Rick's Free Auto Repair Advice

Carbon buildup in Gasoline Direct Injection engines

What causes carbon buildup in Gasoline Direct Injection engines

What is Gasoline Direct Injection?

Gasoline direct injection (GDI) can cause carbon buildup problems. It differs from traditional multiport fuel injection (MFI)in several ways. First, the tip of the fuel injector is located right in the combustion chamber, so the injector is subjected to very high compression and combustion pressures. That’s different than an MFI injector that’s located in the intake manifold and subject to only manifold vacuum.

An MFI injector is coil driven and releases fuel at approximately 50 to 75-psi. in a conical pattern. An MFI injector can inject gas at any point in the 4-stroke cycle because it relies on the intake valve to time the delivery of the fuel into the cylinder. GDI injectors, on the other hand, can only inject fuel during the intake and compression stroke. This reduces the amount of time available for the fuel to vaporize.

GDI injectors also operate at much higher pressures. The fuel rail provides fuel at 150 to 3,000-psi. The high pressure is needed to counteract the high compression and combustion cylinder pressures. The much higher pressure results in a finer mist that more rapidly vaporizes, causing an evaporation “cooling” effect that helps prevent fuel detonation.

Rather than relay on a coil and pintle, GDI injector utilize piezo technology. A stack of piezo crystals are located in the center of the injector. When energized, the crystals change size, opening and closing the flow of fuel. This allows the GDI injector to open and close up to five times faster than a solenoid style MFI injector.

In addition, GDI employs several injection events.

Cut away illustration of a fuel injector used on an engine with gasoline direct injection

Piezoelectric GDI injector

The injector can be commanded to inject a certain amount of fuel at a low pressure during the intake stroke and then inject more highly pressurize fuel during the compression stroke.




carbon accumulation of valves

In an MFI system, the injector sprays fuel onto the back side of the intake valve

gasoline direct injection

In a GDI system, the fuel sprays directly into the cylinder

What causes the gasoline direct injection carbon buildup?

At shutdown, any remaining fuel and

dirty valves

Buildup on the back face of an intake valve

oil vapors rise into the intake manifold through open intake valves. As the vapors cool, they form carbon deposits on the back face of the intake valves and the throttle plate and throat of the throttle body. In an MFI system, the detergents in the fuel constantly cleans the backside of the intake valves.

Since GDI systems don’t have that cleaning feature, carbon can build up to the point where it causes hard starting and misfires. At that point, a decarbonization procedure is required to remove the deposits.

Carbon problems vary by engine design. To combat this buildup, engineers have had to alter GDI spray patterns, spray timing and, in some cases, even intake valve design. Car makers have also made significant changes to the positive crankcase ventilation (PCV) system to scavenge more crankcase blow-by and prevent those oil and fuel vapors from depositing in the intake manifold. Plus, they’ve had to change piston ring tension to reduce blow-by in the first place.

GM has addressed the buildup problem by altering fuel injection timing and valve opening timing. If too much fuel is injected during the intake phase when the intake valve is open, the high cylinder temperature can cause the fuel to contact the intake valve and form soot. Once soot accumulation starts, it builds rapidly. So GM changed valve and GDI injector timing to reduce the contact time and prevent soot formation.

The industry reports that GM and Ford engines are among the least affected by carbon buildup problems in their GDI engines.

On other brands, the amount and severity of buildup is directly related to the quality of the fuel, driving habits and engine operating temperatures. GDI engines that encounter lots of short trips and cold starts tend to have more problems with carbon buildup.

What are the symptoms of carbon buildup?

• Lower power due to decreased airflow past the buildup on the intake valves.

• Lower MPG due to lower airflow efficiency

• Poor acceleration power.

Fuel induction service and decarbonization services

Once the buildup has reached the point where it affects engine performance, a fuel induction service or decarbonization service is required. In severe cases, the cylinder head must be removed and manual cleaning is required.

Fuel induction service involves the carbon buildup in engineintroduction of a solvent vapor through the air intake system with the engine running. The solvent attacks the buildup on the back face of the intake valves.

Two cleaners can be used and both can be applied by a DIYer. One cleaner is GM’s Top Engine cleaner (ACDelco Upper Engine & Fuel Injector Cleaner 88861803 –CLEANER). The cleaner is available as a liquid or spray can. The other is CRC’s Intake Valve and Turbo Cleaner.

DIYers can perform this service on their own. In conventional parlance, DIYers often refer to this procedure as Seafoaming their engine. The typical Seafoam procedure involves pouring Seafoam directly into the vacuum line to the vacuum brake booster. Because Seafoam contains combustible solvents, that is NO LONGER recommended by car makers because it allows unmetered air into the engine. The PCM/ECM immediately responds to the unmetered air by boosting fuel trim, resulting is excessive dumping of unburned fuel and Seafoam combustible solvents directly into the catalytic converter. That excess fuel can result in cat converter temperatures in excess of 2,000°F, causing catastrophic converter meltdown failure.

See this post for more warnings about Seafoaming your engine.

The GM and CRC cleaners should be injected into a vacuum line in short bursts with the engine running at 2,000 RPM. After you empty the container, turn off the engine at let it heat soak for at least one hour. Heat soaking is VERY important to the success of the cleaning. After an hour, drive the vehicle at highway speeds for at least ten minutes and drive the vehicle every day for approximately six more days.

Should you agree to routine Fuel Induction Service?

Most car makers do NOT recommend routine fuel induction service. In fact, their factory service bulletins warn dealers not to perform these cleaning services as a routine “recommended service.” In the car repair industry, these “dealer recommended” services are referring to was “wallet flushing services,” since they benefit only the dealer.

Fuel induction cleaning should be performed only on certain engines where buildup is diagnosed as the cause of engine performance issues. Then fuel induction cleaning is justified to remove the carbon buildup and restore the engine to original performance.

See this post for more information on fuel induction cleaning.

See this post for more information on fuel injector cleaning

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat

Custom Wordpress Website created by Wizzy Wig Web Design, Minneapolis MN