Causes of carbon buildup in GDI engines

Jan. 1, 2020
Google “gasoline direct injection problems,” and it won’t take you long to see that many early adopters of this technology faced issues with severe carbon buildup occurring on the stem and throat of the intake valve.
Google “gasoline direct injection problems,” and it won’t take you long to see that many early adopters of this technology faced issues with severe carbon buildup occurring on the stem and throat of the intake valve. Narrow that search to just issues with carbon deposits, and you’ll notice that some makes seem to be more affected than others. What causes this build up, and what can be done to prevent it in those platforms that seem to be the most susceptible?

How GDI Works
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, tells Motor Age, “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.”

Safety First

Our experts offer these precautions when servicing/repairing GDI systems.

  • 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.

As for multiport fuel injection (MPFI) systems, they can inject at almost any time during the engine cycle, relying on the intake valve to time the delivery of the fuel/air mixture to the combustion chamber, adds Dave Sant, senior warranty engineer with Delphi’s Powertrain Systems.

“The injection window for a GDI system is limited to the intake and part of the compression stroke, which requires higher instantaneous injector flow rates and reduces the time available for evaporating the fuel between injection and combustion,” he states. “Fuel delivery is controlled by two key factors: injector pulse width and fuel pressure.”

Injector driver circuitry is vastly different as well. For most Port Fuel Injection (PFI) systems, the injector driver is simply an on/off switch that provides a ground path for the negative, or control, side of each injector coil which operates at 13.5 volts (system voltage), Sant adds.

“For GDI systems, the injector drivers control voltage and current. The GDI injector driver will provide a boost voltage of 50 to 65 volts DC along with a boost current of more than 10 amps,” he says. “The boost portion of the pulse only lasts for a few hundred microseconds, at which point the supply voltage returns to around 13. 5 volts and hold current drops to approximately three to four amps for the remainder of the injector pulse.”

This boost voltage is necessary to overcome the extreme fuel pressure the GDI system uses and to avoid lags in the injector’s opening response. Reducing the current flow during hold prevents overheating of the injector coil and allows the magnetic field to dissipate more rapidly at the end of the injector event, decreasing closing response.

Another type of injector is used in many GDI designs and features a design you may not be familiar with.

“Conventional injectors are dated by piezo technology,” says David Tenpenny, BG Products technical service advisor. “A piezo crystal stack located in the center of the injector changes in size when energized. This minute change in crystal size is used to accurately control fuel volume, spray application time and fuel pulse speed. Piezo crystal injectors can switch five times faster than a solenoid injector.”

Lindgren adds, “In the GDI time domain, there may be several injection events, delivering the correct amount of fuel at the exact moment needed throughout the combustion event. Piezo-driven systems have the fastest response time.”

The Carbon Issue
“GDI systems are showing drivability issues related to carbon buildup on the intake valves as early as 3,000 miles,” says Tenpenny. “This is causing hard starting and random misfire codes. Since fuel is not sprayed on the backside of the intake valves where fuel detergents can do their job, deposits build quickly.”

According to a U.S. patent application filed by Volkswagen in 2002 (and referenced by AutoObserver in June 2011), these deposits are described as a sticky coating of oil and fuel eleme
nts that serve as a base for future deposits, kind of like a stalactite in a cave. These deposits significantly reduce engine performance and can even lead to catalytic converter damage, or damage to turbochargers when bits of this hardened deposit break off and enter the exhaust.

As a side note, that is something you may want to keep in mind when trying to decarbon any engine that has excessive buildup. The pieces you break loose have to go somewhere.

But not all makes suffer equally from this problem. It seems the European makes have struggled more with this issue than the domestic manufacturers. Lindgren tells us that “some carbon buildup is inherent to all GDI systems. It is normal. There were some applications at the advent of GDI, which had more buildup than expected. This was mainly due to engine calibration, spray pattern design and perhaps to some extent the design of the valve seat.”
Not All Gasoline is Created Equal

Several automakers have cited problems with fuel quality as a reason for their product’s tendencies to collect carbon, and even specify in their owner’s manuals that only Top Tier fuel be used. What is Top Tier fuel?

