Carbon Buildup Can Haunt Your Vehicle (and Wreck Your Wallet)

Dan Carney
Nov 22, 2019

The deadliest ghosts are the ones you can't see. That's why carbon buildup can haunt your engine (and your wallet) like the Grim Reaper. It may come in the form of decreased gas mileage, increased oil consumption, and in some cases, catastrophic engine failure.

Enthusiasts who have turned some wrenches in their lives probably have long familiarity with carbon deposits in engines. You’ve likely seen black gunk caked on the spark plugs of older engines, or have even removed a cylinder head to discover carbon built up in the combustion chamber atop the pistons and on the valves.

The source of these deposits in worn-out engines is obvious: it comes from oil leaking past the pistons and valves and into the combustion chamber, where it is partially burned as the engine runs.

The remnants of this partial combustion accumulate on every surface, reducing the space inside the combustion chamber and effectively increasing the engine’s compression ratio beyond its design specifications.

This, along with the hot spots of glowing carbon chunks, can cause damaging pre-ignition engine knock.

A few years back, newer engines began struggling with carbon buildup too, in a less extreme fashion. The paraffin commonly used in gasoline can stick to intake valves on its way into the cylinder, creating buildup that obstructs the flow of fuel and fresh air into the cylinders.

Gas companies have addressed this with premium grades of fuel that have cleaning additives to prevent and remove carbon on the valves from this source.

But now, the very newest generation of engines— those that feature direct fuel injection systems for maximum power and efficiency—are experiencing carbon buildup on intake valves for a new reason. And sometimes you don’t see it coming.

Direct fuel injection sprays gasoline directly into the combustion chamber under very high pressure, bypassing the intake port and intake valve entirely. This yields benefits in terms of managing combustion efficiency.

But it permits an unexpected problem. Even in very new engines, tiny amounts of oil may seep past the valve seals and run down the valves.

On exhaust valves, hot gases exiting the cylinder scorch this oil from the valve stems. But the cold intake side has relied on the gasoline mist entering the cylinder to wash the oil from the intake valves.

In direct-injected engines, there’s no gas passing through the intake port because it’s squirted straight into the combustion chamber. Today’s newest and most sophisticated engines are suffering surprising carbon buildup even at very low miles as a result.

The problem is that while it is not hot enough on the intake valves to burn the oil off, it is hot enough to cook it into carbon, explains Valvoline engineer Michael Warholic.

“These deposits can form quickly, within 20,000 to 25,000 miles, and may cause issues related to fuel economy and overall engine performance,” he warned.

This carbon can accumulate enough to jam the intake valves when they close and prevent the combustion chamber from sealing properly. When this happens, the car’s “Check

Engine” light is likely to illuminate, even if the symptoms are still too mild for drivers to detect, Warholic said.

“It notices a problem long before you do in most cases,” he explained. “A rough idle might be an extreme case.”

Indeed, modern engine management systems are so sensitive that they can detect the valve timing error that results from just a 0.02-percent stretch of an engine’s timing chain, said Warholic.

“You are going to lose power and pressure, so it will take more fuel burn to get more horsepower,” he said.

But that’s only the beginning, because chunks of carbon like little bits of charcoal briquette can break free from the valves and fall into the running cylinder where they can get lodged in the piston rings.

“You do see some of these deposits that are loose and break free and get into the cylinder and interact with the piston rings, which can lead to increased oil consumption,” Warholic warned.

Oil consumption is bad. What is worse is that because modern cars almost never need to have oil added between oil changes, most of us don’t regularly check our cars’ engine oil levels anymore.

“It used to be every time you got gas you’d check your oil,” recalled Warholic. “I don’t think people are very aware of checking oil between changes.” If the oil level gets too low before the driver notices, then a catastrophic engine failure can occur because of carbon buildup on the intake valves.

The solution is to prevent that buildup, or to remove it if it has already accumulated.

Valvoline Modern Engine motor oil is specifically designed to help resist this problem, because it is formulated to withstand the intake port environment without oxidizing and clinging to the intake valves.

“That oil has to be able to withstand oxidation, thermal breakdown, and degradation, or that oil can form deposits over time,” explained Warholic. Other kinds of oil might be very good in other respects, but only Valvoline Modern Engine is specifically designed to help prevent the problem of carbon accumulating on the intake valves of direct-injected engines.

Fortunately, if your car already has carbon on its valves, which can be identified by an inspection at a Valvoline Instant Oil Change center, there is a solution. While early victims of this problem faced engine rebuilds to remove the carbon by hand, Valvoline First Defense is a treatment available at Valvoline Instant Oil Change centers that chemically cleans the valves in minutes.

DIYers might want to try to deal with this themselves, but really all you can do yourself is put in the right oil to help prevent this from occurring. If it has already happened, let a Valvoline store clean it up before anything serious happens and then use Valvoline Modern Engine to help keep those valves as clean as possible.

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About the Contributor
Dan Carney
Nov 22, 2019

A member of the North American Car of the Year jury, Dan is Popular Science magazine's automotive editor, writing car reviews as well as auto industry analysis and commentary. He specializes in analyzing technical developments, particularly in the areas of motorsports, efficiency and safety. He has been published in The Wall Street Journal, The New York Times, Motor Trend, Popular Mechanics, and others.

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Tags: DIY