Replacing Optimax Fuel Injectors: A Step-By-Step Guide

The Mercury Optimax DFI engine is a 2-stroke direct-injected motor made and designed by Mercury Marine. It is a complex motor that combines high-pressure fuel and high-pressure air to inject an atomised mixture of gas directly into the combustion chamber. The Optimax air and fuel injection system is fairly complicated, and maintenance and fuel-related issues are the main causes of engine performance problems. The fuel injectors can be removed and sent out for cleaning and adjustment, which can cost up to $1000. However, it is possible to remove the injectors yourself, which can save a lot of money in labour costs.

Depressurise fuel and air systems

Depressurising the fuel and air systems of your Optimax engine is a crucial step before replacing the fuel injectors, as it will prevent spraying yourself or your equipment with hot, highly pressurised fuel. Here is a detailed, step-by-step guide on how to safely depressurise the fuel and air systems:

Step 1: Shut Off the Engine and Let it Cool

Firstly, make sure to shut off the engine and let it cool down. This is important because many models return unburnt fuel to the tank while it is still hot, so giving it time to cool down will allow the fuel to reach room temperature.

Step 2: Loosen the Fuel Filter Cap

Once the engine has cooled, locate the fuel filter cap and loosen it without removing it completely. This step helps to release any remaining pressure in the system.

Step 3: Locate and Loosen the Fuel Line Fittings

Refer to your owner's manual to locate the fuel line fittings, which are usually found in the engine compartment or on the undercarriage's frame rail. Using a flare wrench, carefully loosen the fittings without removing them. This will help to further release the pressure inside the fuel line.

Step 4: Wait for Any Hissing to Stop and Clean Up Spilled Fuel

After loosening the fuel line fittings, wait for any hissing noises to stop, which indicates that the pressure has been released. Use a clean rag or towel to wipe up any spilled fuel to ensure a safe work environment.

Step 5: Remove the Fuel Lines (If Replacing Fuel Filters)

If you are replacing the fuel filters, you will need to completely remove the fuel lines. Refer to your owner's manual for specific instructions on how to locate and remove the fuel lines for your particular model.

Step 6: Depressurise the Air System via the Schrader Valve

Now, we will focus on depressurising the air system. The air system can be depressurised via the Schrader valve, which is located on the fuel rails. Start by removing the caps from the Schrader valves on both the air and fuel sides of the fuel rails. This step is especially important if your engine has been running within the last 8 hours, as there may still be pressure built up in the system.

Step 7: Confirm Zero Pressure

Even if your engine has not been run recently, it is a good idea to confirm that there is no pressure in the system. You can do this by using a pressure gauge to check the Schrader valves on both the air and fuel sides of the fuel rails.

By following these steps, you will safely depressurise the fuel and air systems of your Optimax engine, allowing you to proceed with replacing the fuel injectors. Remember to always refer to your owner's manual for model-specific information and instructions, as each diesel model may have unique variations.

Disconnect fuel injector connectors

Disconnecting the fuel injector connectors is a crucial step in removing and replacing the fuel injectors in an Optimax engine. Here is a detailed guide on how to do this:

First, it is important to depressurise the fuel and air systems via the Schrader valves on the fuel rails. Even if the engine has not been run recently and there is no pressure, it is a good idea to check and release any pressure that may be present.

The next step is to locate the fuel injector connectors. These are usually held in place by a metal retaining clip. To remove the connectors, depress or pull this metal clip from the back. A small flat-head screwdriver or a right-angle pick tool can be useful here to release the clip without damaging any other components. Be careful not to break the plastic retainer "ears" that may be present.

Once the metal clip is released, the fuel injector connectors should be able to be carefully removed. They may be a tight fit, so patience and caution are important to avoid any damage.

Repeat this process for all the fuel injector connectors. On a 2.5L Optimax engine, there are six fuel injectors and six direct injectors, so twelve connectors in total. The process is similar for 1.5L and 3L models, although there are some differences in the orientation of the direct injector connectors.

After all the connectors have been removed, move on to removing the injector harness. There will be wiring harness retainers that need to be released from the brackets surrounding the cylinders. An auto-body trim tool can be useful for this step.

By following these steps, you will successfully disconnect the fuel injector connectors, allowing you to proceed with the removal and replacement of the fuel injectors in your Optimax engine.

Remove the injectors

To remove the injectors, start by depressurising the fuel and air systems via the Schrader valves on the fuel rails, beginning with the air side. If your engine has not been run for at least 8 hours, this step may not be necessary, but it is still recommended to remove the caps and check.

Next, identify the Oetiker clamps that need to be cut. Begin with the one on the air compressor above the port fuel rail. Then, to remove the fuel injector connectors, simply depress the metal retaining clip from the back. The top and bottom side injector connectors are a tight fit, but they will fit through the metal brackets.

