Where is the bleed screw on a diesel engine?

There isn’t a single universal bleed screw location on diesel engines. In most setups you’ll find a bleed screw on the fuel filter housing, on the fuel lift pump, or on the injection pump. The exact spot depends on the engine model and fuel system design.

The bleed screw is used to purge air from the fuel system after maintenance or filter changes. Bleeding ensures fuel reaches the injectors and the engine will start reliably. Always follow safety practices when working with diesel fuel and hot components.

Common bleed screw locations

These are the places you’re most likely to encounter a dedicated bleed screw or vent valve on typical diesel installations. Checking these points first can save time after maintenance or filter changes.

• Fuel filter housing: Many engines have a small bleed/vent screw or bleed nipple on the top or side of the fuel filter housing. Loosening this while priming releases air until fuel flows steadily.


• Fuel lift pump: Some engines use a hand primer or automatic lift pump with a bleed screw on the body to vent air from the supply line.


• Injection pump: Certain mechanical injection pumps (for example Bosch VE or VP types) include a bleed screw or bleed port on the pump body used during priming and after injector work.


• Fuel rail or injectors (modern common-rail): Some systems provide a service port on the rail or near the injector lines for bleeding, typically accessed with a wrench or tool specified by the manufacturer.


• Other components: Some setups place a bleed screw at the return line or near the water separator if the filter assembly is integrated with air bleed features.

Not every engine has an easily accessible bleed screw. In those cases, priming is performed at the filter head with the built-in primer or by cycling the fuel pump as directed by the manufacturer. Always consult your exact model’s service manual for the precise location.

How to bleed a diesel fuel system — step-by-step

Below is a generic procedure for removing air from a diesel fuel system. Adapt it to your engine’s manual, and follow all safety precautions when handling diesel fuel.

1. Turn off the engine and relieve any residual pressure in the fuel lines if your system has a pressure relief method.


2. Locate the bleed screw or bleed port on the appropriate component (filter head, lift pump, or injection pump).


3. With the engine off, slowly loosen the bleed screw a quarter to half turn to allow air to escape while fuel begins to seep out. Do not remove the screw completely yet.


4. Operate the primer or cycle the hand pump to push fuel through the line. When you see fuel without air, tighten the bleed screw firmly.


5. Repeat as needed until fuel flows steadily with no visible air bubbles. Recheck all fittings for leaks after tightening.


6. Start the engine and listen for rough running or leaks. If the engine stalls or there is continued air chatter, recheck the bleed points and repeat the process.

When the system is properly bled, the engine should start smoothly and run without misfiring. If problems persist, there may be a more complex fuel-system issue that requires professional diagnostics.

Special cases by engine type

Different diesel platforms handle priming and bleeding in distinct ways. Here are quick notes for common categories and what to look for in manuals:

• Older diesel engines with mechanical injection pumps: Bleed addresses the pump and filters; manual priming is typical.


• Inline injection systems (Bosch VE, VP, etc.): Bleed ports are located on the pump and at the filter; ensure air is purged from each line.


• Common-rail diesels: Many modern systems do not use a traditional bleed screw at the rail. Priming is typically performed via the service port, the fuel pump, or a diagnostic tool following the manufacturer’s procedure.

Always refer to the exact engine or vehicle manual for precise bleed locations, procedures, torque values, and safety steps for your model.

Summary

Bleed screws on diesel engines are not universal. Look for a bleed screw on the fuel filter housing, lift pump, or injection pump, and for service ports on the fuel rail in newer systems. Bleeding is a straightforward process of purging air, priming the system, and checking for leaks, but follow the manufacturer’s instructions to ensure safety and proper operation.

How do you clear the air lock on a diesel engine?

To remove an airlock in a diesel engine, you must first ensure the fuel tank is full and then "bleed" the air from the system. This is done by locating and loosening bleed screws on the fuel filter and/or pump, using the manual primer to pump fuel until all bubbles are gone, and then tightening the screws. If the engine still won't start, you may need to loosen the fuel lines at the injectors and crank the engine until solid fuel sprays out, before re-tightening each line. 
This video demonstrates how to bleed air from a diesel fuel system: 58sAurora Generators Inc.YouTube · Dec 17, 2011
Step 1: Prepare the system

• Fill the fuel tank with diesel to ensure there is enough fuel to push the air out. 
• Locate the manual fuel pump primer, which may be a lever or plunger on the side of the fuel filter housing. 
• Find the bleed screw or valve on the fuel filter housing, typically on the top. 

Step 2: Bleed the fuel filter 

• Slightly loosen the bleed screw on the fuel filter housing. 
• Pump the manual primer until fuel flows steadily without any air bubbles. 
• Tighten the bleed screw once all the air is expelled. 

Step 3: Bleed the injection pump (if necessary) 

• Some systems have a second bleed screw on the injection pump itself.
• If the engine still won't start, loosen this screw and use the primer until fuel flows without air.
• Tighten the screw after bleeding. 