According to the website, TopTierGas.com, “TOP TIER Detergent Gasoline is the premier standard for gasoline performance. Six of the world's top automakers (BMW, General Motors, Honda, Toyota, Volkswagen and Audi) recognize that the current EPA minimum detergent requirements do not go far enough to ensure optimal engine performance. Since EPA first established the minimum additive performance standards in 1995, most gasoline marketers have actually reduced the concentration level of detergent additive in their gasoline by up to 50 percent. As a result, the ability of a vehicle to maintain stringent Tier 2 emission standards have been hampered, leading to engine deposits which can have a big impact on in-use emissions and driver satisfaction.”

GM, BMW, Honda and Toyota developed separate standards for what could be considered Top Tier gasoline. Gasoline suppliers can choose to have their fuel certified to these standards and be listed as a Top Tier fuel source.

Tenpenny adds that several factors lead to intake port and valve deposits. “Oil vapor emissions from the PCV (Positive Crankcase Ventilation) system, variable cam timing allowing combustion gases to enter the intake system, and loss in piston ring elasticity allowing (blowby) to overwhelm the PCV system.”

This seems to hold some weight, and might explain why some models have more rapid buildup than others. Tony Chick, principal engineer for European Performance Labs in Stratford, Conn., is referenced in the AutoObserver report as saying that the base problem for those engines most severely impacted can be traced to their breathing systems. Some crankcase ventilation and exhaust gas recirculation systems are just “dirtier” than others.

GM also seems to think that deposit buildup is more likely when fuel timing allows fuel to contact the intake valve in the combustion chamber. This forms a soot that sticks to the valve, and once that initial coating is created, additional formation becomes that much easier.

Ameer Haider, GM’s assistant chief engineer for V6 engines, said in the AutoObserver report, “Our intake cam timing, injector targeting and timing of the injection events are optimized to avoid direct fuel contact on the intake valves.”

Stephen Russ of Ford added, “The technology of injection components, particularly the high pressure solenoid injectors, has quickly matured and excessive valve deposits…should become a thing of the past.”

Bosch’s Lindgren agrees. “As time has progressed, these and other parameters have been optimized and excessive carbon buildup is not much of an issue today (when) compared to the first systems,” he says.

GM and Ford, by the way, seem to be among the least affected by these issues, considering the minimal number of comments posted on Internet enthusiast forums.

Tackling Carbon Buildup
So, what are we as technicians supposed to do? “From the standpoint of the technician, there is nothing that can be done to minimize the buildup of carbon deposits,” says Lindgren.

Volkswagen, which has seen its share of carbon problems in their VW and Audi platforms, tells Motor Age that buildup is a function of fuel quality, driving habits, engine operating temperatures and other parameters. That makes some sense, especially in those instances where the driver is making lots of short trips and has no Autobahn close by. Mitsubishi Motors, the first company to offer GDI as a production car in Japan, told us that fuel quality in the U.S. is one reason they never offered the powerplant here. In fact, they are out of the GDI market entirely — for now. Other makers (Toyota’s D4S, for example) are moving to a marriage of GDI and port injection. It’s a GDI design, but adds an injector to the intake tract. After all, there is no better solvent than gasoline.

As a preventive measure, most of our experts agreed that keeping up with routine maintenance and using quality fuel is about all a consumer can do to minimize buildup. VW even shared the recommendation that the periodic use of a quality injector cleaner would be a good idea.

Deposit levels that impact drivability can be identified using a borescope or by removing the intake so the valves can be visually inspected. Volumetric efficiency testing can provide clues to buildup issues, because the presence of the deposits reduces airflow into the combustion chamber.

“The only correct way to thoroughly clean these deposits is to remove the intake and manually clean (the valves),” Tenpenny says. BG Products offers a GDI cleaning tool and specialized cleaner, and also recommends the use of its BG 44K® and BG MOA® at every oil change. You can find out more about BG’s recommended procedures and products at www.bgfueltest.com.

Cylinder head removal/disassembly might be the only solution for engines exhibiting serious deposit levels. Stay away from traditional cleaning methods, though. You can’t perform a traditional injector cleaning, and the use of conventional cleaners applied through the manifold might result in deposit chunks that could damage turbocharger vanes and catalytic converter substrates. Of course, be sure to check the OEM’s service recommendations and Technical Service Bulletins (TSBs) for model-specific information.

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