Valves are crucial pieces within the valvetrain, combustion chamber and cylinder systems of any engine. When you understand the symptoms of bad valve seals, it can mean the difference between making the correct repairs and further damaging your diesel engine.

In this guide, we’ll discuss what a valve’s function is, how it affects your entire marine boat engine, how to analyze engine valve failures and how you can correct the faults to keep you moving on the water.

What Is the Engine Valve?

Engine valves play a critical role in the performance of all internal combustion engines. They regulate, control and direct the proper amount of air and fuel inside the engine’s combustion chamber for ignition to occur. The precise timing of the valves allows your Detroit Diesel or Cummins marine engine to operate at peak efficiencies during various conditions and speeds.

The most common type of diesel engine valve is the poppet-type. It has a slim stem that leads to a flat-faced piece of metal, which is the head. It looks similar to a plunger or the shaft of a trumpet. The parts of a poppet valve are:


Keeper groove


Seat face


Combustion face

Each poppet valve rests in a circular hole on top of the combustion chamber. The valve seat is what creates a tight seal between the valve and its surface — and the valve spring keeps the component closed by pushing against a retainer.

Engine valves are positioned on the engine’s cylinder head and can be made of hardened metal, such as:

Stainless steel

Low carbon steel

Nickel alloy

Stellite alloy

In more high-performing engines, the valves may be made of titanium. Different configurations are also available to enhance the valve’s durability and performance. For example, special coatings are applied to increase the valve’s resistive properties to heat, wear and corrosion.

Coatings and treatments can range from hard chrome plating and chromium nitride coating to zirconium nitride coating and mirror polish. Because of the different arrangements available, engine valves can withstand the extreme temperatures and conditions of the combustion chamber.

Two types of valves include the intake and exhaust. Depending on what make and model of marine engine you have, the number of valves on each side of the cylinder head can vary. Multi-valve engines can have anywhere from three to five sets of valves to boost fuel combustion efficiency.

The engine’s camshaft controls both the intake and exhaust valves, and their timing of opening and closing is measured by the engine’s timing chain or belt. Valves and their proficiency are crucial for accurate and effective engine operations for your marine diesel engine.

Engine Valve Function and Purpose

The purpose of an engine valve is to seal the combustion chamber and advance the exchange of gases.

Valves allow an air-fuel mixture to pass in and out of the chamber through passages called ports. They work to seal the chamber at precise moments to ignite and release the combusted fuel. On one side of the cylinder, the intake valve allows a calculated amount of air to enter at a specific moment while the exhaust gets rid of the dissipated air on the other side. The more air an engine can move in and out of the cylinder determines how efficient and powerful it is.

Four procedures occur in an internal combustion engine called strokes:

1. Intake stroke: The intake stroke is the first step. The intake valves open and draw air into the chamber where it mixes with fuel.

2. Compression stroke: Stroke two is called the compression stroke, where the intake and exhaust valves close to trap the air and fuel mixture. The piston is forced upward, causing the compound to combust. Extreme pressure increases chemical energy.

3. Power stroke: During stroke three, also known as the power stroke, the compressed mixture spontaneously ignites, converting the air and fuel into mechanical energy. The ignited fuel and air expands, forcing and driving the piston back down the cylinder. As a result, the crankshaft turns.

4. Exhaust stroke: The last stroke it the exhaust. As the piston works its way back up the cylinder after compression and ignition, the exhaust valve opens to release the gases. It will then close as the intake valve starts the process over. The engine valves function at high speeds, forcing the motor to propel your boat.

Intake vs. Exhaust Valves

While both the intake and exhaust valves seal the combustion chamber, they uphold two different purposes — one allows air inside while the second gets rid of the gases.

During the first stroke, the intake valve allows air, or an air-fuel mixture, to enter the chamber. As it opens, the exhaust valve is closing simultaneously. As the last stroke takes place, the exhaust valve gets rid of the expended gases, and the process repeats itself.

The valves are activated by the camshaft, which is driven by the crankshaft — which is controlled by the piston. The entire system is timed in synchronism with a consistent and smooth motion.

The intake is often larger compared to the exhaust. Because the input has a lower intake pressure, it requires a larger opening to push the air inside. It relies on a filter to ensure the clean air reaches the chamber, while also depending on a vacuum to release the right amount of mixture.

In opposition, the exhaust valve has a high exhaust pressure because of the combustion. The exhaust releases a smaller volume of gas, so a larger valve is unnecessary. When the air and diesel mixture ignite and explode, the chemical energy turns to mechanical, creating more pressure and power to push through the exhaust. It’s easier to release the gases from the exhaust valve because of the piston and combustion.

When comparing an engine’s valves up close, the exhaust is often cleaner than the intake because it endures extreme temperatures from the combusted gases. The high temperatures burn off any deposits on the exhaust, while the input collects a buildup of carbon because it receives colder air and gases. The intake is usually dirtier or darker compared to the exhaust.

A Diesel Engine Valve Failure Analysis

Although valves are engineered to last throughout the service life of your commercial marine engine, they can only do so if you adhere to a regular maintenance schedule. Your upkeep should include standard practices to avoid engine valve failure, such as:

–Replacing oil, oil filters and air filters.

–Following the correct service maintenance intervals.

–Replacing the timing chain or V-belt after a particular number of miles or amount of time.

If you don’t make the right parts replacements, the timing chain or belt can tear, causing the valves to become immobile and collide with the piston. Understanding how long engine valves last depends on maintenance and the workload you place on the engine. If you neglect proper upkeep, the damaged valves can cause extreme engine impairment.

