Replacing Fuel Injectors In Mercedes: Step-By-Step Guide For Diyers

Changing the fuel injectors in a Mercedes-Benz can be a complex task. While some people may choose to do it themselves, others may prefer to seek professional help. There are several signs that can indicate that your fuel injectors need to be replaced, such as a rough-running engine and poor gas mileage. It is important to have the necessary tools and safety precautions in place before attempting to change the fuel injectors. Additionally, it is recommended to refer to a repair manual or seek assistance from a qualified mechanic if you are unsure about the process.

Testing fuel injectors

Another way to test fuel injectors is by measuring the resistance with a voltmeter. Checking for continuity by testing the resistance will indicate whether the injector is functioning properly. Additionally, swapping a couple of injectors to determine if the issue lies with the injector or the firing signal can be helpful.

For a more advanced test, you can build your own test station. This involves connecting the injector to a power source, such as a 6V or 12V battery, and observing the spray pattern. A good injector will produce a strong, consistent spray pattern without any dripping or leakage.

It is important to note that some fuel injectors cannot be tested with just a voltmeter. For example, the bench mount tester sold by Mercedessource.com will not work on electrically operated fuel injectors. Their tester is designed for older Bosch fuel injection systems without electrical wires plugging into the injectors.

Tools and parts required

The following tools and parts are required to change the fuel injectors in a Mercedes 300D, 300SD, or 240D:

• Fuel injectors (Monark or Bosio are recommended)
• Heat shields/crush washers (one for each injector)
• Fuel return line (one meter should be enough)
• 17mm combo wrench
• 1/2" drive x 27mm or 1-1/16" deep socket
• 1/2" drive breaker bar
• 1/2" x 3 to 5" extension
• Hook tool (or a flat screwdriver)
• Torque wrench (click type preferably)
• Pliers
• Wire cutter
• Paper towels or rags
• Fire extinguisher
• Chemical-resistant gloves
• Can of compressed air
• White lithium grease

Additionally, for a Mercedes-Benz W124, you will need the following tools:

• 10mm socket
• 15mm and 17mm wrenches
• 6mm Allen
• Flathead screwdriver
• Oil filter wrench

Removing the fuel injector

To remove the fuel injector from your Mercedes, you will need the following tools: a 10mm socket, 15mm and 17mm wrenches, a 6mm Allen, pliers, a flathead screwdriver, an oil filter wrench, and rags. You will also need new injectors and white lithium grease. Before you begin, ensure you have a fire extinguisher and know how to use it, as you will be working on the fuel system. It is also recommended to wear chemical-resistant gloves and have plenty of paper towels or rags for cleanup.

First, relieve the pressure in the system by removing the fuel pump relay and cranking the engine over a few times. This will draw the fuel out of the system and relieve pressure. The fuel pump relay is located in the upper right side of the engine bay, behind the battery.

Next, you will need to remove the crossover air pipe. Then, cut the zip ties on the wiring harness to give yourself room to move them without cutting any electrical lines. Blow all the dust and debris away from the injector ports using a can of compressed air.

Now, you can start to disconnect components. Using a 10mm socket or wrench, remove the single bolt holding the tray in place. Disconnect the wiring from the injectors by squeezing the wire clips and pulling the connectors straight off. Disconnect the vacuum line from the fuel pressure regulator. Move the wiring and tray out of the way towards the top of the engine and remove the dipstick.

To access the two rear 5mm Allen bolts on the injector rail, you will need to remove the oil filter lid. Use an oil filter socket or wrench to do this. There are also two fuel lines attached to the rail. Use a 15mm and a 17mm wrench to break the first line free, and then two 17mm wrenches to break the other line. Always support the fitting with another wrench when separating fuel lines.

There are five 6mm Allen bolts holding the fuel rail to the intake manifold and engine. Some of these bolts may be difficult to reach, so be sure the Allen is well-seated in the bolt head before attempting to remove them. You do not want to strip these bolts.

Now, you can remove the injectors and fuel rail. Pull them straight back and out from the head. You will feel a "pop" when they let go. Be sure to put rags in the open holes in the manifold to prevent anything from falling into the engine.

Take the injectors and fuel rail to your workbench. The injectors are held on by metal clips, which can be pried or pulled off using a small screwdriver. Wiggle the injector out of the rail. It is just held in by the O-ring at this point. Make sure that the O-ring in the fuel rail comes out with the injector. If it remains in the port, use a small screwdriver to remove it carefully, being careful not to scratch the port.

New injectors come with new O-rings. Apply a small amount of white lithium grease and reinstall. If you are reusing your old injectors, be sure to replace both O-rings before installing.

Installing the new injector

To install the new injectors, you will need to follow the removal process in reverse.

First, ensure that the new injectors are complete with new O-rings. Apply a small amount of white lithium grease to the O-rings and reinstall them. If you are reusing your old injectors, make sure to replace both O-rings before installing.

Next, insert the injectors into the fuel rail, ensuring that the O-ring in the fuel rail comes out with the injector. If the O-ring remains in the fuel rail port, use a small screwdriver to remove it carefully, being careful not to scratch the port.

Then, connect the wiring to the injectors and move the tray and wiring out of the way. Reconnect the vacuum line to the fuel pressure regulator.

After that, reconnect the two fuel lines to the rail, using a 15mm and 17mm wrench for the first line and two 17mm wrenches for the second line. Remember to always support the fitting with another wrench when separating fuel lines.

Finally, secure the fuel rail to the intake manifold and engine using the 6mm Allen bolts. Make sure that the Allen is well-seated in the bolt head before attempting to remove it to avoid stripping the bolts.

Once everything is connected and secured, you can start the car and keep it running to clear any remaining air from the lines. Check for leaks at the hard lines, injector-to-head, and the return lines.

Bleeding the fuel lines

Locating the Hand-Operated Prime Pump:

Start by opening the hood of your Mercedes-Benz. The hand-operated prime pump is located on the primary pump, which is situated on the right-hand side of the engine bay. This pump plays a vital role in removing air from the fuel system.

Operating the Hand-Operated Prime Pump:

Once you have located the hand-operated prime pump, it's time to operate it. Start by unscrewing the handle of the pump by turning it counterclockwise. Then, work the pump handle up and down until you feel resistance and hear air escaping from the injection pump. This indicates that you are successfully removing air from the fuel system.

Next, push the pump handle back into the pump and secure it by twisting it clockwise. Be careful not to over-tighten the pump handle.

Cranking the Starter:

After preparing the hand-operated prime pump, it's time to crank the starter to expel any remaining air in the fuel system. Turn the starter for no more than 30 seconds at a time to avoid potential damage. If the engine still doesn't start after several attempts, you may need to take the following additional steps:

Loosening and Retightening Fuel Injector Nuts:

Locate the fuel injectors on the cylinder head and use a 17mm combination wrench to loosen the attaching nuts in a counterclockwise direction. It's important to note that you should not completely remove the nuts.

Crank the engine until you see fuel seeping out of the loosened fuel injector nuts. At this point, use the 17mm combination wrench again to retighten the fuel injector nuts, this time turning them clockwise.

Starting the Engine:

Finally, start the engine and let it run for a few minutes. This will ensure that any remaining traces of air are expelled from the fuel system. After this process, your Mercedes-Benz diesel engine should be free of air in the fuel lines, and you can proceed with any necessary maintenance or repairs.

It's important to note that, after bleeding air from the fuel system, your Mercedes-Benz diesel engine may run roughly and produce excessive smoke from the tailpipe. This is normal, and the smoke should clear as any remaining air is expelled from the system.

Frequently asked questions