Anyone familiar with the internal combustion engine understands that these devices produce carbon. This is a result of using hydrocarbon fuel stocks and lubrication oil within the engine. There are many different types of fuels currently used in the U.S. and aboard; however, the two primary fuel stocks used in the U.S. for on-highway transportation are gasoline and diesel. Either of these fuel stocks will produce carbon as a result of the combustion process within the cylinder.
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The fuel is comprised of chains, rings and branches of hydrogen and carbon. When fuel reacts with oxygen during the combustion process carbon and hydrogen atoms from the fuel disassociate from one another and form new chemical bonds with oxygen. Hydrogen atoms react with oxygen to form dihydrogen monoxide (H2O — water), and carbon atoms react with other oxygen to form carbon dioxide (CO2). If the amount of hydrogen, carbon and oxygen atoms are not in the exact ratio to complete these reactions then some hydrocarbons are not completely combusted. The hydrocarbons that do not combust or do not burn completely either stay as hydrocarbons or form other chemical compounds such as carbon monoxide (CO).
When an organic compound, such as a hydrocarbon-based fuel, has a combustion reaction it produces heat. If there is a lack of oxygen during the burning of the fuel then pyrolysis occurs, which is a type of thermal decomposition that occurs in organic materials exposed to high temperatures. Pyrolysis of organic substances, such as fuel and oils, produces gas and liquid products but also a solid residue rich in carbon. Heavy pyrolysis leaves mostly carbon as a residue and is referred to as carbonization. Pyrolysis can occur rapidly or slowly depending on the temperature. An example of slow pyrolysis is the formation of carbon deposits within the induction system of the engine. Lubricating oils and fuels accumulate in the intake system and, when exposed to heat over a period of time, pyrolysis bakes off some of these oils and fuels as light chemicals and leaves heaver chemicals. Over time this becomes heavy carbonization (carbon deposits).
Same but different
It is important to understand that the carbon produced within an engine is not all the same. The carbon in the combustion chamber is produced under high heat and high pressure. Due to the conditions within the combustion chamber the carbon produced is denser and has low porosity; additionally, the carbon thickness is usually low. The carbon that is produced within the induction system is created under very different conditions than the combustion chamber deposits. The carbon in the intake is produced under low heat and low pressure. Due to the conditions within the induction system the carbon produced has high porosity; additionally, the carbon thickness can be quite high. Thus, due to the conditions that they were produced under, these are two different carbon types.
Another way to produce different carbon types within the engine is the use of different fuel delivery systems. When fueling the engine with a carburetor or port fuel injection the fuel is delivered into the intake manifold of the engine, as illustrated in Figure 1. Thus, the carbon within the intake port area is constantly washed by gasoline. As you already know, gasoline is a very good cleaner and can wash oils and sludge off of parts. Gasoline can remove some of the carbon accumulation from the induction port as well. The gasoline being in contact with the carbon deposit as it is forming will also change the configuration of carbon bonds in the induction system’s carbon deposit.
On modern engines that incorporate the method of Gasoline Direct Injection (GDI), the fuel is delivered directly into the combustion chamber as illustrated in Figure 2. Therefore, there is no fuel available to wash the carbon deposit in the intake manifold, as occurs with the port fuel injection method. This creates a problem in that the carbon deposits will build without opposition. Additionally, the lack of gasoline within the induction system can create a carbon bond configuration that is again quite different. Under these conditions the carbon deposits can become quite large and create drivability problems. On some GDI engines these carbon accumulations that create drivability problems can occur in as little as 15,000 miles. The very design of the GDI engine leads itself to carbon deposit in the induction system. No GDI engine is immune from these inherent carbon deposits.
Some carbon deposits within the GDI intake port area can be as great as ¼- to ½-inch thick as shown in Figure 3. These heavy carbon deposits can cause problems such as; misfiring cylinder(s), hesitation during throttling, low power, rough idle, surging, pinging, fuel trim adaptions, high tailpipe emissions, MAF range or performance DTC and MAP range or performance DTC.
The effects of carbon build up
In order to know if there are carbon deposits in the induction system of the engine you are working on, visual inspection using a borescope is the preferred method. One can find an entry point through a vacuum port or by removing a sensor such as the MAP sensor, or IAT sensor. If these will not provide access, with the ignition key off, the throttle plate can be opened and the borescope can be fed through this opening.