Changing The Fuel Screen On Your 1991 Mercedes 420Sel

If you're looking to change the fuel screen on your 1991 Mercedes 420SEL, you'll first need to locate the fuel filter, which is underneath the car at the right rear. It's a fairly straightforward process, but it's always good to have a guide on hand to walk you through the steps.

One important thing to note is that the metal/rubber connection from the filter to the tank is delicate and can crack easily, so handle it with care. It's also worth checking the rubber fuel lines, as these can crack over time, leading to fuel leaks.

The fuel filter is located underneath the car at the right rear

The fuel filter on your 1991 Mercedes 420SEL is located underneath the car at the right rear. This means that you will need to jack up the car and get underneath it to access the fuel filter. It is important to use jack stands and chocks to support the car and ensure it does not move or roll while you are working.

Once you are safely underneath the car, you will need to locate the fuel filter. It is usually located near the fuel pump, which is behind the rear axle on the passenger side. The fuel filter is a cylindrical or oval-shaped metal canister with fuel lines running into and out of it. There may also be a rubber or metal connector from the filter to the fuel tank, so be careful not to crack or damage this connection when removing the old filter.

Before removing the old fuel filter, place a drip pan or container underneath it to catch any fuel that may spill. Fuel filters can be tricky to remove, especially if they have not been changed in a while, so you may need to use a filter wrench or oil filter pliers to loosen it. Be careful not to damage the fuel lines when removing the old filter.

When installing the new fuel filter, make sure that the fuel lines are securely connected and that there are no leaks. You may need to prime the new fuel filter by pouring a small amount of fuel into it before starting the car. It is also a good idea to check the condition of the rubber fuel lines and replace them if they are cracked or brittle.

Remember to dispose of the old fuel filter properly and in accordance with local regulations.

The fuel pump is located under the car on the passenger side

The fuel pump is an electrically driven device that delivers fuel from the gas tank to the engine. It is responsible for pressurizing and sending fuel through the fuel lines, ensuring the engine receives the required amount of fuel for proper operation.

The fuel pump on a Mercedes 420SEL is typically located in the fuel tank. However, for the 1991 model, it is situated under the car, on the right rear side. This makes it easier to access and service the pump without needing to remove the fuel tank.

The fuel filter is also located in a similar position, underneath the car on the passenger side.

If you need to replace the fuel pump on your 1991 Mercedes 420SEL, you can follow these steps:

• Locate the fuel pump under the car on the passenger side, behind the rear axle.
• Prepare the necessary tools and a replacement fuel pump. You can refer to online resources or a repair manual for specific tools and detailed instructions.
• Ensure the engine is cool before starting work and take appropriate safety precautions.
• Carefully raise the vehicle and securely support it on jack stands.
• Identify the fuel lines and electrical connections leading to the fuel pump.
• Disconnect the fuel lines and electrical connectors from the fuel pump.
• Remove the mounting hardware securing the fuel pump and carefully remove the old pump from the vehicle.
• Install the new fuel pump, ensuring it is securely mounted and all connections are properly attached.
• Reattach the fuel lines and electrical connectors to the new fuel pump.
• Lower the vehicle and start the engine to test the new fuel pump.

It is important to note that working on fuel systems can be dangerous due to the flammable nature of fuel. Always exercise extreme caution and refer to a qualified mechanic if you are unsure about any part of the process.

Adjusting the fuel mixture on a CIS-E fuel injection system is not easy

The CIS system is very reliable but is also known for having a few characteristic problems. The two primary adjustments for the CIS are the idle speed and the mixture adjustment. The mixture can be adjusted by turning a small hex screw that is located in-between the fuel distributor and the sensor plate. Use a long hex key, or the special mixture adjustment tool to turn the screw.

The screw is indexed so that it should click when you rotate it. Turn it clockwise to richen up the mixture, or counterclockwise to lean it out. The mixture should only be adjusted when the car is hooked up to a CO meter. Make sure that you rev the engine for a few seconds before you take a reading from the meter, and that you don't leave the tool in the adjustment screw: the screw itself must be free to move up and down without any weight on it.

The idle speed is adjusted by allowing more air to bypass the closed throttle. Using a large flathead screwdriver, simply rotate the screw clockwise to reduce the idle, and counterclockwise to increase it. The idle should be set to be about 950 rpm.

The CIS system in general is very reliable, and was only bested in 1984 by the introduction of the Motronic engine management system that controls both ignition and fuel injection together.

The CIS meters the amount of fuel provided to the cylinders as a function of the air drawn into the engine. The CIS air flow sensor plate measures the amount of air that is being drawn into the system. This sensor plate is attached to a fuel distributor that evenly meters the amount of pressurized fuel sent to each injector. The system doesn't differentiate among the six injectors: it sends the same amount of fuel to them at all times. Unlike pulsed-injection systems like the Motronic system or even the earlier Mechanical Fuel Injection system, the CIS injectors distribute fuel to the cylinders at all times. The opening and closing of the intake valves control the intake of fuel into and out of the combustion chamber.

