Many service people believe the war with serious *carbon contamination has been won, but fuel *engineers and driveability specialists disagree. They said the problem has just moved downstream from the intake valves into the combustion chambers. Besides the familiar symptoms of spark knock (pinging), dieseling and rough idling, combustion chamber carbon also causes knocking noises when the engine is cold.
Plus, two other factors complicate the situation for both technicians and motorists. First, combustion chamber contamination is no longer a problem only associated with higher-mileage engines or engines with mechanical problems. Carbon accumulation may become a problem on low-mileage engines in good condition.
Second, combating deposit formation in some engines may require using a fuel additive on a regular basis.
Gradual carbon deposit buildup on intake valves and inside the combustion chamber is a normal occurrence. Historically, carbon accumulations only became a problem on higher-mileage engines. Or a relatively obvious mechanical problem, such as worn valve stem seals, worn valve guides or worn piston rings, allowed excessive amounts of oil to enter the combustion chamber. This excessive oil usually forms black, somewhat oily carbon deposits.
Carbon buildup hampers performance and increases emission levels and fuel consumption for several reasons. First, it restricts or upsets normal air flow through the engine. Second, it may disturb a designed-in air swirl needed for efficient combustion.
Furthermore, carbon deposits actually lean out the air/fuel mixture by absorbing fuel molecules from the incoming air/fuel charge.
During the 1980s, another version of carbon contamination began appearing on the intake valves of port fuel-injected engines. This dry, hard carbon buildup usually appeared in cauliflower-shaped formations smaller than oil-induced deposits.
As little as several grams of these deposits can cause cold driveability headaches such as hesitation, stalling and backfiring. This occurs in a cold engine because the fuel injection computer isn't fully responsive until the engine warms up. Instead of using its full range of air/fuel ratio adjustments, the computer holds the ratio within a preset, relatively narrow range. Plus, a cold engine demands a richer mixture than a warm one.
Stringent emission regulations have narrowed the cold-engine air/fuel ratio range so much that a cold engine won't tolerate the fuel-robbing, leaning-out effect intake valve deposits cause.
One of port fuel injection's virtues is the very reason this breed of intake valve contamination occurs only on port-injected engines. You see, spraying fuel from an injector improves efficiency by atomizing the fuel more effectively. Aiming the injector tip directly at the backside of the hot intake valve further enhances efficiency by vaporizing any lingering ``wet'' fuel. Unfortunately, normal intake valve heat can bake some fuel ingredients into carbon deposits.
Worse yet, some ingredients used to keep fuel injectors clean-as well as some aftermarket injector cleaning additives-aggravate intake valve deposit buildup!
Bill Bitting, a former fuels specialist at BMW, is a fuel research engineer with Texaco. ``What the industry has done is move the deposit tendency downstream,'' Mr. Bitting said. ``We used to have carburetor deposit problems, then in port injectors, then the intake valves. Now there are degradations we can see inside the combustion chamber that are the result of some fuel additive packages.''
These chamber deposits have spawned a new condition called combustion chamber deposit interference (CCDI).
CCDI occurs when carbon deposits on top of the pistons strike similar deposits on the cylinder head, creating a cold-engine knocking noise. The knock, which is synchronized with engine speed, has a lower pitch than the familiar, metallic-sounding spark ping. Typically, the knock occurs during cold idle and may persist for several minutes after start up.
Some engineers theorize CCDI is most prevalent after cold starts because excessive piston-skirt-to-cylinder-wall clearance is normal on a cold engine. So the carbon-to-carbon contact aggravates piston slap when the engine is cold.
They also hypothesize that after the engine is shut off for the night, liquid fuel lingering inside the combustion chambers evaporates into a carbonaceous material that swells as it solidifies.
CCDI-induced knock, which has been reported on
Chrysler, Ford, Mazda, and Toyota engines, is so intense, some motorists think severe engine damage is occurring.
Dennis Rosson is senior engineer and technical service manager at BG Products Inc., a Wichita, Kan.-based manufacturer of automotive chemicals. Mr. Rosson, formerly with BP America research, said the consensus among fuel engineers is that CCDI is a byproduct of small-squish combustion chamber designs.
The small-squish design became popular because its small volume promotes extremely efficient combustion, reducing emissions and improving performance and fuel economy. The design is particularly effective at reducing pesky hydrocarbon (HC) emissions. Traces of unburned fuel increase HC levels.
Squish area or region is the minimum clearance measured between the piston top and the cylinder head with the piston at top dead center position. Experience shows engines with squish areas of 0.040-inch or less are most susceptible to CCDI. (Note: The 0.040-inch dimension is approximately equivalent to the combined thickness of 10 human hairs.)
The squish region dimension on some new engines is as small as 0.020 inch. Due to manufacturing tolerances, some engines BG technicians analyzed had squish regions in the 0.008- to 0.012-inch range, Mr. Rosson said.
Tiny squish regions allow carbon accumulations that would be harmless on other powerplants to cause CCDI on small-squish engines, he added.
A technician encountering cold engine knock faces the same test options as with intake valve deposit symptoms. He can remove the spark plugs and inspect the combustion chamber with a borescope. A borescope is an illuminated microscope designed to search for internal engine problems.
Mr. Rosson noted that CCDI, in its earlier stages, often appears as an orange- or copper-colored resinous material inside the combustion chamber. Later, it just looks like common black carbon deposits.
Some driveability specialists run a carbon-removal chemical through the engine and note its effect on the knock. They said that when CCDI is present, a high-quality chemical makes an immediate impact, reducing or eliminating the knocking noise.
To ease the task of decarbonizing fuel-injected engines, companies such as BG Products and Wynn's offer adapter kits that allow techs to feed the carbon-cleaner directly into the engine via the PCV hose.
The photos on page 9 show an engine before and after being treated with BG Products' 44K decarbonizer.
Many techs use injector-flushing equipment to pump decarbonizing chemicals into the engine via the injectors. If you do this, be sure the chemical is harmless to injector windings. Some popular decarbonizers use butylcellosolve, which can dissolve the insulation on injector windings, causing them to short out and fail.
Technicians who favor butylcellosolve-based decarbonizers claim that when diluted with gasoline, the ingredient won't harm injector windings. But chemists urged techs to avoid these carbon cleaners altogether.
Some experts said the most practical approach is to explain CCDI to the owner and sell him several containers of an effective and automaker-approved decarbonizing fuel additive for use with each fill up.
Lastly, technicians can use a shell blaster to remove combustion chamber carbon. Kent-Moore Tool Div., SPX Corp., in Warren, Mich., makes an air-powered blaster that removes intake valve carbon deposits. The device shoots finely ground walnut shells at the valves.
Removing the spark plugs or injectors would allow a tech to shell-blast carbon from the piston tops of an engine with CCDI problems. The plumbing on the Kent-Moore device is designed to suck the mixture of carbon and walnut shells out of the combustion chamber.
Fuel experts said CCDI trouble will persist unless some technological breakthrough occurs or gasoline producers switch to improved formulations.