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Gasoline Direct Injection (GDI) is used on a variety of late model engines: Audi, BMW, GM, Ford, Hyundai, KIA, Lexus, Mazda, MINI, Nissan, Porsche, VW and others. GDI sprays fuel directly into the combustion chamber under high pressure, rather than spraying fuel under low pressure into the intake ports in the cylinder head. GDI increases fuel economy and power 15 to 25 percent, but there is a downside that is now becoming apparent as these engines accumulate miles. For more information on this subject, see Gasoline Direct Injection (GDI).
The problem is carbon deposits are building up on the inlet side (top) of the intake valves. The deposits create turbulence and can restrict airflow into the cylinders causing performance and driveability problems such as hesitation, stumbling, misfiring, even hard starting. The thicker the carbon deposit buildup on the valves, the worse the driveability problems.
GDI sprays fuel directly into the combustion chamber so the fuel completely bypasses the intake valves. Consequently, detergents and cleaners that are added to gasoline to prevent intake valve deposits from forming in port fuel injection engines never have a chance to do their job in a GDI engine. The inlet side of the intake valves are never in direct contact with the fuel so the detergents cannot wash away the deposits. Because of this, fuel detergent additives that are either in gasoline from the refinery or are added to the fuel tank have almost no effect on preventing or removing intake valve deposits in GDI engines. The additives work in regular fuel injected engines, but not GDI engines.
Intake valve deposits form as a result of oil slowly seeping past the intake valve guide seals and down the valve guides. A tiny amount of oil is necessary to lubricate the guides, but when oil reaches the hot surface of the valve, it can stick and burn forming heavy black carbon deposits that gradually build up over time. The higher the mileage on the engine and the greater the wear in the valve guides and seals, the faster the accumulation of black carbon deposits on the intake valves. Low viscosity motor oils (such as 5W-20 and 0W-20) may make the problem worse because they are thinner (to reduce friction) and flow more easily down the valve guides. Conventional motor oils also have a lower flash point than synthetic oils, which can also increase the formation of deposits over time.
In 2020, motor oils were reforumulated to reduce the buildup of deposits on intake valves in late model engines with Gasoline Direct Injection. Motor oils that meet the new deposit-reducing specifications will have an American Petroleum Institute (API) rating of "SP".
Motor oils that meet the International Lubricant Standardization and Approval Committee (ILSAC) specifications were also revised in 2020 to reduce intake valve deposits in GDI engines. Motor oils that meet these new specifications are rated "GF-6".
The point here is that if your vehicle has an engine with Gasoline Direct Injection, or a dual injectin system (GDI and port injection), you should be using a motor oil that meets the new "SP" or "GF-6" specifications to reduce the risk of intake valve deposit buildup.
Another contributing factor to the formation of intake valve deposits is unburned fuel vapors and oil vapors being siphoned back into the intake manifold through the Positive Crankcase Ventilation (PCV) system. This is done to control crankcase emissions and to remove moisture from the oil (which helps prolong oil life). The fuel vapors, carbon particles and oil droplets that the PCV system routes back into the intake manifold are reburned in the engine to reduce pollution. But these same vapors can also form carbon and varnish deposits on the intake valves.
The more blowby an engine has due to cylinder and piston ring wear, the greater the volume of crankcase vapors that are pulled back into the engine by the PCV system. High mileage engines typically have more blowby than low mileage engines, so the build up of intake valve deposits is usually faster.
An engine that is experiencing driveability and performance problems as a result of intake valve deposits may or may not set any Diagnostic Trouble Codes (DTCs) and turn on the Check Engine light. If the engine is misfiring bad enough, it may set a P0300 random misfire code or individual cylinder misfire codes. However, many other factors can also set misfire codes so a misfire code alone is not necessarily an indication the engine has dirty intake valves.
You cannot see intake valve deposits directly because the valves are inside the cylinder head. The only way to see deposits on the intake valves is to remove the intake manifold and peer into the intake ports in the cylinder head - unless you have a fancy tool like a boroscope or a fiber optic video camera that can be inserted into the combustion chamber through the spark plug hole or snaked down the intake manifold to inspect the valves. Few automotive technicians have this kind of equipment and probably wouldn't use it even if they had it because they would proceed with the assumption that the valves are dirty and need to be cleaned.
How fast the intake valves get dirty does not seem to be a function of fuel quality or how much ethanol alcohol is in the gasoline. Rather, it appears to be influenced most by how often the engine oil is changed. Oil vapors and combustion byproducts that are drawn back into the intake manifold through the PCV system seem to contribute most to carbon deposits on the intake valves.
