How to change spark plugs

How to Change the Spark Plugs on a Mercruiser Engine

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Keeping your boats spark plugs up to date and maintained is an important part of keeping your engine running smoothly and performing at its best. Your spark plugs are an important part of the engines operation, and should be changed on a regular basis to ensure it’s getting the proper spark it needs. We’ve laid out a step-by-step guide on how to change the spark plugs on a Mercruiser engine.

The concepts in this guide can also be applied to other engines as well. On top of that, we’ve also added additional information that will help you determine how often to change them as well as how to inspect them for warning signs of underlying engine issues. To skip straight to the instructions, simply click on the button below:

Spark Plugs 101

If you’ve ever asked yourself, “what is a spark plug?” or, “what does a spark plug do?”, we’ve put together some information about them below. If you’d like to get to the point and go straight to the instructions on how to change them, click on the button above.

What Are Spark Plugs?

Spark plugs are devices on your boats engine that deliver a spark to the cylinder area. This is where the rubber meets the road. They are built to take the spark that is created by the coil and passed through the spark plug wires, or directly from a coil pack, and deliver it to the inside of the engine. Below is a diagram that shows the parts of a spark plug.

Parts of a Spark Plug

How do Spark Plugs Work?

As the engines rotating assembly rotates, the pistons go through different strokes. Once a piston is on the combustion stroke, the cylinder cavity will be full of an air and fuel mixture. At this point, the system delivers a spark to ignite this air and fuel mixture. This causes an explosion that pushes the piston in the cylinder area and rotates the crank.

The spark that is delivered happens at the end of the spark plug that is inside of the cylinder combustion chamber. On this end, there is a central electrode that receives current, and a ground electrode that this current arcs to, creating the spark. On the opposite side there is a terminal either a spark plug wire, or coil pack, is connected to depending on the style engine that is in your boat. The most common method is delivery through spark plug wires, but newer engines also have coil packs mounted directly on top of the spark plugs.

Why is it Important to Change the Spark Plugs on an Engine?

As your engine is used, hours of spark, fuel, and combustions will eventually cause a spark plug to be worn out. A spark plug can be worn out in many different forms, but the end result of each scenario is the inability for the plug to have a clean strong spark. If the spark on a spark plug becomes weak, or non-existent, the air and fuel mixture in the combustion chamber will not ignite fully, or even at all. If this mixture does not properly ignite, your engine will not produce power on all cylinders, run rough, and have less power.

The best way to avoid this is to do a routine spark plug change, however, there are some other issues with an engine that may cause certain types of spark plug failures or fouling. These different failure modes are discussed below.

How Often Should You Change the Spark Plugs on a Boat?

Most manufacturers recommend changing the spark plugs on an inboard engine once every 300 hours or every 3 years, whichever comes first. This is the recommendation that Mercruiser gives as well for their engines specifically. If you don’t put a significant amount of hours on your boat, we still recommend inspecting the spark plugs each year not only to make sure they are in good shape, but to also ensure there are no other issues with your boats engine. Checking a spark plug can be useful as they can show signs of issues such as running rich, running lean, oil leaks, and more.

Steps to Change the Spark Plugs on a 4.3l Mercruiser

On a 4.3l Mercruiser engine there are six cylinders, therefore, six spark plugs. In the guide below we show where these are located and how to change them. Other engine spark plug changes are performed in a similar matter, they are typically just located in a different position or have a different number of plugs. For example a 5.0l engine has 8 cylinders, therefore 8 spark plugs, but the procedure is the similar.

1. Access the Engine

The first step to change your boats spark plugs is to access the engine. This is done differently depending on your boat, but if you have a 4.3l, typically there is an engine cover or hatch that can be lifted away from the engine to perform maintenance on it.

2. Locate and Remove the Spark Plug Wires

The spark plugs will be evenly distributed on both sides of the engine. In this example, the 4.3l Mercruiser engine is a 6 cylinder engine so there will be 3 spark plugs on each side of the engine. Below are pictures of the spark plug locations for both the port and starboard side of a 4.3l engine. An 8 cylinder engine will be similar except 1 additional plug on each side.

Grip the spark plug cable boot firmly and pull straight out while twisting back and forth. The metal clip inside will simply pop off and the boot will pull off of the plug.

Tip: Keep track of the location of each plug wire. This can be done by labeling the wires as they are removed. It is important they go back onto the same plug they came off of or the engine will not run properly, if at all.

