Mercedes Mullins
Diesel engines are known for their ability to withstand extreme conditions, but they are susceptible to air in their fuel systems. This can cause issues with starting the engine, reduced engine power, or even the inability to start the engine altogether. While modern diesel engines are better equipped to handle this issue, older diesel engines may require manual intervention, such as priming the fuel system or bleeding air out of the fuel system. Turning up the fuel on a diesel engine can be achieved by adjusting the fuel screw, but it is important to exercise caution as it can lead to increased internal heat and potential damage to the pistons.
| Characteristics | Values |
|---|---|
| How to turn the fuel up on a diesel | Turn the screw clockwise in small increments. |
| How to prime the fuel system | Turn the ignition on (Run position) for 30 seconds, but do not start the engine. |
| What to do if the engine doesn't start | Repeat the priming process until it starts. If it doesn't start after several attempts, take a break and then try again. |
| What to do if the engine starts but stalls | Wait for one minute before attempting to start it again. |
| What to do once the engine starts | Allow it to idle for several minutes and check for any leaks before driving. |
| How to prevent air from entering the fuel system | Use an AirDog® fuel system, which removes entrained air from diesel fuel. |
| How to bleed air out of the fuel system | Loosen the bleeder screw with a wrench, allowing air and bubbles to escape. Re-tighten the screw once the system is free of air bubbles. |
| Why it's important to prevent air from entering the fuel system | Air in the fuel system can delay or prevent diesel fuel from reaching the injectors, making the engine difficult to start. |
| Why you might want to increase the fuel supply to an engine | To get more power. |
| Warning when turning up the fuel on your engine | This will cause more internal heat, which could melt your aluminum pistons. Be conservative with this adjustment and carefully watch your EGT (Exhaust Gas Temperature). |
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What You'll Learn
Manually prime the fuel system
Modern diesel engines have made priming the fuel system a lot easier, and in some cases, the process is handled by the trucks themselves. However, for older cars, the process may require more manual intervention and the use of tools.
To manually prime the fuel system, you must first ensure that there is a reasonable amount of fuel in the system. It is difficult to prime the system without fuel in the tank, and the system may draw air if there isn't enough fuel. The next step is to locate the bleed screw, which is a small valve found in various locations in the fuel system of a diesel engine. Its purpose is to allow trapped air to exit the system, ensuring that only pure fuel runs through the lines. The common locations of the bleed screw include the fuel filter housing, the top of the engine, the fuel pump, and the fuel injectors. It usually has a hexagonal head with a small hole on top for opening and closing with a tool, such as a wrench or a screwdriver. When opening the screw, have a container and a rag ready, as the release of air pockets may also cause some fuel to escape.
Once the bleed screw is located and opened, the next step is to cycle the key to prime the fuel system. This involves turning the ignition on (to the "Run" position) for about 30 seconds without starting the engine. This allows the pump to prime the system. Then, turn the ignition off and crank it for about 15 seconds. If the engine does not start, repeat the process until it does. According to some sources, it may take anywhere from 2 to 10 key cycles, waiting until the fuel pump shuts off (~20 seconds) for each cycle. It is important to note that working with the battery on can be a safety hazard and may cause electrical damage, so it is recommended to disconnect the battery before performing any maintenance on the fuel system.
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Identify key components of the diesel fuel system
The key components of a diesel fuel system work together to inject a specific amount of pressurised and atomised fuel into the engine cylinders at the right time. Here are the key components of a diesel fuel system:
Fuel Tank
The fuel tank stores enough fuel to keep the engine operational. It is closed to avoid foreign particle contamination and has vents to enable air to enter and replace the fuel being used. It also requires three additional openings: one for filling the tank, one for discharging fuel, and one for drainage.
Fuel Lines
Fuel lines are pipes that carry fuel from the tank to the engine. They can be heavyweight, medium-weight, or lightweight, depending on the pressure they need to withstand.
Fuel Filters
Fuel filters are essential for removing impurities, water, and debris that could damage the engine. Most systems require multiple stages of filtration. Simple fuel systems use gravity or air pressure to move fuel from the tank to the injection pump, while modern high-speed diesel engines use fuel transfer pumps.
Fuel Pumps
Fuel pumps pressurise the fuel and send it to the injectors. Mechanical or electronic fuel pumps are used, depending on the engine's design and age.
Fuel Injectors
Fuel injectors deliver a precise amount of atomised and pressurised fuel into each cylinder. This results in increased power, improved fuel economy, decreased engine noise, and smoother operation. Modern fuel injectors use piezoelectricity, allowing them to handle very high pressures with extreme precision.
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Bleed air out of the diesel fuel system
Bleeding your diesel engine involves extracting air from inside your car's fuel system. Diesel fuel systems are designed to maintain pressure ratings from 10,000 pounds per square inch (PSI) to 30,000 PSI. This pressure is necessary to pump the fuel into the engine, allowing it to function correctly. Air can prevent the buildup of sufficient pressure, which hinders the timing of the fuel injection process. This presence of air bubbles within a diesel fuel system is known as an airlock.
Airlocks commonly occur following routine maintenance to the fuel line, fuel filter replacements, or when your fuel tank runs out of fuel. They can also occur when there is a small leak in your fuel system. Before attempting to bleed the air from the fuel system, ensure you have determined how air entered the fuel system so that it does not re-enter after completing the bleeding process.
- Gather the tools and other materials needed for the job.
