What Causes Diesel Engine Slobber?

“When diesel engines are started cold, and more worse when the ambient temperature is chilly, “Fuel Slobber” occurs. This was a regular occurrence in the 1970s and 1980s, when most Cummins engines did not have variable timing. Because the Caterpillar 425-B had improved timing for cold idle engines, the fuel slobber wasn’t as terrible. The STC-444 Cummins engines also had enhanced idle timing, making them significantly cleaner. Many owner operators thought the fuel pouring from the flex line just behind the turbocharger was engine oil, however it was actually fuel and water combining with the soot in the exhaust system, which resembled oil. When the engine reached the proper operating temperature, which normally didn’t happen until the vehicle was driven, the problem would go away.

Diesel engines run cool due to their huge cooling systems and the large amount of air that passes through them while idling. A modern diesel engine produces roughly 67 percent nitrogen, 11 percent carbon dioxide, 11 percent water vapor, and 9 percent oxygen as a result of burning fuel. It makes up less than 1% of the total “all of the “bad” pollutants for which diesel exhaust has recently gained notoriety. On a cold start of a diesel engine, the water vapor in the exhaust system may be cold enough to condense into liquid water – notably on vehicles with vertical exhaust stacks. This liquid water can flow down the stack at numerous clamped joints, causing a mess on the chrome elbows, and possibly even leaking out of the flex pipe.

This is a good example “If the vehicle is put to work, the “fuel slobber” problem usually goes away in a few minutes, but if it is left idling, especially on a chilly day, the liquid water can be quite a nuisance. Most of us think that liquid water is bad for modern diesel particulate filters, especially if the engine is turned off before it is hot enough to transform the liquid into vapor and expel it from the filter. This is a good example “Slobber” can also leak through the connection joints of the exhaust manifold and flow down the side of the engine. Simply spritz the slobber with soap or penetrating oil and wipe it away with a towel if this happens (many penetrating oils are also very good cleaning solvents that will also help to keep the engine from rusting).

Blow-by can potentially cause damage to your truck’s engine and structure. The amount of blow-by produced by an engine increases with the horsepower and the amount of time it is operated. Blow-by is just compression passing through the piston rings, and it occurs in all engines. The Oil Trap, which attaches to the bottom of the blow-by tube, is a great way to trap the oil residue that can be found in the blow-by gases. Every 10,000 miles, this Oil Trap will catch around half a cup of oil, which is better kept in a container than smeared all over the underneath of your truck.

Drive a motorcycle down the strip in Las Vegas while it’s raining to see the effects of blow-by on the pavement. When you step into the road, it will try to pull you out from under you. That is something I can tell you from personal experience, because holding up an Ultra Classic with a passenger in the back seat is difficult! Many mechanics will tell you that if you see blow-by coming out of the tube, the engine is worn out, but this is not true; oil consumption, not blow-by, is the way to tell if an engine is worn out. When one gallon of oil is spent every 2,500 miles, the engine is considered worn out and must be rebuilt, but not until then.

Let’s talk about the small 3/8-inch blow-by tube positioned on top of just one of the three valve covers, which I haven’t mentioned in a long time, but now that the Big Cam Cummins engines are regaining popularity, let’s talk about it. This may have sufficed when the NTC350 and 400 horsepower engines were popular, but not any longer, especially since we began raising the power to 700 and 800 horsepower. With the rise in horsepower came an increase in blow-by, so we began inserting a breather tube on each of the three valve covers, which fixed the problem.

Blow-by gases must evacuate the engine, and if the blow-by tube is insufficiently large, they will attempt to escape up the turbocharger drain tube. The oil pouring out of the turbocharger’s bearing housing is whipped and foamy, and it drains by gravity. Keep in mind that all turbocharger drain tubes must be less than 30 degrees from vertical, otherwise the oil will be driven past the compressor and turbine wheel seals.

