Running out of fuel can be annoying in any vehicle, but it poses unique challenges for common rail diesel engines. Unlike old-school diesel engines with mechanical fuel systems, modern common rail diesels have extremely high fuel pressures and inject timing that is computer-controlled to fractions of a second. Air getting into the system can throw everything out of whack and leave your high-tech engine struggling.
Fortunately, bleeding a common rail diesel engine to remove trapped air is a straightforward job—if you know what you’re doing. Trying to start the engine over and over with air in the fuel lines will only leave you frustrated. As a reliable diesel engine parts supplier, I will teach you how to get your diesel engine back up and running by removing air pockets from the high-pressure fuel system.
How Common Rail Diesel Fuel Systems Work
Before jumping in, let’s take 30 seconds to understand what’s happening inside a common rail diesel engine. As the name suggests, there is a “common rail” that stores pressurized fuel and supplies it directly to the injectors.
A high-pressure fuel pump pressurizes the fuel to intense pressures. We’re talking over 2,000 psi when the pedal is to the metal. For reference, even late model direct-injected gasoline engines typically operate below 2,000 psi.
At the proper time, the engine’s computer tells the injectors to spray a precise amount of fuel into the cylinder. Timing is controlled electronically to one ten-thousandth of a second.
When everything is working properly, this high-tech fuel injection allows common rail diesel engines to produce abundant torque while maintaining emissions compliance.
What Causes Air in the Fuel System
Air doesn’t compress like a liquid. When pockets of air make their way into the high-pressure fuel rails instead of diesel, you encounter problems. At best, performance will suffer from weak combustion events. At worst, the engine won’t run at all due to no fuel entering the cylinders.
By far the most common cause of air in the fuel system is running the tank dry. Diesels draw fuel from the bottom of the tank. As the last fraction of diesel leaves, air gets sucked through fuel lines. The lift pump will actually pull vacuum once the pickup runs dry!
Even a mostly full tank can have suction issues if the pickup tube is mounted too high. Grading the terrain or squatting the suspension while rock crawling allows air at the top of the tank to enter the fuel lines. Installing a transfer tank helps prevent problems, as does filling up often.
Air also enters after changing diesel fuel filters or working on other components in the high-pressure system. Anytime a line is disconnected, air takes the path of least resistance into the lines. It’s unavoidable.
Finally, check your diesel for external leaks if you encounter hard starting issues when the tank is full. Porous hoses and cracked injection lines allow air to mix with fuel under vacuum. Replace defective parts once the system is bled.
How to Bleed a Common Rail Diesel Engine: Without Special Tools
Diesel purge tools and vacuum fillers simplify the job, but aren’t necessary on most consumer automotive diesel engines. Only medium duty trucks or heavy equipment typically need such specialty tools for priming fuel systems.
The good news is that a basic set of hand tools is all you need for most Duramax, Cummins 5.9/6.7, or Power Stroke engines. Large shop rags, a diesel fuel transfer jug, and a bucket for catching fuel round out the standard gear list.
Let’s get started bringing your dead diesel back to life.
Step 1: Add a Full Tank of Fresh Fuel
Attempting to bleed the fuel system multiple times on a near-empty tank only leads to frustration as air is reintroduced. Begin by filling up with diesel, using winterized fuel if temperatures warrant. The dew point is especially critical with bio blends absorbing more atmospheric moisture.
Adding a fuel treatment like Power Service or Howe’s clears water and stabilizes dying batches of diesel fuel. That $10 insurance primes the pump for vigorous bleeding efforts and continued smooth-running.
Consider adding a biocide if your diesel sits for long periods between use. Harmful bacteria accumulation often plays a hidden role in hard starting issues.
Step 2: Check Mechanical Lift Pump Operation
Diesels constantly recirculate fuel that isn’t needed for combustion using mechanical lift pumps. The device sits at the back of the high-pressure fuel injection pump and provides reliable feed. Its pulsations purges air through the system.
