CARBON DEPOSITS: Cleaning Up What's Left Behind

Motor, May 2008 by Thompson, John

Engine carbon deposits have a measurable effect on performance, emissions and fuel economy. Routine carbon cleaning has been shown to prevent these problems, and remedial cleaning removes more severe deposits that have already formed.

Many technicians and managers are well aware that severe combustion chamber carbon buildup can create significant driveability issues with todays engines. It's all too rare, however, for them to focus on the fact that carbon buildup and slowly deteriorating injector performance is a gradual process that not only affects engine performance but fuel economy as well. Combining the reality of today's high fuel cost with the fuel injector cleaning and decarbonization services your shop offers creates a genuine opportunity for preventive maintenance business. Despite the opportunity, the vast majority of these potential PM sales remain untapped.

The ever-increasing price of fuel in recent years has created a very emotional hot button. Increasing your share of profitable PM sales while saving your customers real dollars every time they pull up to a gas pump is truly a win-win proposition for all. Promoting carbon and injection cleaning services to the top of your PM sales focus will make real bottom line sense to both shop and customer.

Optimum cylinder combustion depends on the correct air/fuel ratio for engine operating conditions. With a stoichiometric 14.7 parts air to 1 part fuel, the fuel is the most variable and critical factor of the ratio. Fuel is supplied to the cylinders by the injectors. Each cylinders individual injector is not only required to deliver a specific and precise amount of fuel, but the fuel must also be in a wellatomized form. To maintain optimum combustion efficiency, the injectors must be operating very close to OE design specifications, and hard or active carbon deposits within the combustion chamber must be at a minimal level.

Fuel injectors are designed to operate through several billion cycles during their useful life. Even if a customer drives only 12,000 miles per year, each injector on the engine will need to pulse approximately 18 million times. That's a fantastic amount of use for any mechanical device. Despite this incredible load, most injector designs rarely fail due to mechanical or electrical faults. The most common problem relating to injectors is restriction. Even slight restrictions will skew both the injector's atomization quality and the fuel volume it's able to deliver at a given engine load and rpm.

Given time, contaminants in fuel tanks, fuel lines or the fuel rail-or even in the fuel itself-will always restrict injector flow; that's a fact. Foreign particles such as rust will also accumulate within the injector filter or fuel filters to effectively reduce fuel flow. Extremely small rust particles may even pass through the tiny injector filter itself, causing altered spray patterns as well as reduced injector volume; they may even prevent the injector pintles from seating properly (see photo 1 on page 50).

Whether a pintle is sticking on or off its seat, overfueling of cylinders will always occur. If an injector's pintle is off its seat, not only will the corresponding cylinder be flooded with fuel, but also the PCM (via O2 sensor feedback) will reduce fueling to other cylinders, causing a lack of performance (and a reduction in fuel economy), and creating the potential for engine, piston or ring damage. On the other hand, if a stuck pintle never opens, that cylinder will receive no fuel at all and the PCM will try to correct a lean bank issue by overfueling the rest of the cylinders on that O2 sensor bank. These scenarios are common on vehicles whose fuel systems have not been regularly maintained. Injectors need to be very clean for optimum system performance and fuel economy.

Although a PCM (in closed loop) can alter injector flow by reducing injector pulse width, it cannot control a single faulty individual injector. Just one inefficient injector will affect the overall performance and fuel efficiency of an engine. Aside from issues relating to fuel quality, the environmental heat injectors are subject to will invariably cause internal as well as injector tip clogging. Every day, unburned fuel additives adhere to injector pintles and orifices and will eventually alter injector flow volume and fuel spray patterns. After an engine is stopped, the injector tips become a heat sink and will bake residual fuel and/or fuel additives onto the nozzle tips. Eventually, this will cause such symptoms as lack of engine performance, leaking injectors and damage to other components such as O2 sensors and catalytic converters when multiple cylinders are overfueled to compensate for one or more underfueled cylinders as the PCM attempts to maintain stoichiomeby. But way before these issues become severe, a significant reduction in your customer's fuel economy will occur.

Part of the fuel injector's job is to atomize fuel by physically turning the liquid fuel supplied to the fuel rail into very tiny droplets. But in order for the fuel to be fully combusted and release as close to 100% of its energy as possible, it must be vaporized by the back of a hot intake valve. Only after vaporization can the fuel effectively mix with oxygen to form an efficient combustible mix. Even in a brand-new engine, total vaporization of fuel will never take place. Over time, the problem of inefficient atomization from restricted injectors will build carbon deposits on the valves. Because carbon deposits are a very poor heat conductor, the fuel vaporization process eventually will become less and less effective and, as a consequence, will reduce individual cylinder combustion efficiency, waste fuel, decrease performance and create undesirable emissions.