Maximizing GDI system efficiency

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Maximizing GDI system efficiency

Dieter Lorentz and Ryan Barr are Senior Training Instructors, Bosch Automotive Training

Engine management systems can be divided into two main categories: manifold or port injection, and gasoline direct injection (GDI) systems. These systems have many similarities, and major differences between the systems are primarily fuel delivery and control and operating pressures.

The need for higher fuel economy and lower emissions has been the driving force behind the changes to these systems since their inception. In the last six years a shift has been underway — many manufacturers are downsizing engine displacement and utilizing direct injection coupled with turbocharging to increase fuel economy without sacrificing engine performance.

Both of these systems use electronic throttle control, returnless fuel systems, and a majority of vehicles incorporate variable valve timing and lift and variable geometry intake manifolds. Manifold or port injection and direct injection systems utilize many of the same sensors to operate.

In the manifold injection system, the air/fuel mixture is generated outside the combustion chamber, in the intake manifold. The fuel injector sprays the fuel directly onto the intake valves where together with the intake air it is drawn into the cylinder.

GDI engines generate the air/fuel mixture in the combustion chamber. During the intake stroke, only combustion air flows past the open intake valve and into the cylinder. Using a high-pressure pump that is driven off of the camshaft, fuel is then injected directly into the cylinders under a variable high pressure, which can be over 2,000 psi.

In a typical manifold injection system, as the engine speed and load change, fuel pressure remains constant. The powertrain control module (PCM) will control the injectors for a longer period of time to allow for the additional fuel required. At full load, the on time of the injector is as much as 20 milliseconds long.

In a direct injection system, the length of injector on time as well as the pressure is controlled, and a GDI system does not utilize as much time to inject the same required amount of fuel as in a manifold system, so the open time of the injector is increased, and also the fuel pressure is increased using the high pressure pump.

Because of the higher pressures involved, the fuel rail and high pressure components are different. GDI systems use a low-pressure in-tank pump similar to a manifold type injection system, to supply the high-pressure pump with fuel.

Diagnosing and repair

Any problems associated with these systems are similar, and all of the basics apply.

Fuel delivery and volume, air leaks, etc., affect both of these systems. A good starting point, if there is a problem, is to check the scan tool data.

False air will have the same effect on either of these systems, causing drivability issues like hesitations or poor idle quality. Possible problems can show up as fuel trim issues like low fuel delivery due to a restricted fuel filter or line.


Many times technicians will pull a fault code P0170 (fuel trim malfunction) or P0171 (system too lean). These codes are universal in diagnostic information between manifold injection as well as gasoline direct injection. By monitoring the fuel trim values on a scan tool, the technician will be able to gain insight into what the PCM is correcting for.

Intake manifold leaks and other false air issues are best located by using a smoke tester, such as the Bosch SMT 300, to identify the leak.

On GDI systems, proper fuel pressure controls are required in order for the PCM to keep the engine in fuel control parameters, and low pressure can result in the above mentioned P0170 or P0171 OBD-II codes.

GDI high side fuel pressure readings, which can range between 500 to 2,150 psi, are made through the scan tool using information from the fuel rail pressure sensor.

CAUTION: Additional safety procedures are necessary when servicing a GDI system. The technician must ensure that all pressure has been reduced from the system. This can be done by removing the fuse for the low pressure fuel pump while the engine is running and waiting until the engine stops running while monitoring the fuel pressure reading on the scan tool to ensure the pressure has dropped in the system.

Software issues can also cause drivability problems. With any drivability issue, technicians should always check for vehicle OE manufacturer’s technical service bulletins (TSBs.) Many times problems can be resolved by simply updating the vehicle’s PCM to the latest software release, even if there is no TSB for the specific concern. By using a J2534 programmer such as Bosch’s Flasher Pro and visiting the OEM Web site, one can, for a fee, reflash the PCM and update it.

Both systems make use of variable valve timing to increase engine performance and lower emissions.

These systems can set camshaft sensor codes that can be related to lack of oil changes or improper oil types being used.

High mileage vehicles can have excessive slack in the timing chain and tensioners or have debris in the camshaft actuator and or passages. This can cause slow apply or cause the cam timing not to advance. Correcting this can be a matter of cleaning the oil passageways and control valves or replacing the worn-out components.

In addition, a technician can utilize a scan tool to manually advance the camshaft and see the corresponding change in values between the “desired camshaft timing” versus “actual camshaft timing.”

This is a quick test that can be performed before possible engine/camshaft actuator disassembly takes place.

GDI systems are also starting to show drivability issues related to carbon buildup on the intake valves, and this is causing hard starting and random misfire codes. Since the fuel is not sprayed on the backside of the valves where the detergents can do their job, deposits build up. This can be corrected by thoroughly cleaning the intake manifold and intake valves.

Adding a fuel injection cleaner to the gas tank will not solve the issue. Additionally there have been some reports of camshaft lobe failures due to the high loads the high-pressure pump places on the lobes. Using the correct grade and type of oil the vehicle manufacturer specifies is critical to ensure a long service life of all components.

Many customer complaints will be lack of performance followed by a check engine light.

A technician may pull several codes such as: P2293 (fuel pressure regulator performance), P0087 (rail pressure too low) as well as fuel trim lean codes.

These fault codes all indicate that a lack of fuel delivery is being provided to the fuel rail, most likely caused from excessive pump/camshaft lobe wear.

Modern fuel injection/engine management systems have become quite complex. The systems must perform many calculations and accurately control the engine for maximizing performance, reliability, and improved fuel economy, all while providing low emissions. Paying attention to basic service procedures, following the vehicle manufacturer’s recommendations and correcting any problems as they arise can keep these systems operating at peak efficiency.



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