Consider this: if blow-by is attempting to escape through the turbo drain tube, the turbo oil will be driven out of the turbo seals rather than draining down the tube. As a result of the limitation in the blow-by tube, many turbochargers are replaced. The filter prior to the blow-by tube on newer ISX Cummins engines must be updated. Never attach a heater hose on the bottom of the blow-by tube, since the hot gases will soften the rubber, and the wind under the vehicle will drive the hose horizontal, choking off the blow-by gases. Now, a turbocharger is being blamed for oil leaks when it’s simply that hose you place on the blow-by tube.

I write these articles to get you thinking; I want you to be aware of what’s going on under the hood of your truck while you cruise down the highway, especially when ascending a mountain. Stay watch for additional information about this next month as we prepare to start doing a bi-monthly video on YouTube and on our website.

What causes engine slobber?

That your engine isn’t slobbering like Fido isn’t the case. Excessive crankcase blow-by is a symptom of engine slobbering. Condensation can be detected oozing from the exhaust manifold joints in some circumstances. When an engine is driven for long periods of time with low loads, slobbering is common.

When a new or freshly manufactured engine is being run in, this condition is more prone to occur. The piston rings may not seal properly if the engine is not driven under load, resulting in oil carryover into the combustion chamber and cylinder wall glazing. Low engine temperatures can also lead to incomplete combustion, resulting in low exhaust temperatures and unburned fuel condensate in the exhaust manifold.

To avoid engine slobber and probable engine damage, it’s critical to follow correct running-in procedures while rebuilding an engine. See the individual service procedures for your engine in for additional information.

What is turbocharger slobber?

Turbos do not have oil seals, with the exception of the carbon face positive oil seals employed in turbos used in carbureted engines prior to the widespread use of fuel injection. Because the turbo’s compressor was frequently exposed to vacuum due to being applied to a throttled engine in what was known as a pull-through turbo system, positive oil seals were utilized in those early applications. Pulling air through the carburetor rather than blowing it through was referred to as the pull-through system. Because they both had virtues, these two systems were constantly argued as to which was the best. Fortunately, both, as well as the carburetor, are no longer in use.

On both the compressor and turbine ends, a piston ring is present. They get their name from the fact that they resemble very small piston rings. Their principal purpose, like piston rings, is to prevent boost and exhaust gas pressure from entering the oil drain chamber and, as a result, pressurizing the engine’s crankcase, which is an undesirable state.

A cutaway section of the turbine end of a bearing housing demonstrates some of the design features of a typical bearing housing and how it prevents high heat from the engine’s exhaust from migrating into the oil and causing damage to the bearing system. The backplate (1) creates a dead air space between the turbine area of the bearing housing surface and the turbine gases (2). The bore area of the seal ring is formed in a suspended casting part, which creates a long heat channel for the heat to move (3). As long as oil is available, the oil pushed off the turbine-end flinger has an opportunity to cool it. The bearing bore is also isolated in its own section of the casting (4), and is not connected to the high heat area directly. (Image credit: Diesel Injection Service Company, Inc.)

The piston rings in a turbocharger are essentially dynamic gas seals that prevent pressure from entering the engine’s crankcase via the turbo’s oil drain cavity from either the compressor or turbine end. On the compressor side, the smaller ring is used, whereas on the turbine side, the bigger ring is employed. (Image credit: Diesel Injection Service Company, Inc.)

The oil is shot off at tremendous speed as it leaves the thrust bearing area. The thrust bearing (2) is shrouded by an oil deflector (1), which directs the oil into the bearing housing drain cavity, preventing the area of potential leakage from being swamped with oil. (Image credit: Diesel Injection Service Company, Inc.)

The compressor cover (1), bearing housing (2), turbine housing (3), compressor wheel (4), and turbine wheel and shaft assembly are the five major components of a turbocharger (5). The remaining pieces, such as bearings, seals, and oil control components, are crucial to the assembly’s overall operation. Understanding these components and how they work is important for designing and installing a solid turbo system, as well as turbo troubleshooting, maintenance, and rebuilding. (Image credit: Diesel Injection Service Company, Inc.)