- Familiarize yourself with the engine and identify the key components of its diesel fuel system, including the fuel tank, fuel lines, fuel filter, and fuel injectors. Review the engine's fuel system layout and connections.
- Work with the fuel filter. Confirm that you have turned off the engine and removed the ignition key from the cylinder.
- Locate the fuel filter and determine whether it has a built-in bleeder screw. If it does, position a clean rag or towel beneath the bleeder screw to catch any fuel spills, then use a wrench or bleeder screw key to slowly loosen the screw in a counterclockwise direction. If the fuel filter lacks a bleeder screw, consult your vehicle's manual to understand your system's specific ins and outs.
- Cover all venting units to prevent spills. Crank the engine until all air is forced out and fuel is present. The engine will start to pop on one or two cylinders.
- Tighten the injector lock nut one at a time to tell by sound which cylinders are firing properly. Run the engine until it runs smoothly. This will bleed the other injectors.
- Tighten the bleed plug. Attempt to start your engine and evaluate its condition. Repeat the above steps for the primary filter, fuel pump, and injection lines if the engine doesn't start or runs poorly.
- Dispose of any spilled fuel, clean up, and return your tools to their proper location.
If you encounter any difficulties or persistent issues after repeating the bleeding process, consult a professional diesel mechanic or contact the manufacturer for further assistance.
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Adjust the fuel screw
Adjusting the fuel screw on a diesel engine will change how fast or slow the engine idles and how smoothly it runs. The basic process for adjusting a fuel screw is similar for all small engines, whether it's a car, motorcycle, scooter, ATV, or any other vehicle with a carburetor.
First, locate the carburetor by finding the engine's air filter. The carburetor is the part of the engine that the air filter is attached to. The air filter on a car is usually large and round, and sits on top of the carburetor, while on a motorcycle, the air filter usually comes off the side of the engine and faces the rear of the bike.
Once you've found the carburetor, look for the fuel screw. This is usually a flat-headed, slotted, gold-colored brass screw. Now, start the engine and let it run for around 5 minutes to warm it up to normal operating temperature. It's important to always make adjustments with a warm engine so that you can listen to how the adjustments affect the engine's idle speed.
Now, you can begin to adjust the fuel screw. Turning the screw clockwise will decrease the amount of fuel flowing to the engine, while turning it counterclockwise will increase the fuel flow. Keep turning the screw clockwise until the engine starts to sound rough. Listen for a rough rising and falling sound instead of its normal idling sound. Once you've reached this point, turn the screw counterclockwise until the engine runs smoothly again.
Finally, re-adjust the idle speed screw so that the engine is idling at the correct RPM. And that's it! You've successfully adjusted the fuel screw on your diesel engine.
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Monitor the exhaust gas temperature
Monitoring the exhaust gas temperature (EGT) of a diesel engine is important for several reasons. Firstly, it helps to prevent engine damage caused by overheating, especially in air-cooled engines used in some cars and motorcycles. Secondly, it provides information about the vehicle's air-fuel ratio (AFR). At a stoichiometric air-fuel ratio, the exhaust gas temperature differs from that in a lean or rich air-fuel ratio. At a rich air-fuel ratio, the exhaust gas temperature either increases or decreases depending on the fuel type.
High temperatures, typically above 1,600 °F or 900 °C, can be a warning sign of dangerous conditions that may lead to catastrophic engine failure. To avoid this, an EGT gauge or sensor is used in conjunction with a thermocouple-type pyrometer to monitor the exhaust gas temperature. This allows the driver or pilot to adjust the fuel-air mixture accordingly, which is particularly important in light piston aircraft with manual mixture controls.
In diesel engines, exhaust gas temperature sensors play a crucial role in monitoring the temperature of the diesel particulate filter (DPF) to establish the correct temperature for regeneration, reducing harmful emissions. It is common to have multiple sensors fitted at different points in the exhaust system, including before and after the turbocharger and particulate filter. These sensors come in two types: one with a positive temperature coefficient (PTC) sensing element and the other with a negative temperature coefficient (NTC). The main difference between them is how they measure temperature. NTC sensors have high resistance at low temperatures and low resistance at high temperatures, while PTC sensors exhibit the opposite behaviour, with resistance increasing as temperature rises.
It is important to note that exhaust gas temperature sensors are prone to failure due to exposure to excessively high temperatures, severe vibration, and contamination from fluids such as oil or antifreeze. A faulty sensor can negatively impact the vehicle's after-treatment system and trigger the check engine light. Therefore, regular maintenance and diagnostics are necessary to ensure accurate readings and prevent potential engine issues.
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Frequently asked questions
You can increase the fuel supply to a diesel engine by adjusting the fuel screw. This is located within the injection pump, which can be accessed either from the side or the top. Turning the screw clockwise will increase the fuel supply. However, it is important to be conservative with this adjustment, as increasing the fuel can cause more internal heat, potentially melting your pistons.
You will need a good quality 5/16th allen wrench to adjust the fuel screw.
It is recommended to make small adjustments to the screw, as a small adjustment makes for a large increase in fuel. A turn of 1/6th (from 12 o'clock to 2 o'clock) is a conservative adjustment that will increase the fuel rate. A turn of 1/4 is also possible but may be less safe.
It is important to increase airflow when increasing the fuel supply, or you will produce a lot of black smoke without any extra power. Additionally, you should carefully watch your exhaust gas temperature (EGT) when making this adjustment.
