Fixing Low Fuel Rail Pressure In Lbz Duramax Engines

Low fuel rail pressure is a common issue with Duramax trucks, and it can leave you stranded if not addressed. The check engine light (code P0087) may come on, and you might experience low power, poor transmission shifting, increased smoke, and high exhaust gas temperatures. To fix low fuel rail pressure, you can start with simpler solutions, such as changing the fuel filter, inspecting fuel lines, and changing the fuel pressure relief valve. If these don't work, you may need to check the fuel pressure regulator, CP3 or CP4 injection pump, or even replace the injection pump. Adding a lift pump can also help prevent low fuel rail pressure issues.

Check for leaks or damage in fuel lines

When troubleshooting low fuel rail pressure in a Duramax engine, it is important to check the fuel lines for any leaks or damage. This is because issues with the fuel lines can cause disruptions in fuel delivery, leading to low rail pressure.

To begin, locate the fuel lines that run from the fuel filter to the injection pump and from the CP3 to the fuel rails. Inspect these lines thoroughly for any signs of damage, cracks, leaks, or collapse. Fuel lines can decay, crack, or collapse over time, leading to issues with fuel delivery. It is important to replace old, cracked, or collapsed fuel lines with new, rigid, and bio-friendly high-vacuum fuel lines.

Additionally, check for any signs of fuel leaks around the fuel filter head and the O-rings. Leaks in these areas can cause a loss of prime, leading to low fuel rail pressure. If you notice any leaks, consider rebuilding the fuel filter head by replacing the O-rings with a rebuild kit. This is a relatively inexpensive and straightforward task.

Another area to inspect is the fuel module in the tank. The welds holding the tubes in the fuel module can fail, leading to leaks. To access the fuel module, you may need to drop the fuel tank or remove the pickup bed. If you suspect an issue with the fuel module, consider consulting a mechanic or a dealer for further diagnosis and repair.

Finally, pay close attention to the quick-connect fittings and flex lines. These areas are prone to leaks and can be challenging to work with. If you notice any leaks around the quick-connect fittings, consult a mechanic or a forum for guidance on safely releasing and replacing these fittings.

Remember to perform a thorough inspection of the entire fuel system, as leaks or damage in any part of the system can contribute to low fuel rail pressure.

Change fuel pressure relief valve

To change the fuel pressure relief valve, follow the steps outlined below:

Firstly, it is important to understand the common symptoms of low fuel rail pressure, which can leave your vehicle stranded if not addressed promptly. These symptoms include reduced engine power, a "change fuel filter" message, poor transmission shifting, increased smoke output, high exhaust gas temperatures, and a check engine light with code P0087.

If you are experiencing these issues, start by performing the cheaper and easier repairs first, such as changing the fuel filter if it hasn't been replaced recently (within the past 5,000 miles). Inspect the fuel lines from the fuel filter to the injection pump and CP3 for any leaks or damage.

Now, to specifically address the fuel pressure relief valve:

• Diagnose the issue: Before making any changes, it is important to confirm that the fuel pressure relief valve is indeed the source of the problem. Perform a "bottle test" to check for leaking bypass fuel. On the driver's side fuel rail, locate the "U" shaped 3/8 hose near the back of the rail and the turbo. Remove this hose from the fuel rail and plug it. Then, attach a 3/8 hose to the fuel rail nipple and place the other end in a bottle. Drive the truck aggressively, including several wide-open runs. If there is fuel in the bottle afterward, your fuel pressure relief valve is leaking main pressure.
• Consider your options: There are a few different approaches you can take to address the issue. One option is to simply replace the fuel pressure relief valve with a new one. This can be a costly option, with the actual FPRV part costing between $350 and $400. Alternatively, you can try shimming the valve, which involves adding washers to help the valve hold pressure. This is a cheaper option but may not be a long-lasting solution. Another option is to use a "rail plug," which is a solid plug that completely seals the valve, eliminating any fuel pressure relief function. This option costs around $35-$40 but removes the safety feature of fuel pressure relief.
• Acquire the necessary parts: If you decide to replace the fuel pressure relief valve, you can source the part from either the dealership or an aftermarket supplier. The GM part number for the FPRV is #97371618, and it corresponds to the Bosch part number BOSCH 1 110 010 013 8973716180. You can also find aftermarket options from vendors and suppliers online or at auto parts stores.
• Install the new valve: Once you have the necessary parts, it's time to install the new fuel pressure relief valve. You can refer to online resources, such as forums and YouTube videos, for step-by-step installation guides specific to your vehicle model. Ensure you have the correct tools and safety equipment before beginning the installation process.
• Test and monitor: After installing the new valve, take your vehicle for a test drive and monitor the fuel rail pressure. Compare the actual and desired fuel rail pressure values to ensure they are closely aligned. If you continue to experience significant deviations, there may be other underlying issues contributing to the problem.

Remember, it is always recommended to consult a qualified mechanic if you are unsure about any part of the process or if you encounter complications during the repair.