Oil control is primarily accomplished through oil control and deflection, which keeps the oil away from potential leakage sites. Oil cannot access the engine’s compressor or turbine during a boost condition due to pressures in the corresponding housings. The oil that has been drained from the bearings is no longer under pressure and is allowed to drain back into the oil pan through the bearing housing’s oil drain cavity. When a turbo is claimed to be leaking oil, it’s usually because it’s been idled for too long and there isn’t enough housing pressure to keep the oil from leaking into the housings. Turbo slobber is the name for this disorder. Other factors that contribute to oil leakage are discussed in Chapter 10.

Several various design modifications of certain turbo parts, such as turbine wheel material, different turbine housing availability, enhanced thrust bearings for high-load applications, and so on, have been described in this chapter. It can be difficult to tell if any of these options are necessary or even available for your turbo model. One good place to get this information is from your local turbo expert. Many turbo professionals with years of knowledge can assist you with some of your questions. A list of contacts can be found in the source guide at the back of the book.

How do you fix wet stacking?

The most easy method is to always operate the generator set at a load that reaches the diesel’s designed operational temperature, which is about 75% of full load. If wet stacking has not yet reached the point when carbon buildup can only be eliminated by a significant engine overhaul, built-up fuel deposits and carbon can be removed by running the diesel engine at the required operational temperature for many hours.

The following load bank solutions should prevent wet stacking from happening again:

  • Automatic auxiliary loading – When the diesel generator set is the principal source of power, this approach is frequently used. When just the lighter loads are present, the “auxiliary load bank” will be activated, and when the greater load is attached, it will be deactivated.
  • Facility manual load bank – A manually operated system for use with minor loads and when the bigger load is also manually initiated, as specified for the automated load bank. The load bank can also be utilized to load test a system that is used largely for standby power.
  • Portable load bank – The diesel generator set distributor is frequently the best qualified to perform system maintenance. It’s fairly normal for a standby generator system owner to outsource comprehensive maintenance and have a scheduled maintenance (PM) contract with a full-service generator set supplier these days. The distributor will bring in a portable load bank during a regularly scheduled planned maintenance call to run the generator at a load that maintains the designed operational temperature. Portable load banks range in size from a few kilowatts to 3 megawatts and are transported on big trailers.

Wet stacking prevention is just one part of diesel generator maintenance. Download our Keys to Running Your Generator Efficiently guide for more generator maintenance ideas. Contact our parts and services department directly for immediate generator maintenance guidance, or leave a message in the chat box on the bottom right.

How do you prevent wet stacking?

Wet stacking is quite widespread in the field of diesel engines. If you don’t run your engine at recommended operating temperatures 100% of the time, you’re going to have some wet stacking. When diesel generators are not used at least 60% of the time, they run at suboptimal temperatures, which means the engine never achieves the temperature required to burn off the surplus fuel and carbon deposits, resulting in “wet stacking” in the exhaust system.

How Do I Know It’s Happening?

Black ooze surrounding the exhaust pipe or continuous black exhaust smoke are frequently the first signs of wet stacking. If you operate your generator below the recommended operating temperature, run it at less than 60% load, use the incorrect air-to-fuel ratio, leave it idle for lengthy periods of time, or run it with too much or too little fuel in the tank, you can presume it’s wet stacking.

How Can I Prevent It?

To avoid wet stacking, exercise your generator according to NFPA and manufacturer guidelines (at least once a week with at least a 60% load), run your engine at optimal temperatures, keep the fuel tank full, have a qualified technician maintain your generator at regular intervals, and make sure the internal temperature of your generator reaches manufacturer recommendations if you’re operating in cold conditions.

If you observe a buildup of gasoline and soot particles in your engine, the solution is sometimes as simple as running it at maximum power for a few hours to burn them off. Higher particle levels may need the use of a load bank to simulate a full load on your generator, as well as the use of a competent technician to complete the load banking procedure.