My advice is to change your oil every 3000 miles if you only do short trip stop-and-go city driving, or change your oil every 5000 miles if you do mostly highway driving. If you want to minimize carbon buildup on the intake valves, don't push your oil change intervals to 7500 miles or longer unless you are using a high quality full synthetic oil (which usually has less volatility than conventional motor oil). Also, use a motor oil that meets the latest "SP" or "GF-6" specifications to help keep your intake valves clean.
Changing your oil regularly will help minimize the carbon buildup on the valves, but eventually they may still get dirty. If that happens, it may be necessary to clean the valves every 25,000 to 30,000 miles with an aerosol cleaner that is sprayed into the intake manifold.
If you think the intake valves on a GDI engine are dirty but you do not want to go though all the work of removing the intake manifold and cylinder head, you can attempt to clean the valves using the following procedures:
Get a bottle of liquid engine top cleaner, intake system cleaner or carburetor cleaner (such as Sea Foam), or a specialized product such as CRC GDI Intake Valve Cleaner or BG Gasoline Direct Injection Cleaner to clean the intake valves. Follow the directions on the product, or proceed as follows:
With some products, the cleaner is sprayed into the throttle body while the engine is running. Others recommend disconnecting the PCV hose from the PCV valve, or using any other large vacuum hose that connects to the intake manifold so you can slowly pour the cleaner into the hose while the engine is running (you will probably need a small funnel for this). Run the engine at fast idle (say 1000 to 1500 RPM) while feeding the cleaner into the intake manifold.
Depending on how dirty the intake valves are and how effective the cleaning chemical is, the process may take 10 to 20 minutes or more to remove the carbon deposits. You may also have to repeat the cleaning process more than once to completely remove the deposits.
If this cleaning process fails to do the job because the carbon deposits are so thick, you may have to try a more direct cleaning approach. This requires removing the intake manifold so the cleaner can be applied directly to the valves. You may need to refer to the factory service information for detailed step-by-step removal procedures for the intake manifold.
CAUTION: If you have to disconnect any fuel lines to remove the intake manifold, make sure all residual fuel pressure inside the lines has been relieved before opening any lines.
Once the intake manifold has been removed, look into each port to see which valves are closed and which valves are open. The cleaning process will start with all of the valves that are CLOSED. Once those valves have been cleaned, rotate the engine to close the remaining valves that were open. The reason you want the valves closed when you clean them is so the cleaning chemical and carbon deposits don't fall down into the engine's cylinders.
Use an aerosol product that can loosen and remove carbon such as brake cleaner (CRC Green works well), Sea Foam or Intake Manifold Cleaner on the intake valves.
Spray the cleaner directly into the intake port so that it puddles on top of the valve. Let it soak for about 30 minutes to loosen the deposits. You can also use a small brush or pick to scrape at the deposits while the cleaner is working. After 30 minutes, soak up the cleaner residue with a rag or paper towels. After the cleaner has completely evaporated and the carbon residue has dried, you can use a shop vacuum to suck the debris out of the ports.
CAUTION: Do NOT use a shop vacuum if any liquid solvent is still present in the intake ports. Most areosol solvents are highly flammable, and might explode if ignited by a spark from the vacuum's electric motor. Also, do not smoke will using a flammable aerosol cleaner! And make sure there is adequate ventillation because solvent fumes can be toxic.
Now rotate the crankshaft to close the remaining valves that are open, and repeat the cleaning process as needed on the other valves that are now closed until all of the valves have been cleaned.
If the intake valve deposits are so thick and hard that chemical cleaning does not work, you can try blast cleaning the valves with an air gun blaster and soft blasting media such as walnut shells, baking soda or plastic beads. Seal or tape all other openings on the top of the engine so blast media and residue can't get into the crankcase, coolant or oil passages. The blast residue can then be sucked out of the intake ports with a vacuum once the valves have been cleaned.
CAUTION: Make sure the valves are CLOSED before spraying blast media into the intake ports, and NEVER use any kind of hard blast media such as sand (silica), glass bead or metal beads as these can cause severe damage to the rings and cylinders if any blast media gets past the valve.
If all else fails, the last resort is to remove the cylinder head, disassemble all of the valves and clean them by hand with a wire brush, bead blaster or other blast media, or to soak the valves in a hot tank or ultrasonic cleaning tank.
Some car dealers want to replace the entire cylinder head with a new one if the valves are badly carboned up. But this is an unnecessary expense because in most cases the valves can be disassembled, cleaned and reassembled in the existing cylinder head once the head has been removed from the engine. Disassembling a cylinder head and cleaning the valves takes more time and labor, and may require some special tools such as a valve spring compressor, gear puller or overhead cam removal tools. But it can save the cost of replacing the entire head. The only time head replacement would be recommended is if the engine has a lot of miles on it (say well over 100,000 miles) and the head has other problems such as worn valve guides and/or seats, cracks or other damage.
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