Valves can also damage from operator neglect. For example, if you’re operating at a lower gear but moving at high speeds, it can bend or break the engine valves.

So, what causes engine valve damage? Two common failures can occur — bent or burnt valves.

Bent Diesel Engine Valves

The most common type of valve failure is when the valves break after contacting or hitting the pistons. As a result, the valves can no longer seal the chamber, which allows excessive air and fuel to reach inside while the exhaust isn’t closed all the way.

Most engines are interference engines, meaning the piston moves up past the lowest point where the valves travel downward during a normal process. A timing belt or chain times the engine components through the crankshaft. Then, the crankshaft drives the pistons, which affects the camshaft, which affects the valves.

When the timing of the chain or belt is disrupted, the pistons can collide with the valves. If there is an incorrect synchronization in your Detroit Diesel, Cummins or another engine, don’t turn it on. Running the marine engine can cause more damages to the pistons, cylinder head and bores.

Bent valves are a result of a faulty timing belt or chain that you can replace according to your manufacturer’s manual.

Burnt Diesel Engine Valves

Burnt valves are the second most common type of failure. When the valves experience extreme temperatures, it can often cause damage to the component’s material through burning or corrosion — occurring most often on the exhaust valves because they experience a harsher undertaking.

The burnt valves won’t be able to seal because of cracking or burning. Combustion gases will escape through the valve seat and valve when they aren’t sealed. The hot gases start to burn the valve’s edge, and over time, it will eat away at the valve head, causing it not to seal. It will affect the performance and compression of your commercial marine boat as well as fuel consumption.

Signs of Engine Valve Failure

Because valves are in constant motion and subjected to extreme temperatures, they naturally wear over time. However, valve failure can happen faster if your marine diesel engine is under severe conditions or you neglect to care for it.

If you suspect there is an engine valve failure occurring within your marine engine, there are a few signs to look for:

A cold engine start: A valve cover coated with oil will leak past the faulty seal and into the chamber, causing a puff of blue-white smoke out the exhaust pipe.

Excessive oil consumption: If your valves are in poor condition, the issue can lead to high levels of oil consumption.

– Exhaust smoke: Deteriorated valves will cause blue-white smoke that lasts longer than normal during acceleration and engine start-ups.

Impact loading: Valve failure can occur from impact, often damaging even the strongest points on the valve system.

Poor operating techniques: Stop-and-go operations and idling can wear the valve seals.

Recessed valves: Recession is a result of extreme operating temperatures and deposits of oil and sludge that cause the components not to seal.

Bent valves are often more serious compared to burnt components because their failure can lead to the destruction of other engine parts and affect the performance of your commercial boat.

Causes of burnt valves include:

–Combustion gases escaping at a single point past the valve.

–Defective valve clearance.

–Faulty valve sealing with the head valve seat — carbon buildup can affect the sealing and cause abrasion.

–High amounts of confined heat.

–Inadequate cooling of the cylinder head.

–Using a dry fuel that doesn’t lubricate the valve seat properly.

Indications you may be dealing with a burnt valve may involve signs like:


–Less power


–Rough idle

Operating with burnt valves can result from your marine engine overheating or the use of incorrect fuel.

Keep an eye out for different signs that may indicate your engine has broken, bent or burnt valves. At the first signs of a mishap, diagnose the problem to avoid further damage.

How to Correct Engine Valve Failure

Resolving an engine valve problem begins with the understanding of the combustion process and the various mechanisms involved. Several ways you can prevent valve failure include:

–Checking the valve’s tightness or its sealing properties.

–Cleaning the cooling system for efficiency.

–Cleaning the carbon deposits and corrosion off the engine valves.

–Inspecting valve clearance.

–Using high-quality fuel to avoid carbon buildup.

Before you try to fix any internal complications, analyze the situation. Your valves can be broken, worn, cracked or burnt. Even a marine engine’s valve guides and seats can endure damage. To asses the problem:

1. Inspect any abnormal conditions on the valvetrain components after disassembling the cylinder head.

2. Check for signs of wear or additional damage, like broken pieces, cracks and carbon buildup.

3. Examine each valvetrain part to determine which ones need replacing or reconditioning.

4. Be aware of your manufacturer’s guidelines, and refer to the specs of each valve part.

During your breakdown, it’s smart to conduct a leak down test. A leak down test will determine if your valves are faulty by allowing fluids to pass through at the incorrect times. During the process, the diesel engine will rotate until the tested cylinder achieves top dead center. The valves close during the first part of the test.

Then, air will inject into the cylinder. A leak down gauge will help determine how much pressure the cylinder can hold. If leaking is present, you may be able to use a borescope to inspect the valves. You may also have to remove the valve cover to check the valvetrain.

Determining the cost and time it takes to complete a marine engine valve repair is subjective to the amount of damage. There can always be more to uncover too, like a faulty valve guide or seat.

Engine valve replacement is necessary if you’re experiencing the following:

–Consistent noise from the engine

–Excessive oil consumption

–Extensive valve damage

–Poor engine performance

Performing a DIY valve repair is cost-effective and doable when you have the right materials. Define which valves and other components need replacing. It’s also an opportune time to replace other critical engine parts because you will have to break down the system. Here are a few simple steps:

1. Allow your marine engine to cool and open the bonnet to view the cylinders.

2. Disassemble the head gasket.

3. Remove the valves from the screw threads.

4. Conduct a complete inspection of the valvetrains’ conditions.

If your commercial diesel engine has hidden valves, you may need to reference the expertise of a professional. In the case you’re unable to perform other intricate restoration and repair steps, refer to a reputable mechanic that can rebuild your diesel engine.

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