Port Side Spark Plug Locations

Spark Plugs Port Side

Starboard Side Spark Plug Locations

Spark Plugs Starboard Side

3. Remove the Old Spark Plugs

How to change spark plugs

Required Tools

The spark plugs can be accessed by simply using a ratchet, extension, and socket. A standard deep socket can be used, however we recommend a spark plug socket to keep from dropping the plug during removal. In some cases if the space around the spark plug is very tight, a thin walled socket is needed. Typically a 12 point socket will have a thinner wall than a 6 point socket.

Some people also use a spark plug wrench or a spark plug socket and extension combo with built in swivels if the plugs are in a tough spot or oriented at a tricky angle. The advantage of having the socket and extension combo is the ability to use a ratchet wrench for more torque when removing, as well as a torque wrench for installation.

In the 4.3l configuration the spark plugs are not sitting horizontally so it is tougher for debris to get into the socket holes, but it’s still recommended to make sure the plugs and surrounding area are free of loose debris to keep it from entering the hole once removed. A quick blast of compressed air should make quick work of this.

Tip: If you remove a spark plug and put a new one back in as soon as you remove it, there will be less of a chance for foreign matter to get into the spark plug hole versus pulling them all out at once.

4. Install the new Spark Plugs

Required Tools

Required Supplies

  • Spark plugs

Spark Plug Torque:

Spark Plug Torque: 15 ft lbs (20 Nm)

The spark plugs will go back in the exact way they came out using your tool of choice for removing them. It’s very important to start threading the spark plug into the hole by hand. This will ensure the threads on the plug and head do not get cross threaded.

Once the threads have been started, the spark plug can be fully screwed down, then torqued to 15 ft lbs using a torque wrench. It’s important to use a torque wrench when tightening the spark plugs down to ensure the threads don’t get damaged or the plug does not break off in the head. If this happens, your quick spark plug change could end up taking much longer and potentially costing more money as well.

5. Reinstall the Spark Plug Wires

Required Supplies

Install the spark plug wires onto the same spark plug locations they came off of. Once ready to install, firmly grip the spark plug wire boot and push directly over the plug. Give the back of the boot a push toward the plug and you should feel a slight click or pop when the terminal inside the boot pops onto the spark plug terminal.

Tip: Before putting the plug wires on, it’s a good idea to put some dielectric grease inside of the boot and or on the terminal of the spark plug itself. This will keep moisture out of the connection and also help keep corrosion from forming on the connection ensuring good contact and low resistance between the plugs and wires.

Different Types of Spark Plug Wear

It’s important to understand the symptoms of a bad spark plug. Knowing how to tell if a spark plug is bad, and how to read a spark plug can make the difference in helping your boats engine perform better, and potentially save it from further damage. We put together a few signs of bad spark plugs and what they mean below.

Carbon Fouled

How to read a spark plug

When you hear someone referring to a spark plug as “carbon fouled”, this usually means the plug has dark or black soot on it. From an engine issue stand point, this can be caused by a dirty air filter as well as too rich of a air to fuel mixture.

Oil Deposits

Signs of a bad spark plug

If the electrode on the spark plug has black, oily deposits on it, this is what is referred to as an “oil-fouled” spark plug. Typically, this buildup on a spark plug indicates some kind of oil leak into the combustion chamber of the engine. This can happen in a few different ways. Oil lubricates all moving parts in the combustion chamber from your pistons below, to the valves above. The seals around the valve stems as well as the rings around the pistons keep oil from entering the combustion chamber. If either of these are warn, oil can leak by them and make its way into the area where your spark plug electrodes are located, causing them to be oily.


Fouled spark plug

A spark plug that has charring on the end, typically white, is a sign of a spark plug that is running too hot. There are a few scenarios that cause this. An overheating engine is one of them. Be sure to check your engines cooling system and temperatures to make sure they are all in the proper ranges. A spark plug with the incorrect heat range could also cause this, make sure you have the proper spark plugs for your engine. A loose spark plug, incorrect ignition timing, or an air to fuel ratio that is too lean can also cause this burning on a spark plug.

Make sure you check these items if you are experiencing signs of a burned spark plug to make sure your system is operating properly.

Worn Electrode

Worn spark plug

If you’re changing or inspecting your spark plugs and see that they have worn electrodes, this may not be an indication of any major issues. Typically, a worn electrode simply signifies that a spark plug has been used beyond its run time and it’s time to replace it. In the future, it may be good to replace your spark plugs sooner before the electrodes start to break down.

Final Thoughts

Changing the spark plugs on a boat is a pretty simple job. We hope you now have more confidence in doing this yourself now that you have read through our guide. Your boat will thank you and run smoother with a fresh set of spark plugs. Always remember to check your plugs after you have removed them, or do a yearly inspections to spot any kind of issues they may be trying to notify you of. This could save money and headaches down the road if spotted in a timely manner. Happy boating!

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