Tech tips come from a variety of sources, and some are certainly better than others. And as the technical editor for the magazine, I get them fairly often.
Recently, my friends at Bosch sent me one regarding the increasing problem of carbon build-up in GDI (Gasoline Direct Injection) engines that I knew I had to share. So, with all due credit to Stephen Albert, Product Manager – Port Fuel and Gasoline Direct Injection Systems for Robert Bosch, LLC, here it is!
GDI engines typically build carbon on the back of the intake valve (!) and along the intake runner (2). (Photo Courtesy: Bosch)
Addressing Carbon Buildup in GDI-Equipped Vehicles
Driven by strict emission laws and a growing demand for low fuel consumption, an increasing number of vehicles equipped with gasoline direct injection systems (GDI) are now being seen on roads today. However, a common complaint about these vehicles has been the buildup of carbon deposits inside the cylinder head.
What causes carbon deposits in GDI-equipped vehicles?
The cause of carbon buildup can vary by vehicle make, as engine management systems can mitigate the carbon buildup in certain vehicles by something as simple as an ECU update. Essentially, carbon buildup occurs due to a combination of low quality fuel, short, frequent cold weather trips or positive crank case ventilation, which may leave an oily layer around the intake valve. These factors, in conjunction with the operating temperatures of the engine casing, lead to a buildup of carbon deposits.
Timing may also have an effect if the intake valve stays open long enough to be exposed to unburned fuel. Finally, the location of the GDI injectors in the cylinder head plays a significant role in carbon buildup because it does not allow for the fuel sprays to reach the back of the intake valve, making it difficult to keep the intake runners and valve clean.
What is the impact of carbon buildup on drivability, performance, emissions and fuel economy?
With regard to drivability, one of the more common symptoms is a lack of power, especially when driving at higher speeds. Other symptoms could include cold stalling and cold-start difficulties, including rough running when the engine is cold. There is also the possibility of a failed NOX emissions test and/or excessive ping on acceleration.
In some cases, carbon buildup may cause one or more cylinder misfire codes (P030X), which will result in fuel inefficiency and poor performance. This occurs when an insufficient number of cylinders fire while the vehicle is in operation.
If the presence of carbon deposits is not detected early enough or the buildup is severe, the check engine light may turn on and the vehicle is likely to fail inspection.
How is soft carbon different from hard carbon? How long does it take for soft carbon to turn into hard carbon?
Soft carbon is sooty and more easily dissolved than hard carbon and can be remedied with additives, while hard carbon is baked on and is more labor intensive to address. Hard carbon requires manual cleaning such as media blasting.
How long it will take the soft carbon to turn to hard carbon will depend on the motorist’s driving habits and the initial cause of the buildup.
What steps can technicians take to remove soft carbon and/or hard carbon?
Soft carbon can be addressed by following the manufacturer’s preventative maintenance schedule along with a fuel additive that may be added to the vehicle’s fuel system before warm up. The technician can also utilize a fogger system via the throttle body. The fogger is connected to a pressurized dispenser filled with a chemical blend formulated to dissolve baked-on carbon deposits from the valves and passageways.
For hard carbon, the process is more involved. The most common procedures call for removing the intake manifold to expose the head’s ports, and rotating the engine by hand in its normal direction of rotation so that the intake valves for one of the cylinders are completely closed, then utilizing some form of media blasting.
Can checking certain components when a car comes in for service help to avoid carbon buildup? For example, the high pressure fuel pump, pressure sensor, etc?
Checking spark plugs is a great way to start because it may help to reduce the amount of unburned fuel in the combustion chamber. Follow the regular maintenance schedule. Look for telltale signs for product failure such as the check engine light and poor vehicle performance.
Also, oil changes are extremely important. Customers should always use the manufacturer’s specified oil to prolong the life of GDI components such as the high pressure fuel pump. If the wrong oil is used or the vehicle does not follow the manufacturer’s recommended oil change intervals, it may result in the failure of the high pressure pump or of other GDI components.
What should technicians do to prepare to service and repair GDI systems?
Technicians should educate themselves on the vehicle manufacturer’s maintenance schedule to ensure prolonged life of GDI system components. They should especially learn to identify system failure symptoms so they can be proactive in sharing the information with their customer when servicing these vehicles. Learning about the tools needed to service these vehicles is also important. At the end of the day, technicians are savvy and will build on the principles they already know. By doing so, they will simplify their approach to servicing a complex system.
Should a technician educate customers on the nature of the GDI system and guide them in taking the right steps to avoid buildup of carbon deposits?
Yes, I would highly recommend that technicians share the knowledge with their customers that bring in GDI-equipped vehicles for service. Technicians can advise them to follow the manufacturer’s schedule for service, and take a few proactive measures to have their vehicle inspected between services. GDI is a great technology and if serviced properly is nothing to be intimidated by, as a consumer or as a technician. In fact, techs can help their customers enjoy driving a GDI-equipped vehicle.
For information on GDI, visit: http://www.boschautoparts.com/auto/fuel-injectors/gasoline-direct-injection.
GDI, or gasoline direct injection, is not a new idea. It was first tried in 1925. As its name implies, GDI differs from multiport injection in several substantial ways. First, and most noticeable, is the placement of the fuel injector directly into the combustion chamber.
Clayton Lindgren, product manager and technical specialist for Bosch Engine Systems, explains it this way, “The main differences comparing port fuel injection to gasoline direct injection are with respect to system pressure, injector location, injector timing and fuel spray. Port injectors are exclusively coil driven and utilize a system pressure of 300 to 500 kPa (kilopascals, or roughly 40 to 75 psi for us non-engineer types). They are located in the intake runner.”
“In the time domain there is a single injection event. Their spray pattern is conical and always targets the intake valve(s),” Lindgren adds. “On the other hand, GDI injectors may be coil or piezo driven. These injectors have system pressures of 1-20 MPa (Megapascals, approximately 150-3000 psi). The higher pressure increases mass flow and creates a finer fuel mist, both of which contribute to the trend of engine downsizing while retaining power output.”
That kind of pressure can seriously hurt you if you don’t follow some safety guidelines. When troubleshooting, servicing or repairing GDI components, keep these tips in mind.
- Always allow the engine to cool before servicing a GDI fuel system.
- Always read and follow the OE specific service precautions and recommendations.
- Always disconnect the negative battery cable prior to performing fuel system service. Some GDI equipped vehicles cycle the in-tank fuel pump to cool down even when the vehicle is off and the key is out of the ignition. This strategy is used to maintain a minimum fuel pressure to assure acceptable start times.
- Always follow the manufacturer’s recommendations regarding the re-use or replacement of high pressure fuel lines.
- Always replace the fuel injector O-rings and tip seals after an injector has been removed. Be sure to install the O-ring and back-up ring in the proper locations (back-up ring below O-ring).
- Use a torque wrench to tighten all fuel fittings to the values in the service manual.
- When evaluating fuel system pressure, use the output from the fuel pressure sensor on the fuel rail as instructed in the service information. Do not attempt to add a fuel pressure gauge to the high pressure side of the fuel system.
Got a tip or case study you’d like to share? Send it to me at [email protected]. See you next month!