Through 1979, CIS was a semi-open-looped system, which means that it didn't monitor the output gases from the engine to see if the air/fuel mixture was set at the appropriate levels. In 1980, emissions requirements forced the introduction of an oxygen sensor into the system, which helped the engine to better run at the appropriate mixture level through the regulation of fuel pressure inside the fuel distributor.

One of the most common problems for the older CIS cars is the existence of vacuum leaks. The entire system works almost entirely on vacuum. Any vacuum leaks will cause problems in regulating the mixture, and may also result in an uneven idle. To check for vacuum leaks, spray some carb cleaner or starting fluid around the hoses and injectors of the fuel injection system. If the engine idle increases, then you probably have a vacuum leak that needs to be tracked down. Make sure that you have a fire extinguisher handy: if there is arcing in the spark plug wires, you may ignite the carburetor cleaner.

Another common problem is the warm-up regulator (WUR), or control pressure regulator. Contrary to popular belief, the WUR regulates fuel pressure in the system at all times. Cars that experience difficulty adjusting the mixture on may be suffering from a defective WUR. This failure may also contribute to starting problems.

Rubber fuel lines may be cracked

If the rubber fuel lines in your 1991 Mercedes 420SEL are cracked, it is important to address the issue promptly to prevent fuel leaks, which can be dangerous. Here are some steps you can take to deal with cracked rubber fuel lines:

Inspect the Damage

First, thoroughly inspect the rubber fuel lines to identify the extent and location of the cracks. This will help you determine the best course of action for repairing or replacing the lines.

Replace the Rubber Fuel Lines

The most reliable and durable solution is to replace the cracked rubber fuel lines with new ones. You can purchase replacement fuel lines from automotive parts stores or online. It is important to use fuel lines specifically designed for automotive fuel systems, as they need to withstand high pressure and be compatible with fuel types. Ensure that you select the correct diameter and length of fuel line for your vehicle.

Temporary Repairs

If you are unable to replace the fuel lines immediately, there are some temporary repair methods that can help you get back on the road, but these are not long-term solutions:

• Rubber Hose and Clamps: You can use a section of rubber hose with a similar inner diameter to the fuel line and secure it over the cracked area with hose clamps. This method may not be suitable for high-pressure fuel systems, and it is important to use fuel-rated hoses to prevent leaks.
• Plastic Line Repair Kit: Some automotive parts stores sell repair kits for plastic fuel lines. These kits usually include barbed fittings that you can insert into the cut fuel line after softening it with hot water. This method may not be suitable for all types of fuel lines, and it is important to follow the instructions carefully.
• Epoxy or Tape: In some cases, you can use plumbing epoxy or specialized tape, such as heat-resistant race tape, to seal small cracks or pinhole leaks in the fuel line. However, this is not a permanent solution, and it may not be suitable for high-pressure fuel systems.

Safety Precautions

When working with fuel lines, it is crucial to prioritize safety. Always wear appropriate safety gear, such as gloves and eye protection. Ensure the vehicle is parked in a well-ventilated area, and never attempt to repair or replace fuel lines while the engine is running. If you are uncomfortable or unsure about performing these repairs, it is best to consult a professional mechanic.

Remember, while temporary repairs can be useful in an emergency, it is essential to replace the rubber fuel lines with new ones as soon as possible to ensure the safe and reliable operation of your 1991 Mercedes 420SEL.

Metal/rubber connection from filter to tank is easy to crack

When changing the fuel screen on a 1991 Mercedes 420SEL, it is important to be careful when handling the metal/rubber connection from the filter to the tank, as it can easily crack when old. This connection is made of rubber and metal, and over time, the rubber can become brittle and crack, especially when handled roughly.

To avoid cracking this connection, it is recommended to use caution and a gentle touch when handling it. It may be helpful to use a soft cloth or gloves when handling the connection to avoid any accidental damage. If the connection does crack, it will need to be replaced, as a cracked connection can cause fuel leaks and other issues.

In addition to the metal/rubber connection, it is also a good idea to inspect and replace any rubber fuel lines under the car that may be cracked or deteriorated. These rubber fuel lines can also leak fuel if they are not in good condition. It is recommended to purchase fuel injection-grade fuel hose from an auto parts store and replace any lines that show signs of wear or damage.

When working on the fuel system of a 1991 Mercedes 420SEL, it is important to take your time and be careful when handling the various components, especially the metal/rubber connection from the filter to the tank, to avoid any accidental damage or leaks.

Overall, the metal/rubber connection from the filter to the tank on a 1991 Mercedes 420SEL is a delicate component that requires careful handling to avoid cracking. By using caution and replacing any worn or damaged parts, you can help ensure the fuel system is safe and functioning properly.

Frequently asked questions