What Happens if I Don’t Address it?

Unburned fuel will begin to build up in your exhaust system if you do not address wet stacking at scheduled maintenance intervals. This can clog your injectors and reduce the performance of your generator. These gasoline deposits can also cause backpressure, degrade your engine’s surface, and reduce the overall system’s efficiency. These negative effects will dramatically reduce equipment life, resulting in higher repair expenses.

Wet stacking can also have an impact on the quality of your engine oil. The pistons do not meet the cylinder as they should since your engine is running at sub-optimal temperatures. Unburned fuel may leak into the oil pan, causing the oil to become diluted. This reduces the effectiveness of your oil to protect your engine and increases wear.

Wet stacking increases pollution, and most localities have laws prohibiting wet stacking-related smoke emissions. If the EPA discovers this, it can result in significant fines.

Is wet stacking normal?

Wet stacking has a number of negative impacts on a diesel engine generator, and if left unchecked for a long time, it can lead to lower engine performance or permanent engine damage, necessitating a costly significant engine overhaul.

How do pilot injections reduce combustion noise?

With the widespread usage of common-rail fuel injection systems in diesel engines, the pilot injection approach has received more attention for reducing pollutants emissions and combustion noise. Pilot injection tactics result in a leaner and more homogeneous mixture in the combustion process, which partially fulfills Premixed Charge Compression Ignition (PCCI). As a result, partial PCCI can be applied to the combustion process of diesel engines using a pilot injection technique (PPCI). Pilot injection raises the in-cylinder temperature before main injection, which minimizes the ignition delay of the main spray and, as a result, the combustion noise, allowing the PPCI combustion model to be extended to high-load operation. However, because the mechanism of pilot injection impacts on combustion noise is not thoroughly known, it is difficult to determine the lower combustion noise among various pilot injection settings, making correct selection of pilot injection parameters problematic. Experiments were carried out on a single-cylinder DI-diesel engine with pilot and main injection under high load operating circumstances for this research. To investigate the impacts of pilot injection on combustion noise, a unique approach of synthetic in-cylinder pressure levels (CPLs) in various frequency ranges was developed. The findings show that the high frequency combustion noise is mostly influenced by pilot spray combustion, and that the later the pilot injection timing, the higher the combustion noise. When the time between pilot and main injection is short, increasing the pilot injection quantity increases the high-frequency combustion noise. In the meantime, because the pilot injection quantity has increased, the main injection quantity has decreased, resulting in lesser combustion noise in the middle frequency band.

What causes generator wet stacking?

Let’s start by defining generator wet stacking. Wet stacking is a phrase that refers to a diesel engine dripping a thick, dark liquid from its exhaust pipes, or, as they’re more commonly known, “wet stacking.” “stacking” The issue is created by running the engine at low revs for long periods of time, allowing unburned gasoline and soot to enter the exhaust system. The word is now used to describe an engine that isn’t totally burning all of the fuel provided to its cylinders. This condition can drastically decrease engine performance if left unchecked for an extended length of time.

A diesel engine produces only enough power to drive its accessories and overcome internal friction when it is not under load. Spark plugs are not used in a diesel engine. It relies on the cylinder’s hot compressed air to evaporate and ignite the fuel. Conditions for combustion are less than optimal when the air is cooler than the design temperature. The fuel ignites and begins to burn, but it does not totally burn. Soot—small, hard particles of unburned carbon—remains as evaporated fuel. Fuel vapors condense in the exhaust system and combine with soot to generate a dark, thick liquid that resembles engine oil. It could be dripping from the exhaust ports or oozing from the turbocharger. The name “liquid on the exhaust stacks” comes from the sight of liquid on the exhaust stacks “Wet stacking,” says the author.

What is the best way to tell if my generator is Wet Stacking? If any, or all, of the following circumstances exist, your generator is most likely inefficient:

Expense – Excessive wet stacking will reduce engine life by several years and cause it to fail before it is scheduled to be replaced.

