Replacing Fuel Injectors In Your 2003 S500: A Step-By-Step Guide

If you're looking to replace the fuel injectors on your 2003 S500, you've come to the right place. This job can cost upwards of $1500 at a shop, but with the right tools and know-how, you can do it yourself at home and save some money. The fuel injectors on your S500 are all attached to a common fuel rail that stretches around the engine. Before beginning any work on the fuel system, it is important to relieve the pressure in the fuel rail. This can be done by briefly running the engine with the fuel pump fuse removed, or by attaching a hose to the Schrader valve on the rail to catch the released fuel. Once the pressure is relieved, the next step is to remove the wiring loom and the fuel rail, taking care to clean the area around the injectors and avoid getting any debris in the fuel injector holes. At this point, the injectors can be unclipped and removed by gently pulling downward. When installing the new injectors, be sure to lubricate the O-rings with engine oil to prevent damage and ensure a tight seal. Reassemble the fuel rail and wiring, and you're almost done! The final steps are to start the engine and check for any leaks around the injectors.

Remove engine cover and disconnect fuel injector wiring

To remove the engine cover of a 2003 S500, you will need to first pull the front part of the cover towards you and then lift the larger part from the rear, which will pivot forward.

Now, to disconnect the fuel injector wiring, you will need to:

• Remove the airbox/engine cover and two intake tubes.
• If the car has been run recently, unscrew the Schrader valve on the passenger side. Get some towels and insert a screwdriver to release fuel pressure.
• Use a 17mm open wrench on the fuel delivery line on the driver's side rear of the engine. There will be some residual fuel.
• Remove the fuel rail cover by clipping the zip ties and then releasing the clips. There is also one 8mm bolt in the back.
• Disconnect the electrical connectors.

At this point, you will have access to the fuel injectors and can proceed to manually disconnect the fuel injector wiring on the suspected cylinder. It is recommended to double-check the cylinder order and, if necessary, restart the engine and re-read the codes to ensure you have disconnected the correct cylinder.

Depressurise the fuel rail

Depressurising the fuel rail is an important step in replacing the fuel injectors on a 2003 S500. Here is a detailed, step-by-step guide on how to safely depressurise the fuel rail:

Prepare the Engine:

Start by turning off the engine and allowing it to cool down. This is a crucial safety measure as you will be working with fuel, which can be highly flammable. Make sure the engine is cold before proceeding to the next steps.

Relieve Fuel Pressure:

The fuel rail can contain significant pressure even when the engine is off. To relieve this pressure, you have a few options:

• Use a Hose and Container: Attach a hose to the Schrader valve on the fuel rail. This will allow you to direct the pressurised fuel into a container, preventing spills and reducing the risk of injury.
• Remove the Fuel Pump Relay: Alternatively, you can remove the fuel pump relay, usually located in the fuse box under the right rear seat. After removing the relay, simply start the engine. The existing fuel in the rail will be sucked out, and the engine will stall after a few seconds as no more fuel can be supplied.
• Loosen the Fuel Filter Cap: Loosen (but do not remove) the fuel filter cap to release any remaining pressure in the system.

Release Pressure from Fuel Line:

Use a flare wrench to loosen the fuel line fittings, which are typically located in the engine compartment or on the undercarriage's frame rail. Loosen the fittings without removing them to release the pressure inside the fuel line. Wait for any hissing sounds to stop, and clean up any spilled fuel.

Final Steps:

If you are replacing the fuel injectors, you will need to remove the fuel lines completely. Be sure to consult your owner's manual for model-specific information and additional safety precautions. Remember to work with caution and always put your safety first when working with pressurised fuel systems.

Disconnect the wiring loom

To disconnect the wiring loom, you will need to remove the engine cover. The engine cover is in two parts: pull the front part towards you, then lift the larger part from the rear and it will pivot forward.

Next, you will need to remove the plastic shroud from the wiring loom. This can be done by unclipping the shroud with a flat-head screwdriver and removing any cable ties that may be present. There is also a single 8mm bolt at the rear that needs to be removed.

With the shroud out of the way, you can now access the fuel injectors and disconnect the wiring. It is important to note that you only need to unclip the shroud and move the wiring loom out of the way, you do not need to unplug anything in the wiring loom (apart from the fuel injectors themselves).