Check CP3 or CP4 injection pump for debris

If you're experiencing low fuel rail pressure, it's important to check your CP3 or CP4 injection pump for debris. This is because the injection pump is responsible for pressurising the fuel that goes to the fuel rails, and debris can cause issues with this process.

The CP4.2 pump, in particular, has been known to fail due to a lack of lubrication between components, which can cause the roller tappets on the actuators to wear down over time. This can lead to catastrophic failure and expensive repairs, as the entire fuel system may need to be replaced.

To check for debris, you will need to remove the injection pump from the engine. This process can be time-consuming, as the pump is located at the very front of the Duramax engine's valley and requires removing multiple parts to access it. Once you have removed the injection pump, you can inspect the fuel pressure regulator for any signs of debris or metal shavings. If you notice any debris, it is important to have all fuel system parts cleaned, the pump replaced, and the injectors tested or potentially replaced.

Additionally, you can also look for signs of wear or damage on the roller tappets and actuators. This could indicate that the pump is not receiving proper lubrication, which can lead to future issues.

By regularly inspecting and maintaining your injection pump, you can help ensure that your Duramax engine runs smoothly and avoid costly repairs down the line.

Check fuel pressure regulator for debris

To check the fuel pressure regulator on a CP3 or CP4 for debris, you will need to remove the regulator from the pump. This can be done by loosening the screw that holds the regulator in place and carefully lifting it out. Once the regulator is removed, you can inspect the screen for any debris or metal shavings.

If you find debris on the screen, it is important to clean it before reinstalling the regulator. This can be done with a soft brush or compressed air. Make sure that the area you are working in is clean and free of debris to avoid any contamination.

It is also recommended to check the rest of the fuel system for debris if the regulator is found to have debris on it. This includes the fuel lines, injection pump, and fuel rails. Any debris found in the system should be removed, and the system should be flushed to prevent future issues.

Additionally, if the CP4 fuel pressure regulator is found to have metal shavings on the regulator screen, it indicates a failure that will require further action. In this case, all fuel system parts must be cleaned, the pump replaced, and the injectors tested or potentially replaced.

By following these steps and performing a thorough inspection and cleaning of the fuel pressure regulator and fuel system, you can help ensure the proper functioning of your LBZ Duramax engine and prevent issues related to low fuel rail pressure.

Change fuel filter

Changing the Fuel Filter on a Duramax Diesel Engine

Note: *This guide is specifically for the 6.6L Duramax Diesel engine (LB7, LLY, LBZ, LMM, LML). The fuel filter housing is located on the passenger side of the engine.*

Tools Required:

• Filter wrench
• Strap-style filter wrench (optional)
• Water sensor wrench (optional)
• Channel locks
• Small section of 1/4" fuel hose (optional)
• O-ring seal
• Clean engine oil

Step-by-Step Guide:

• Remove the center air intake tube (between the air filter housing and turbocharger inlet mouthpiece).
• Remove the passenger-side inner fender (secured by a series of plastic body clips).
• Optional* Attach a small section of 1/4" fuel hose to the water-fuel separator drain valve located at the bottom of the fuel filter.
• Open the bleed screw at the top of the fuel filter housing, then drain the water separator into a suitable container and properly dispose of the fuel.
• Close the drain valve and remove the fuel hose.
• Locate and disconnect the water-in-fuel sensor electrical connector.
• Remove the fuel filter by rotating counter-clockwise (as if you were looking from the bottom up). Be sure that the water-in-fuel sensor wire does not become entangled while unscrewing the filter. Keep the filter positioned upright until removed from the vehicle to ensure fuel is not spilled.
• Drain the filter once removed and verify that the gasket did not stick to the fuel filter housing flange.
• Remove the water-in-fuel sensor assembly from the bottom of the fuel filter using an appropriate wrench, socket, or pliers.
• Replace the o-ring on the water-in-fuel sensor assembly (provided with the new fuel filter). If the water-in-fuel sensor assembly is damaged, do not reuse.
• Lubricate the water-in-fuel sensor o-ring with clean engine oil, then install snug onto the new fuel filter; do not overtighten.
• Lubricate the new fuel filter gasket with clean engine oil, then install it at the top of the fuel filter assembly.
• Do not fill the filter with diesel fuel; the fuel filter housing has an integrated hand pump that will be used to pump diesel from the fuel tank into the filter.
• Install the replacement fuel filter, once again ensuring that the water-in-fuel sensor connector does not become entangled with its surroundings. Install snug by hand only, do not tighten with tools.
• Locate and open the bleed valve towards the driver's side of the fuel priming pump. If desired, you can remove the bleed screw for inspection of the o-ring.
• Repeatedly pump the plunger on the top of the fuel filter housing until all air escapes and diesel fuel begins to flow out of the bleed screw orifice. Immediately close the bleed screw; do not overtighten, the screw is plastic. Clean up any spills so that leaks can be easily detected.
• Reinstall the inner fender and intake tube.
• Start the engine and check for leaks. If the truck runs rough for an extended period of time (idle should smooth out within 30 seconds), turn the engine off, re-bleed the system at the fuel filter housing, and check for leaks (including the bleed screw o-ring).

Frequently asked questions