The first GDI engine to reach production was introduced in 1925 as a low-compression truck engine. Several German cars used a Bosch mechanical GDI system in the 1950s. However, it wasn’t until Mitsubishi began using them for production cars beginning in 1996 that the technology began to become popular among the OEMs.
Today, over half of the vehicles being produced are GDI designs.
As the name implies, GDI systems inject the fuel directly into the combustion chamber.
As a result, GDI does not have the valve cleaning action that port injection provides when fuel is introduced to the engine upstream of the cylinder. This lack of a cleaning action can cause carbon deposits to build sooner in the intake tract and especially on the back of the intake valves, with some makes more prone to this issue than others. As the deposits accrue over time and mileage; the engine’s ability to breathe can be affected.
The carbon also acts as an insulator, so the valves can heat up and fail. Over time, the carbon can fall off the valves and make its way to the bottom end of the engine, causing wear and damage to the moving parts. Carbon deposits may even flake off and pass through the combustion chamber and into the exhaust. This can lead to blockage of or damage to the catalytic converter. If the engine is turbocharged, there is a chance the carbon could damage the turbine fins in the turbo.
And in some instances, carbon that gets trapped between the piston and cylinder wall can lead to a condition known as low-speed pre-ignition (LSPI). LSPI is an abnormal combustion event caused by the higher cylinder pressures common in turbocharged GDI engines running in low-speed, high-torque conditions. In these instances, the heated carbon debris can act as the source of ignition, altering timing and often resulting in piston damage.
The problem tends to be worse in GDI engines that are used mostly for short trips because the intake valves never get hot enough to burn off the deposits. European makes that use a breather-style crankcase ventilation system also appear to have more problems than other manufacturers using a Positive Crankcase Ventilation or PCV system.
Symptoms related to carbon buildup include:
- Loss of power especially when driving at a higher speed
- Poor acceleration
- Cold stalling
- Engine misfires
- Reduced fuel efficiency
- Check engine light turned on
- Rough running
- Engine judder at idle speed
There are a few methods you can use to confirm that excessive carbon build-up is the cause of your customer’s driveability problem, and I’ll show them to you in this edition of “The Trainer”.