While still on the port rail, remove the connectors on each direct injector (DI). A small screwdriver or right-angle pick tool may be useful here. Repeat this process for the starboard rail side and direct injectors.

Now, move the injector harness out of the way. There are four 'Christmas tree'-style wiring harness retainers that need to be released from the brackets surrounding cylinders 2/6 and 1/5 side injectors. An auto-body trim tool can be used here. Once these four retainers are freed, you'll need to disconnect both the water pressure and pitot sensor connectors.

Then, cut the radius head tie wrap at the water inlet to the compressor and remove the hose. Open the double hose retainer and cut the radius head tie wrap on the back of the compressor. It is critical that the airline be attached to the rails.

Once all these steps are completed, lift and move the injector harness out of the way. Follow the fuel hoses from the rails to their origin on the VST. Cut the Oetiker clamp, clamp the hose prior to removal to minimise fuel spill, and remove the hose from the nipple. Drain any fuel into a suitable container. Repeat for the starboard fuel rail.

Now that all the lines, connectors, and hoses have been removed, you are ready to remove the fuel rails. Remove the two nuts holding the fuel rails in place, then carefully slide the fuel rails off, ensuring the direct injectors remain in the head.

Finally, remove the direct injectors. Some may be difficult to remove, while others may come out easily. Rotate the DIs while pulling straight out and be prepared in case one suddenly releases. Make sure you have a firm grip, as dropping a DI can be costly. As you remove them, mark their location on the bottom side of the connector with a Sharpie.

And that's it! You have now successfully removed the injectors.

Clean the injectors

To clean the injectors, start by depressurising the fuel and air systems via the Schrader valves on the fuel rails, beginning with the air side. If your engine has not been run for at least 8 hours, this step is not necessary, but it is still recommended to remove the caps and confirm.

Next, identify the Oetiker clamps that need to be cut and start with the one on the air compressor above the port fuel rail. Then, remove the fuel injector connectors by depressing the metal retaining clip from the back. The top and bottom side injector connectors are a tight fit, so be patient and careful.

Once the connectors have been removed, move the injector harness out of the way. There are four "Christmas tree" style wiring harness retainers that need to be released from the brackets surrounding the cylinders. An auto-body trim tool can be helpful here. Then, disconnect the water pressure and pitot sensor connectors.

Now, you can start to remove the fuel rails. Remove the two nuts holding the fuel rails in place, then carefully slide the fuel rails off, ensuring the direct injectors remain in the head. Place the assembly out of the way and drain the rails.

At this point, you can start to remove the direct injectors (DIs). Be patient, rotate the DIs while pulling straight out, and be prepared in case one suddenly releases. Make sure to mark their location (e.g. cyl 1, 2, etc.) on the bottom side of the connector.

After all the components have been removed, it is important to ensure that your work area, hands, and tools are clean and free of dust, dirt, grime, and debris before beginning the reassembly process. Reassembly is basically the reverse of the removal process.

It is recommended to have a bona fide, OEM Mercury Service Manual for your engine before starting this procedure, especially for reassembly.

Reinstall the injectors

Now that your fuel injectors have been cleaned and serviced, it's time to reinstall them. This process will be the reverse of the removal process.

Firstly, ensure your work area, hands, and tools are clean and free of dust, dirt, grime, and debris. This is important as you will be working with freshly serviced components.

Before installing the direct injectors, coat the o-rings with oil. You can use Premium Plus oil or a similar product. This will help to lubricate the injectors.

Next, carefully install the direct injectors by hand. Be patient and gentle during this process, as some may be difficult to insert. It is important to have a firm grip to avoid dropping the injectors, as this can be costly.

Once all the direct injectors are in place, it's time to move on to the fuel rails. Ensure that the direct injectors remain in the head while sliding the fuel rails back into position. Secure the fuel rails by tightening the two nuts that hold them in place.

Now, reconnect the fuel injector connectors. Simply depress the metal retaining clip from the backside to release the connectors. Be careful not to break the plastic retainer "ears".

After that, reconnect the injector harness by attaching the "Christmas tree"-style wiring harness retainers to the brackets surrounding the cylinders. An auto-body trim tool can be helpful for this step.

At this point, you can also reconnect the water pressure and pitot sensor connectors. On 3-liter models, you will only have the water pressure connector on the coil plate.

As you work, be sure to replace any removed radius head tie wraps with exact replacements. Common tie wraps are not suitable substitutes.

Finally, torque all fasteners to the proper specifications according to your service manual.

And that's it! You have now successfully replaced your Optimax fuel injectors.

Frequently asked questions