Priming A Diesel Truck: Post-Fuel Filter Change Guide

Changing the fuel filter in a diesel engine is a common maintenance activity. However, some vehicle owner's manuals do not explain what to do after the fuel filter has been changed, which can lead to issues with starting the engine. To avoid this, it is necessary to prime the fuel filter to ensure fuel is circulating in the system. This can be done manually, by pumping the primer handle and turning the air-bleed screw until the hissing noise of escaping air stops, or by turning the ignition key to run for several seconds, then off, and repeating up to four times.

How to Prime a Diesel Truck After Changing the Fuel Filter

Priming the fuel filter manually

To prime the fuel filter manually, you will need to remove any air bubbles that have been introduced into your fuel system. You will also need to prime the fuel filter to ensure fuel is circulating in your fuel system once again. Avoid cranking the engine in an attempt to prime the pump, as this can wear down your battery and, in some cases, fail to recirculate your fuel.

The majority of diesel engines have a manual primer pump and an air-bleed screw that can be used to bleed the air and prime the pump. To operate these manual features, pump the primer handle (this can take anywhere from 50 to 500 pumps) while turning the screw until the hissing noise of escaping air stops. Once this occurs, tighten the screw and try to turn the engine.

If your diesel engine has two filters, you will need to repeat the process for the secondary filter, which is usually found in the engine bay under the hood. Make sure to wipe down the filter cap and sealing surface on the housing, and install the new O-ring that comes with the new filter. Ensure that the new O-ring is properly seated against the ridge all the way around the cap, as this will prevent diesel fuel filter leakage.

Finally, install the new filter, and use a torque wrench to tighten the cap to the manufacturer's specifications. Before driving the truck, you will need to bleed any air bubbles and prime the fuel system to purge any air that may have gotten into the system while the filter caps were off. This is an important step to prevent vapor lock, which is when liquid fuel is converted to gas vapour before it gets to the fuel rail.

Priming using ignition

To prime your diesel truck using ignition, start by turning the ignition key to the "run" position for several seconds. Then, turn the key off and repeat this process up to four times. This will allow you to restart the engine after a few minutes. On the fourth or fifth attempt, crank the engine, and it should start. Ensure you let the engine run for 30 seconds before turning it off and inspecting the fuel filter for any leaks.

It is important to note that the number of times you need to cycle the key may vary. Some people report needing anywhere from 2 to 10 cycles, while others suggest a minimum of 7 cycles. You'll know you've built enough pressure when you can no longer hear air in the system.

Additionally, it is recommended to fill the fuel filters as much as possible when installing them. If you installed the filters dry, remove them and fill them with clean, filtered fuel.

Bleeding air bubbles and priming the fuel system

Manual Priming Method:

• Locate the manual primer pump and air-bleed screw: Most modern diesel engines have a manual primer pump and an air-bleed screw that can be used for bleeding air and priming the fuel pump.
• Pump the primer handle and turn the screw: Operate the manual primer pump by pumping the handle, which may take anywhere from 50 to 500 pumps. Simultaneously, turn the air-bleed screw until you no longer hear the hissing sound of escaping air.
• Tighten the screw and attempt to start the engine: Once the air has been bled, tighten the screw back in place and try to turn on the engine.

Priming Using Ignition:

• Turn the ignition key to "Run": For some diesel engines, priming can be done by simply turning the ignition key to the "Run" position without cranking the engine.
• Repeat the process several times: Turn the ignition key to "Run" for a few seconds and then turn it off. Repeat this process up to four times.
• Crank the engine: On the fourth or fifth attempt, crank the engine, and it should start.
• Inspect for leaks: Let the engine run for about 30 seconds, and then turn it off. Check the fuel filter for any signs of leaking.

It's important to note that the number of repetitions required for effective priming may vary depending on your vehicle's make and model. Always refer to your vehicle's maintenance manual for specific instructions. Additionally, make sure to fill the fuel filters as much as possible when installing new filters.

Filling the fuel filters

Before filling the fuel filters, it is essential to have a basic understanding of the fuel system in a diesel truck. Most diesel trucks have two fuel filters: the pre-filter and the secondary filter. The pre-filter is located between the fuel tank and the fuel pump, usually along the frame rail. This filter captures any contaminants in the fuel before it reaches the pump. The secondary filter is typically found in the engine bay, and its job is to further clean the fuel before it reaches the injectors.

Now, when it comes to filling the fuel filters, there are a few different approaches that you can take:

One method is to fill the new fuel filter with a clean fuel source before installing it. This helps ensure that the filter is completely full and can start filtering fuel immediately. However, it is crucial to fill the filter from the outside and not pour fuel directly into the centre, as this can bypass the filtering process and introduce contaminants into the system. Some screw-on filters come with a plug for the centre to prevent this from happening. For cartridge filters, you can carefully pour clean diesel towards the outside of the cartridge until it is full.

Another approach is to install the new fuel filter without manually filling it and instead rely on the truck's lift pump to fill it. This method ensures that only filtered fuel enters the filter and reduces the risk of contamination. To do this, simply install the new filter, turn the key in the ignition to the "on" position a few times, and let the lift pump fill the filter. This process may take a while, so be patient and give the pump enough time to do its job.

It is worth noting that some people choose to add fuel additives to their tanks and fuel filters to improve engine performance and fuel efficiency. While this may provide some benefits, it is essential to exercise caution and choose additives that are compatible with your diesel engine. Always refer to the manufacturer's recommendations and consult a professional if you are unsure.

Remember, the process of filling the fuel filters may vary slightly depending on your truck's specific make and model. Therefore, it is always a good idea to refer to the vehicle's maintenance manual for detailed instructions on priming and fuelling procedures.

Removing the old O-ring

Locate the O-ring: The O-ring is a small, round seal typically made of rubber or plastic, and it is located between the filter cap and the filter housing. It is responsible for creating a tight seal to prevent fuel leaks.

Prepare the necessary tools: You will need a pry tool or a flathead screwdriver to remove the old O-ring. Have these tools ready before beginning the process.

Loosen the filter cap: Before removing the O-ring, you will need to loosen the filter cap. Use a ratchet and socket to do this carefully. Make sure not to apply too much force, as you don't want to damage the cap or the surrounding components.

Remove the old O-ring: Once the filter cap is loosened, use your pry tool or flathead screwdriver to carefully pry out the old O-ring. It may be stuck in place, so work slowly and gently to avoid damaging it or the surrounding surfaces.

Clean the area: With the old O-ring removed, take the time to thoroughly wipe down the filter cap and the sealing surface on the housing. It is important to remove any dirt, debris, or residue that may be present to ensure a proper seal with the new O-ring.

Inspect the old O-ring: Before discarding the old O-ring, take a moment to inspect it for any signs of damage or wear. This will help you identify any potential issues that may have arisen during its use and allow you to choose the appropriate replacement.

Choose the correct replacement: O-rings come in various sizes and materials, so it is important to choose the correct replacement for your specific diesel truck. Refer to your vehicle's maintenance manual or consult a certified mechanic to ensure you select the right O-ring for your truck's make and model.

Now that you have removed the old O-ring, you can proceed to install the new one. Remember to take your time and ensure that the new O-ring is properly seated and sealed to prevent fuel leaks.

Frequently asked questions