Direct-injection petrol technology means better performance and efficiency. But there's a dark side...

Black grimy sludge. From the outside, the gasoline-direct-injection (GDI) engine appears shiny but it hides a dirty secret: severe carbon fouling of the intake ports and valves. It can be compared to the lungs of a chain-smoker, but worse. The first time the driver may be aware of this problem is when the engine-check light comes on or they experience a dramatic reduction in performance. Try breathing through a straw and you get the idea. We investigate carbon build-up in GDIs, how it happens and whether you can avoid it.

1. Port vs. direct injection

The fuelling technology of a petrol engine has transformed dramatically in the last couple of decades. From the humble mechanical carburettor to single-point injection, followed by multipoint and now direct injection. The enabler for the rapid evolution is the electronic control unit (ECU) and the goal is to keep increasing efficiency and power while lowering emissions.

Multipoint injection

Multipoint refers to the change from a single-point injector, where the carburettor was located feeding all the intake runners to the cylinders, to an injector per intake runner. The idea is to have the injected fuel closer to the combustion chamber and so reduce response time and provide accurate fuelling for each cylinder in the correct stoichiometric air-fuel ratio. Another advantage is the fuel has enough time to mix with the incoming air before entering the combustion chamber. (Unknown at the time, the detergent properties of petrol helped to keep the intake runners and the intake valves clean from sooty deposits.)

A negative of multipoint injection is the cooling effect of the evaporating fuel (latent energy) cools only the intake port and valves, and does little in cooling the intake charge on the way to the combustion chambers.

Direct injection

If it is beneficial to have the fuel injection as close as possible to the combustion chamber, why not have it in the chamber itself? This is why direct injection was developed where the injector tip protrudes into the combustion chamber. Response time is now increased but the amount of time for mixing air and fuel is decreased. Therefore, injector pressures are much higher and clever “tumble and swirl” techniques are used to move the intake air swiftly around the combustion chamber to promote air-fuel mixing. The main advantage is the latent energy of the evaporating fuel now cools the charge in the combustion chamber, combating the onset of auto-ignition (knock) and allowing a much higher compression ratio that increases efficiency. This is especially true when it comes to turbocharged engines where the intake air is still hot after being compressed, even after the intercooler. Note the fuel does not pass the intake valve, which is relevant to the carbon build-up problem as the cleaning effect of petrol is lost on these surfaces.

What causes carbon build-up?

Carbon is the main element found in fossil fuel and oil. Burning either causes a sooty residue. For carbon build-up to form, deposits need to be present near metal surfaces at the correct temperature. Exhaust valves run much hotter than intake valves and burn off the carbon before layers can start to form. This is not the case on the intake side. But how does carbon get to the intake side when only clean air is digested? The next section explains the paths to the intake ports for sooty particles.

2. Blowby

The engine crankcase runs mostly at positive pressure, as some of the combustion gases leak past the rings and end up pressurising the cavity below the pistons. The spinning crank and conrods also do a good job of promoting airflow in the crankcase. This air pressure is controlled by a pressure control valve (PCV) venting excess pressure. The oily gases cannot be vented to the atmosphere (owing to emissions control), so it is piped back into the intake to burn in the following combustion cycles. This enables the oily substance to cling to the intake port runners and valves.

3. Exhaust-gas recirculation (EGR)

The idea of the EGR valve is to route some of the burnt combustion gases back to the intake side, for the main reason to lower NOx emissions. The principle is burnt exhaust gases are inert and do not contribute to the next combustion events during part-load running and, therefore, lower the peak combustion temperatures (and NOx emissions). The problem is carbon particles are plentiful in spent combustion gases and the stream of exhaust gas does a pretty good job fouling the intake runners and valves.

4. Valve overlap

More commonly employed in naturally aspirated as opposed to turbo engines, this technique keeps the exhaust valve open during part of the intake stroke (intake valves also open, i.e. valve overlap) to allow the remaining exhaust energy going down the exhaust pipe to help suck in the fresh intake charge for increased volumetric efficiency (or cylinder filling). The disadvantage is some of the exhaust-gas particles have time to swirl out of the intake valve and attach to metal at optimum temperature for carbon build-up.

5. Oil leaking past valve-stem seals

There is always a tiny amount of oil which seeps past the valve-stem seals and onto the shaft of the valves. This problem gets  worse in high-mileage vehicles and the carbon build-up accelerates.

6. How to prevent and remove carbon build-up?

Ensure your car’s servicing is up to date because wear on the engine accelerates carbon build-up (increased blowby and oil passing valve-stem seals). Most of the carbon build-up forms during part-load driving. If the vehicle is used mostly in town, take it on the open road once in a while. Use the best quality oil specified for the powertrain, as the high-grade oils come with a cleaning additive to prevent the build-up phenomenon.

If your engine is already in need of intake and valve cleaning, take it to a specialist. For minor build-up issues, a solvent and brush can be used but major carbon deposits require the head to be removed and the carbon blasted with crushed nut shells so as not to damage the aluminium-alloy. 

A saving grace for the latest GDI engines is many manufacturers are opting for both port and direct injection to have the best qualities of both systems during part-load and full-throttle operation. This means a double set of injectors per engine but at least the fuel flow over the intake valves is returned during part-load conditions.

Why not diesel?

Diesel engines are also directly injected and employ similar technologies such as EGR. Although carbon build-up does occur, it is by no means of the same magnitude as found in GDI engines. The reasons are:


For carbon build-up to occur, the temperature of the metal surfaces must be in a sweet-spot range. Too hot and the carbon burns off; too cold and there is less adhesion. Diesel engines run unthrottled (excess air); therefore, the intake valves are cooler than in petrol engines. Because of the excess air and longer expansion stroke of a diesel (higher compression ratio), the exhaust gases are cooler, too.

Little intake vacuum

The throttle employed in a petrol engine causes a vacuum in the intake manifold under part-load conditions. This accelerates the movement of carbon particles to this region. Diesel engines run unthrottled and do not have this issue.

Is carbon build-up an engine killer?

In most cases, the engine may only experience a loss of performance as the intake air is restricted. In a worst-case scenario, the sludge can cause plastic swirl flaps to break inside the intake, leading to parts being digested by the engine. Harder carbon flakes can break off and enter and exit the combustion chamber and make their way to fast spinning turbo blades, which will have disastrous consequences.