My car has one of the first few direct-injection engines. At the time I bought it, the technology was claimed to be state-of-the-art, offering better power and fuel economy.

But I have recently discovered that the latest version of the car comes with a modified injection system that has an additional injector mounted in the intake passage of each cylinder. In other words, it has both direct and the traditional manifold injection. Does this mean that my car with only direct injection is flawed in some way?

With direct injection, the injectors are mounted in the cylinder head. They spray fuel into the combustion chamber. The advantage of this is that fuel can be metered more accurately, timed more precisely and the spray directed for maximum combustion efficiency.

Engines with direct injection are indeed more fuel-efficient and produce more power than an equivalent engine with manifold-injection. But there is a weakness with such a design.

At the end of combustion, there is a split second when the intake valve opens, allowing a small amount of blowback.

Over time, this causes a build-up of carbon around the stem of the intake valve. It sticks quite stubbornly to the valve and, ultimately, affects the air flow through the intake.

The usual symptom is unsteady idling but, more significantly, there will be an increase in fuel consumption plus a slight drop in engine performance.

To overcome this, engineers incorporated a secondary injector in the manifold, so air mixed with a small quantity of fuel flows past the valve stem, cleaning away any blowback deposits in the process.

Removing carbon on high-mileage direct-injected engines would become necessary at around 80,000km.

It may be possible on some engines to clean the inlet valve stems by removing the intake manifold and accessing the carbon through the port.

For a complete job, however, it would be better to remove the cylinder head and, while you are at it, clean the entire combustion chamber too.