen under the bonnet
A hiss or whoosh as you rev the engine often means a boost leak.
Note when it smokes
Only under hard acceleration, all the time or just at idle each point somewhere different.
Check for warning lights
An engine light or DPF light means codes are stored and worth reading.
Keep an eye on fuel use
Rich running usually shows up as worse mpg before anything else.
Is black smoke from a diesel normal?
A light puff under hard acceleration on an older diesel can be. A modern diesel with a working DPF shouldn't show visible black smoke at all.
Can a blocked DPF cause black smoke?
Not usually from the tailpipe, because the filter traps the soot. Black smoke from a DPF-equipped car suggests the filter is cracked or has been removed.
Can a dirty air filter cause black smoke?
Yes. A badly clogged filter starves the engine of air. It's a cheap part and worth ruling out before looking at anything more expensive.
How do I stop black smoke from my exhaust?
Find out why the engine is running rich. A diagnostic reads the fuelling and boost data, and a smoke test shows whether air is escaping.
Find the cause of the black smoke
We'll check boost, air flow and fuelling where your vehicle is parked for £100 + VAT, or carry out a smoke test for £120 + VAT if a boost leak looks likely. Either way you'll know what's making the smoke before spending money on parts.
Questions? Call 0161 694 7331 or email sales@sgrautomotive.co.uk
Eccles, Manchester M30 0SF