Manage these engine exhaust system components to keep your diesels firing on all cylinders.

Illustration by Brett Affrunti

Compare the volume of the waste fumes generated by a working diesel engine to the quantity coming out of the exhaust and you’ll always find a small difference. While almost all the fumes the engine produces exit through your exhaust system, managing the missing ones can improve both the cleanliness of your engine room and the performance of your engine.

Blow-By

While the exhaust system expels nearly all of the fumes produced by the engine’s combustion, it’s not possible for it to capture 100 percent of the gases. A diesel engine works this way: As the piston moves toward the top of the cylinder, it compresses the air, creating heat that ignites the fuel supplied by the injectors. On the one hand, the piston must fit tightly enough in the cylinder to contain the pressure; on the other hand it cannot fit too tightly or it would seize in place. A tolerance of a few thousandths of an inch is required, but even a gap that small would allow far too much exhaust gas to escape past the pistons. To reduce that gap, a set of rings is used. The rings fit into grooves on the pistons and press against the cylinder walls to contain the pressure.

Despite this ingenious arrangement, even in a healthy engine a small amount of gas blows by these rings, hence the term “blow-by.” This gas, mixed with fuel residue, slips down into the gap between the piston and the cylinder walls and makes its way into the crankcase under the engine. As the engine continues to run, the blow-by fumes accumulate and begin to pressurize the crankcase. Every engine manufacturer specifies an acceptable range of crankcase pressure, measured in inches of water. Crankcase pressure provides a reliable indicator of worn rings or worn cylinder liners and should be measured as a part of every prepurchase survey (in concert with oil samples).

To avoid a buildup of pressure the crankcase comes with a vent. As the hot gas works its way into the crankcase, it picks up a mist from the lubrication oil. As the gas exits through the crankcase vent, it leaves an oily film that will slowly coat the engine room surfaces. On a turbocharged engine, these oily fumes will foul the turbo and air coolers, reducing engine performance.

Puke Jugs

Boats with older engines often divert the fumes from the crankcase into a container placed low in the bilge. As the air exits the crankcase and enters the jug, the oil coalesces and collects in the container (affectionately referred to as a “puke jug”). For less than $50 you can purchase a vented “breather tank” designed to filter the air and collect the oil. Who wouldn’t prefer a breather tank over a puke jug?

On this older engine, the vent line from the crankcase has been placed into a one-gallon plastic jug. While some oil will be captured, contaminated fumes will exit into the machinery space, slowly creating a mess and fouling the turbocharger.

On this older engine, the vent line from the crankcase has been placed into a one-gallon plastic jug. While some oil will be captured, contaminated fumes will exit into the machinery space, slowly creating a mess and fouling the turbocharger.

While better than simply venting the crankcase into the engine room, this arrangement remains an “open” system, allowing vapors to escape while only marginally capturing the oil.

Closed Crankcase Ventilation Systems

In the 1980s Walker Engineering introduced a closed crankcase ventilation (CCV) system to the marine market. The product worked so effectively that a number of engine manufacturers made the system standard. Today it is rare to find an engine without the Walker product or one of their competitors, such as Parker Hannifin’s Racor CCV.

Both the Walker AIRSEP and the Racor CCV connect the breather tube from the crankcase to the air intake for the turbocharger. The suction created by the turbo creates a small amount of vacuum inside the crankcase. A separate regulator controls the amount of vacuum. When an engine with blow-by pressurizes the crankcase, the added pressure can create oil leaks. Conversely, with a small amount of vacuum, nuisance leaks are avoided.

Before entering the turbocharger, the fumes pass through a filter that coalesces the oil out of the fumes and redirects it back to the crankcase. The fuel byproducts that remain are fed into the turbo where they mix with ambient air drawn in through the air filter. The water vapor helps cool the incoming air, and the absence of oil residue keeps the turbo components from fouling.

The turbocharger on the right pulls air into the filter on the left. The black hose with a blue stripe on top of the filter connects to the crankcase. The negative air pressure created by the flow into the turbo creates a small vacuum inside the crankcase through this hose.

The turbocharger on the right pulls air into the filter on the left. The black hose with a blue stripe on top of the filter connects to the crankcase. The negative air pressure created by the flow into the turbo creates a small vacuum inside the crankcase through this hose.

While the systems function similarly, the Walker and Racor CCVs use slightly different approaches. The Walker system combines the air filter and the coalescing filter into a single unit mounted on the turbo while the Racor system uses a standalone coalescing filter mounted away from the turbo inlet.

Closed systems have minimal but critical maintenance requirements. The external portion, the air filter, must be cleaned or replaced annually or after every 250 hours of operation.

If the air filter becomes clogged the engine will be starved for air and performance will suffer. The blocked filter will increase the vacuum in the crankcase until the regulator takes over and creates a bypass to reduce the pressure. While most systems include a restriction indicator to alert you to this condition, you should follow the recommended service intervals regardless of whether excessive restriction is indicated.

To service your air filter, remove it and brush it lightly to remove any loose particulates. Next, spray on the cleaning solution provided by the manufacturer and let it sit for 10 to 15 minutes. Finally, rinse the air filter from the inside out using fresh water under low pressure. After allowing the filter to dry, apply the filter oil provided as part of the service kit and you are ready to reinstall.

If the oil coalescing filter clogs up, two problems develop. First, the oil begins to accumulate in the filter housing, which can lead to oil finding its way into the turbo and fouling the turbine fins. As the oil builds up in the filter, it restricts the connection that creates vacuum pressure in the crankcase. Once that happens, the crankcase will start to pressurize from blow-by gases, displacing gaskets and seals and creating oil leaks.

This cannister houses the coalescing filter to capture oil. The drain line at the bottom returns oil to the crankcase. The small clear cap on top include a red pop up indicator for excessive restriction from a clogged filter.

This cannister houses the coalescing filter to capture oil. The drain line at the bottom returns oil to the crankcase. The small clear cap on top include a red pop up indicator for excessive restriction from a clogged filter.

The internal filter that captures oil must also be serviced regularly. This filter usually has a longer service interval, typically 500 to 750 hours of operation. These filters cannot be cleaned and must be replaced to avoid clogging.

A CCV system will keep your engine room cleaner, extend the life of your turbo, reduce turbo noise and protect the environment. Fortunately, these systems require minimal maintenance, which can be easily handled by a boat owner. Given the list of benefits, the modest cost and the ease of maintenance, it is easy to see why these systems have become commonplace in modern engine rooms. 

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