Step 4: Bleed the injectors (if still needed) 

• For persistent airlocks, you will need to bleed the individual injectors. 
• Using a wrench, slightly loosen the fuel line nut where it connects to the injector. 
• Crank the engine for a few seconds. You will hear a hissing sound as air and fuel escape. 
• Tighten the nut as soon as you see a steady stream of fuel without air bubbles. 
• Repeat this process for each injector, one at a time. 

Step 5: Start the engine 

• After bleeding all the necessary components, try to start the engine. 
• It may start roughly at first but should smooth out. 
• If it still doesn't start, you may need to repeat the process, especially the injector bleeding steps. 

This video shows how to bleed air from the fuel lines and injectors: 1mError Code GuyYouTube · Aug 24, 2017

How to start a diesel engine after running out of fuel?

To start a diesel engine after running out of fuel, first add at least 4-5 gallons of fuel. Then, cycle the ignition key to the "on" position for 30 seconds to prime the fuel pump, repeating this several times. If it doesn't start after a few cycles, try cranking the engine for up to 15 seconds, followed by a 30-second rest, repeating until it starts. 
Step 1: Add fuel and prime the system 

• Add at least 4-5 gallons of fresh diesel fuel to the tank. 
• Turn the key to the "on" position (without starting the engine) for 30 seconds. This allows the electric fuel pump to push fuel through the system and force some of the air out. 
• Turn the key off, then repeat this "on/off" cycle about 3-5 times. 

This video demonstrates how to manually prime the fuel system of a diesel engine after it has run out of fuel: 50sD 4 DieselYouTube · Aug 24, 2020
Step 2: Attempt to start the engine 

• After cycling the key, try to crank the engine. Do not crank for more than 10-15 seconds at a time to avoid damaging the starter. 
• If the engine does not start, wait for 30 seconds to a minute before trying again to give the starter motor time to cool down. 
• Repeat the process of cycling the key and then cranking until the engine starts. 

Step 3: After the engine starts

• The engine may run roughly or produce white smoke initially, which is normal due to the remaining air. This should stop after a few minutes of idling.
• Allow the engine to idle for several minutes to fully clear any remaining air before driving.
• Check for any leaks in the fuel lines or at the filter before you go. 

If the engine still won't start

• If the engine still won't start after multiple attempts, there may be a more serious issue. It may be best to have the vehicle towed to a professional for diagnosis. 
• More advanced methods, such as manually bleeding the fuel lines at the injectors, may be necessary if the vehicle has an older mechanical fuel system. 

Where to bleed a diesel engine?

To bleed a diesel pump, you'll need to locate the fuel pump, usually near the engine, and identify the bleed screw. Open the bleed screw, and use the manual hand pump or lever on the fuel pump to pump fuel until a steady stream of fuel, without air bubbles, flows out.

How to remove air in a diesel engine?

To bleed air from a diesel engine, locate the manual fuel pump lever on the lift pump, operate it until air bubbles disappear from the fuel, and then check and tighten any bleed screws on the fuel filter(s). If the engine still won't start, you may need to loosen the injection lines at the injectors, crank the engine until fuel appears, and then retighten the lines.
 
Step 1: Prepare to bleed

• Turn off the engine and ensure no leaks are present. 
• If you have a new fuel filter, install it with a little oil on the new gaskets to help seal it. 
• Fill a spin-on filter with clean diesel fuel before installing to speed up the process. 
• Place clean rags or towels around the work area to catch any spilled fuel. 

Step 2: Bleed the filter and pump 

• Locate and open the bleed screw on the fuel filter closest to the fuel tank. 
• Turn on the fuel supply valve, if applicable. 
• Use the manual priming lever on the lift pump to pump fuel through the system. 
• Pump until you see a steady flow of fuel without air bubbles coming from the bleed plug. 
• Close the bleed plug tightly. 
• Repeat this process for any other filters or bleeder screws. 

This video demonstrates how to bleed a diesel fuel system using a hand pump: 1mError Code GuyYouTube · Aug 24, 2017
Step 3: Bleed the injectors 

• If the engine still runs poorly or won't start, you may need to bleed the lines at the injectors. 
• Use two wrenches to slightly loosen the injection line nut on one of the injectors (about one turn). 
• Crank the engine for a few seconds. Air and fuel will be forced out. 
• Once a steady stream of fuel comes out, tighten the nut securely. 
• Repeat this process for the other injectors one at a time. 

This video shows how to crack the injectors to bleed the air out: 59sfarmcardsYouTube · Aug 2, 2023
Step 4: Start the engine 

• Once the lines are bled, try starting the engine. You may need to repeat the injector bleeding process if it doesn't start immediately.
• Avoid running the engine out of fuel again, as this is the primary cause of air in the system.