Once the wiring loom is disconnected, you can proceed with the rest of the fuel injector replacement procedure.

• It is recommended to double-check the cylinder order before beginning the procedure. You can do this by manually disconnecting the fuel injector wiring on the suspected cylinder, restarting the engine, and re-reading the codes.
• Before removing the fuel rail, depressurize the system by attaching a hose to the Schrader valve on the rail or removing the fuel pump relay (located in the fuse box under the right rear seat) and starting the engine.
• Clean the area around the fuel injectors before removing them to prevent any dirt or grime from falling into the fuel injector holes.
• When removing the fuel rail, apply gentle and equal pressure on all injectors to lift it out of place.
• When reassembling, lubricate all O-rings with engine oil to prevent damage.
• Before replacing the engine cover, start the engine and check for any gas leaks around the injectors. If there is a leak, it is likely due to a damaged or old O-ring.

Remove the fuel rail

To remove the fuel rail of a 2003 S500, you will need to relieve the pressure within it. This can be done by briefly running the engine with the fuel pump fuse out.

Next, you will need to remove the upper plenum on the intake manifold, along with any cables, hoses, or wiring that may restrict access. This includes the wiring loom, which can be removed by simply unclipping the shroud with a flat-head screwdriver and removing a single 8mm bolt at the rear.

Once you have clear access to the fuel rail, the next step is to remove the supply and return lines, as well as the electrical connectors, from the fuel rail. Be sure to mark the supply and return lines so that you can reconnect them properly later.

With the lines and connectors removed, you can now focus on the fuel rail itself. The fuel rail is held in place by bolts; the number and type of bolts may vary depending on your vehicle, but for a 2003 S500, you will likely need to remove eight 10mm bolts and one 8mm bolt.

After removing the bolts, gently lift the fuel rail. It should come out easily, but if it is stuck, you can use a pry bar or a long flat-head screwdriver to gently lever it up. Be careful not to damage the injectors or any other components during this process.

Once the fuel rail is removed, you can proceed to replace or service the fuel injectors as needed. Remember to lubricate any O-rings with engine oil before reinstalling them.

Clean the area around the injectors

To clean the area around the injectors of your 2003 S500, you'll need to gather some tools and materials. These include a fuel injector cleaner, wrench, rags or cloths, and protective goggles and gloves. It is also recommended to park your vehicle in a well-ventilated area, away from direct sunlight or any open flames, and to let the engine cool down before you begin.

Step 1: Disconnect the Fuel Pump and Pressure Regulator

Turn off the fuel pump by removing the relevant fuse or relay. Then, start the engine and let it run until it uses up the remaining fuel in the lines and dies. This step is crucial for depressurizing the fuel system and ensuring your safety.

Step 2: Locate and Disconnect the Fuel Injectors

Fuel injectors are typically located near the engine intake manifold. Refer to your vehicle's owner's manual for the exact location. Once located, carefully disconnect the electrical connectors from each injector.

Step 3: Remove Any Bolts or Fasteners

Using your wrench, remove any bolts or fasteners that hold the fuel rail in place. Be gentle during this process to avoid damaging any components.

Step 4: Lift the Fuel Rail and Remove the Injectors

Gently lift the fuel rail and carefully pull out the injectors. It is important to handle them with care to avoid any damage. Place them in a safe area, ensuring they do not roll off your workspace and onto the ground.

Step 5: Clean the Area Around the Injectors

Use a rag or cloth to wipe down the area around the injectors. Be thorough and ensure that all dirt, grease, and grime are removed. You can also use a degreaser, citrus cleaner, or compressed air to assist in the cleaning process. Pay special attention to any nooks and crannies, as dirt and debris can accumulate in these areas. Additionally, you can use pipe cleaners or Q-tips to reach the hard-to-reach spots.

Step 6: Prepare for Reassembly

Once you are satisfied with the level of cleanliness, prepare for the reassembly process. Have all the tools and materials ready to ensure a smooth and efficient process.

Remember to follow safety guidelines and wear the appropriate protective gear throughout the entire process.

Frequently asked questions