How to bleed a diesel generator

This guide explains how to properly bleed a diesel generator so it operates smoothly and avoids potential issues.

September 24, 2025
BY BISON

Diesel generators provide reliable backup power during outages, ensuring that homes, businesses, and critical equipment continue to run smoothly. However, even the most robust generator can fail to start if air gets trapped in the fuel lines—a common issue known as an air lock.

Air bubbles in the fuel lines can cause starting problems, uneven engine performance, or stalling. Like any mechanical equipment, diesel generators need regular maintenance for optimal performance, and bleeding the fuel system is a key task. Bleeding the air from your diesel generator ensures proper fuel flow to the engine, allowing the generator to start quickly and run efficiently. This guide explains how to properly bleed a diesel generator so it operates smoothly and avoids potential issues.

The importance of understanding diesel generator bleed

A diesel generator depends on a well-designed fuel system to deliver fuel from the tank to the engine. Key components include the fuel tank, fuel lines, filters, injection pump, and injectors. Diesel travels from the tank through the filters to remove dirt, then the injection pump pressurizes the fuel and sends it to the injectors for combustion inside the engine cylinders.

Why do you need to bleed the diesel generator?

Air can enter the fuel system through loose connections, cracked lines, faulty filters, or when the tank runs low. Even a small amount of air can disrupt fuel flow, and diesel engines are particularly sensitive because they rely on high-pressure injection for ignition, and an air lock can interfere with this process.

When air enters the system, it can cause:

Hard starting: Multiple cranks, battery drain, and startup failures.
Irregular engine performance: Uneven fuel delivery leads to misfiring or rough running.
Engine stalling: Persistent air bubbles can shut the engine down, especially under load.
Potential engine damage: Uneven power delivery increases stress and wear on components.
Reduced performance: Limited power output and lower efficiency.
Higher fuel consumption: The engine burns more fuel to compensate for inefficiencies.
Downtime: Poor performance or failure to start can disrupt critical operations.
Safety risks: In emergencies, unreliable power can compromise safety and operational continuity.

Generator needs bleed signs

Air trapped in a diesel generator’s fuel system can cause several noticeable issues. One of the most common signs is difficulty starting the engine. If the generator turns over but struggles to fire up, air in the fuel lines is often to blame.

Rough idling or inconsistent performance is another clue. The engine may run unevenly, shake, or change speed on its own because air bubbles disrupt the steady flow of fuel.

A loss of power under load can also signal a problem. The generator might handle small appliances but fail to keep up when more devices are connected, indicating reduced fuel delivery.

Finally, unexpected engine stalls are a clear warning sign. If the generator suddenly stops running, there may be trapped air in the fuel system that prevents the engine from starting.

How to bleed a diesel generator: Step-by-step instructions

Step 1: Safety and tool preparation

Before starting, turn off the generator, let it cool completely, and disconnect it from any power source. Never work on a running or hot engine—fuel is flammable, and safety comes first.

You’ll need:

Basic hand tools (wrenches, screwdrivers)
Fuel-resistant rags or towels
Gloves and safety glasses
A catch container for spilled fuel
Clean diesel for priming
Manufacturer-recommended tools, if required

Step 2: Locate key fuel system components

Fuel filter – removes dirt and debris from fuel.

Fuel injection pump – pressurizes fuel for delivery.

High-pressure fuel lines and injectors – send fuel to engine cylinders for combustion.

Step 3: Bleed the fuel system

1. Bleeding the fuel filter

Open the vent screw on the filter.

Remove the fuel filter and let the fuel drain into a container.

Use a hand priming pump (if available) to pump fuel pre-pressurize until a steady, bubble-free stream of fuel is seen from the scraper screw.

Close the vent screw securely.

2. Bleeding the fuel injection pump

Loosen the bleed screw on the injection pump.

Prime until fuel flows without air bubbles.

Tighten the screw securely.

3. Bleeding high-pressure fuel lines (if needed)

Only do this if the engine still won’t start!

Loosen injector nuts one at a time and crank the engine briefly until fuel sprays out.

Tighten the nuts immediately.

BISON warning: Never touch high-pressure fuel spray—it can cause serious injury.

Bleeding sequence: Start with low-pressure components (fuel filter) before moving to high-pressure parts (injection pump and lines). Always check your generator’s manual for model-specific instructions.

Step 4: Diesel generator bleeding completed

Bleeding is done when fuel flows smoothly with no air bubbles, and the engine starts and runs steadily. The process usually takes 10–30 minutes, depending on the system and any issues present.

Step 5: Inspection of diesel generators after bleed

System pressure and leaks: Ensure all connections, filters, and lines are secure and leak-free.

Cleaning: Wipe up any spilled fuel, store the tool in a safe place, and dispose of the waste safely.

Startup: Start the generator carefully, listening for odd noises or hesitations.

Performance verification: Let the generator run under load to confirm steady idling, full power, and proper fuel delivery.

Troubleshooting common issues

Even after bleeding, some diesel generators may still have problems. Here’s how to handle them safely:

Persistent air bubbles: If air keeps appearing in the fuel lines, check for loose connections, cracked lines, or a faulty fuel filter. Tighten fittings, replace damaged parts, and bleed the system again.
Fuel leaks after bleeding: If you see fuel leaks, shut off the generator immediately. Inspect all screws, hoses, and fittings. Tighten or replace parts as needed. Never ignore leaks since fuel is highly flammable.
Generator still won’t start: If the generator doesn’t start after proper bleeding, the issue might be beyond air in the system. Possible causes include a faulty injection pump, injector, or other engine components.
When to seek professional help: If you’ve carefully followed the steps but the generator still won’t start or shows serious leaks, contact a qualified technician or the service center. While bleeding is often a simple task with basic tools, persistent issues require expert attention.

Preventive maintenance

Regular fuel filter replacement: Replace the fuel filter according to the generator’s recommended replacement schedule. A clean filter ensures smooth fuel flow and reduces the possibility of air entering the system.
Fuel quality considerations: Always use clean, fresh diesel fuel. Avoid using stale or contaminated fuel, as these can cause clogging, air bubbles, and reduced engine performance.
Proper storage: When not in use, store the generator in a cool, dry place and keep the fuel tank full. This helps prevent air from entering the fuel lines and keeps the fuel in good condition.
Routine inspections: Regularly inspect the fuel system for leaks, loose connections, or damaged hoses. Detecting minor issues early can prevent air from entering and ensure safe generator operation.

Summary

Bleeding a diesel generator involves a few essential steps: Ensuring safety, locating the fuel system components, removing trapped air from the fuel filter, injection pump, and high-pressure lines, and confirming smooth engine performance afterward. When done correctly, this simple maintenance task restores proper fuel flow, prevents starting issues, and keeps your generator running reliably.

As a professional diesel generator supplier in China, BISON is a favorite among dealers for its wide range of reliable and durable generators. Whether you need backup power for home, business, or industrial use, BISON delivers performance and reliability you can trust. Contact BISON today to start a business partnership and enjoy our professional service support!