Unfortunately, aftermarket fuel filtration systems sometimes relocate mechanical lift pumps away from the fuel injection pump. Restriction can occur within these longer return lines. Air pockets result.
Ensure fuel pulses from the supply line when cranking the starter with the key on for 15-second intervals. No solid stream of fuel indicates lift pump issues or severe restrictions in the fuel filter assembly. Consider adding an electric boost pump if flow is insufficient.
Cycling the key repeatedly without starting the engine allows the lift pump to build pressure. Carefully check for leaks as pressure increases within the fuel filter housing.
Step 3: Open Bleeder Screws
With mechanical lift pump operation verified, the next step is to open bleeder screws present on most consumer diesel fuel filter housings. Cummins, Duramax, Power Stroke, and VM Motori engines all employ bleeders for removing trapped air during filter changes.
Place a towel beneath the filter assembly, then give the screw a few counter-clockwise turns using a small flat head screwdriver or needle nose pliers. Expect a solid stream of fuel once air evacuates the housing. Close bleeder screws as soon as bubbles vanish from the flow.
Note that some diesel pickups moved to screw-on canister housing lacking traditional bleeder valves. Purchase a specialty wrench for opening the housing slightly if this applies to your truck. Opening any filter housing allows air to exit while minimizing leakage.
Repeat the process for your vehicle’s secondary fuel filter if equipped. Opening bleeders at both stages ensures no air pockets remain trapped.
Step 4. Flush Injector Lines
Cylinders lacking fuel due to air blockages in injector lines fail to fire. That familiar “hit and miss” stumbling signifies bleeding isn’t complete. Confirm fuel reaches all injectors by opening lines individually.
Step on the primer bulb included on many fuel filter kits to build pressure. Then slightly loosen the inlet fitting at one injector. Allow fuel to flow until the line is fully purged of bubbles before closing it and advancing to the next cylinder.
Having an assistant crank the starter while carefully opening injector lines reduces complications. Note that high pressure common rail designs mandate using a backup wrench to avoid damaging fragile injector tips—disconnected lines spray intense fuel streams!
Step 5. Bleed Rail Supply Lines
Rail crossover tubes distribute high pressure fuel to injectors evenly. Air trapped in these lines starves cylinders randomly rather than consistently. If an injector flush fails, access bleeder valves present on most rail supply tubes.
Their location requires extensive disassembly in cramped engine compartments. Having small hands certainly eases the process! Also beware that spilled fuel runs straight into cylinder heads once valves are opened. Protect sensors and electrical connections from leaks.
As always, place a towel beneath fittings and slowly loosen bleeders one turn at a time. Allow fuel to expel air before closing valves and advancing to the next injector connection. Not all engines have accessible bleed valves — opening injector lines is the only option on some models.
Quick Tips for Refractory Air Bleeding Issues
Diesel engines bang to life once most trapped air evacuates the injection system. But repeat stalling and black smoke bellowing from tailpipes signifies bleeding difficulties continue. Here are some troubleshooting tips for the toughest air lock cases:
- Remove fuel/water separator bowls from the bottom of fuel filter assemblies. Water contamination prevents bleeding in extreme scenarios.
- Check for clogged tank vent tubes restricting fuel flow to fill volume displaced by exiting air.
- consider installing clear fuel filter housings. Air bubbles visibly flowing through filters indicates further bleeding is needed.
- Add a valve for pressurizing filter housings to 7-10 PSI using shop air. This pushes fluid through bleeder valves.
- Replace lift pump assemblies supplying fuel filters if over 5 years old. Weak lift pumps compound hard bleeding.
- Verify sufficient battery voltage reaches injection components. Energized solenoids prevent air pockets.
In Conclusion
Modern common rail diesels throw enough complications even with ideal operating conditions. But understanding the nuances of bleeding temperamental high-pressure fuel systems combats headaches for operators relying on diesel power. Arm yourself with knowledge—and a well-stocked toolkit—before saddling up for adventure travel far from the pump!