Pollution – The amount of smoke emitted by wet stacking is restricted in many urban areas.

Power – Deposits reduce maximum power even before an engine is damaged. An engine that has been prematurely worn will produce less maximum power than it was planned to produce.

Maintenance – A moist stacking engine will require significantly more maintenance than one that is appropriately loaded.

A few hours of operation at a load of about 75% to 100% of the generator’s nameplate rating, raising the exhaust temperature high enough to evaporate the unburned fuel in the exhaust system and blow away the soot, is usually enough to cure wet stacking. If wet stacking has not yet reached the point when carbon buildup can only be eliminated by a significant engine overhaul, built-up fuel deposits and carbon can be removed by running the diesel engine at the required operational temperature for many hours. However, under that load, the exhaust temperature is much over the auto-ignition temperature for diesel fuel, and fuel and soot can ignite within the exhaust system on rare occasions. It’s critical to have a competent generator maintenance specialist oversee the load testing method if a unit has a history of extended operation at low load, or if there’s no documentation indicating it’s been exercised recently at acceptable load.

Additional Load Bank Testing Information: Generator Brochure for Load Bank Testing

Selecting the Right Generator Maintenance Plan (for more information on generator maintenance).

What is turbo wastegate?

If the maximum boost pressure on the engine specification sheet is 2.1 atm, the calculation is 14.68 times 2.1, or 30.83 psi. 0.9869 atm (14.5 psi) is equal to one bar. As a result, a pressure of 3 bar equals 43.5 psi.

Most people use 15 psi as the nominal atmospheric pressure for fast calculations.

With this established, turbocharger boost is the pressure above atmosphere as measured in the intake manifold. At wide-open throttle on a naturally aspirated engine, the pressure in the induction system is considered atmospheric. Due to flow losses, it is little less than that. A vacuum, on the other hand, is any pressure lower than atmospheric.

A turbine and a compressor are the two basic components of a turbocharger. These are known as the hot and cold sides, respectively. The engine’s exhaust is connected to the turbine, which is connected to the compressor via a shaft.

This shaft’s fins are angled in the other direction. When hot exhaust gas escapes the cylinder head, it expands and rotates the turbine wheel, much like a river does with a water wheel. Because the compressor and turbine are on the same shaft, the compressor wheel spins with the turbine. The driving member is the turbine, whereas the driven member is the compressor.

The compressor pushes air into the engine’s induction system, which has two key impacts. It increases the amount of airflow into the engine measured in cfm and raises the pressure that enters the cylinder to above atmospheric pressure (cubic feet per minute).

The boost pressure is the consequence of the cumulative effect of the increased pressure and the mass of the incoming air. The exhaust gas flow and temperature control the turbine speed and, as a result, the boost pressure. It is necessary to adjust the pressure, which is usually performed with the use of a wastegate.

The wastegate

If there was no way to adjust boost pressure, it’s feasible that the cylinder pressure would surpass the engine’s acceptable limits due to a series of circumstances.

The wastegate is used to adjust boost pressure by allowing a specific quantity of exhaust flow to bypass the turbine wheel. It’s just a disk that shuts against a route and redirects a portion of the exhaust flow.

Boost pressure is limited while the route is open. When it is closed, the turbocharger’s full potential may be achieved.

Can a diesel engine get wet?

Wet stacking, if overlooked, can cause serious harm to your diesel engine over time. Unburned fuel will begin to accumulate in your engine, blocking injectors and reducing performance. Deposits can also cause backpressure and limit the performance of the turbo system of the engine. Worse, they’ll degrade engine surfaces over time, reducing the product’s lifespan.

Engine oil is also affected by wet stacking. Because the engine isn’t as hot, the pistons don’t expand as much as they should to contact the cylinder wall. Gases and unburned fuel enter the oil pan beneath the cylinder as a result, diluting the oil. This reduces the oil’s ability to protect your engine and causes it to wear out faster.

Higher pollution and emissions, reduced power, and increased maintenance are some of the other negative consequences.