Pressurizing Duramax Fuel System: The Complete Guide

The Duramax diesel engine has proven to be a reliable design, with many owners reporting hundreds of thousands of miles of trouble-free use. However, some issues have been reported, including injector failure, crankshaft breakage, and fuel system leaks.

One of the most common issues with the Duramax engine is injector failure, specifically with the LB7 version. The primary failure points are internal cracking of the injector body and corrosion-induced failure of the internal ball seat, which can lead to leaking injectors and fuel in the crankcase. It is recommended to replace all eight injectors with genuine Bosch injectors, which have improved durability.

Another issue that has been reported is crankshaft breakage, which can occur in higher horsepower engines or even in stock power plants. This is often due to a combination of excessive rpm and the large external counterweight, as well as the factory firing order that imposes undue stress on the crankshaft.

Additionally, there have been reports of fuel system leaks, which are almost always on the return side of the system. The smaller of the two fuel lines travelling down the frame rail has been known to leak, and the cloth-covered rubber return lines at the injectors can also be a source of leaks.

To pressurize the fuel system, some Duramax owners have drilled out the gas cap and used a rubber-tipped blow nozzle to gently push air into the tank. However, it is important to note that the fuel system contains high-pressure fuel and can be dangerous if not handled properly.

The Duramax LB7's crankcase can be pressurised by drilling the gas cap

To pressurise the fuel system, one can drill out the gas cap and insert a rubber-tipped blow nozzle to gently push air into the tank. However, it is important to note that the gas cap has a vent to prevent pressure buildup, so this will need to be plugged.

One user reported that they pressurised their tank and could hear air purging from the top, but there were no fuel leaks. They were advised to check their oil and make sure it was not making its way into the fuel.

Another user reported that they drilled out their gas cap, used an old Ford cap, and epoxyed an air chuck fitting into it. They then used an air regulator to set the pressure to 7 lbs and left it for 20 minutes, successfully finding a leak at the filter housing.

It is important to note that the fuel system contains high-pressure fuel, so caution should be exercised when attempting to pressurise the system.

The Duramax's high-pressure fuel pump must be able to produce a minimum of 2500-3500 psi while cranking

The Duramax's high-pressure fuel pump is a crucial component of the engine's common-rail fuel injection system. This system is found in virtually all modern diesel-powered vehicles in the US and Europe and involves high-pressure fuel being pumped through and stored in a rail that feeds multiple injectors simultaneously. This is in contrast to older direct injection systems, where fuel was fed at low pressure from a fuel pump to an injection pump, which then ramped up the pressure before delivering it through separate lines to the injectors.

To check the supply side, first stroke the hand pump. If the hand pump is initially soft but becomes hard, recheck the engine's cranking fuel rail pressure. If the pressure is now okay, the issue is likely a fuel supply problem.

If the fuel supply side is okay but the cranking pressure is low, and the hand primer is stiff, there are several potential causes:

• Leaking fuel pressure relief valve
• Leaking injector
• Leaking high-pressure fuel lines
• High-pressure pump issue
• Faulty fuel pressure regulator

The Duramax's CP4.2 pump is prone to failure due to debris and water contamination

The CP4.2 pump is a component of the Duramax engine's fuel system. The pump is responsible for sucking fuel from the tank and creating a vacuum from the fuel tank through the fuel lines and fuel filter. However, the CP4.2 pump has a reputation for being prone to failure, often in a catastrophic manner, due to debris and water contamination.

When the CP4.2 pump fails, metal particulates are sent throughout the entire fuel system, causing instant contamination. This can lead to a range of issues, including difficulty accelerating, reduced fuel efficiency, excessive smoke, power loss, rough idle, and even a no-start condition.

To prevent issues with the CP4.2 pump, it is recommended to take certain precautions. These include:

• Using high-quality, clean fuel and avoiding questionable fuel sources.
• Regularly adding fuel additives to improve lubricity and reduce water contamination.
• Installing a lift pump, such as a FASS or Airdog, to improve fuel filtration and reduce the risk of fuel starvation.
• Monitoring the inlet metering valve for signs of debris or metal shavings.
• Performing regular maintenance and fuel filter changes as per the manufacturer's recommendations.
• Considering a CP3 conversion or the installation of a fuel system saver, a fine mesh screen that can protect the fuel system in the event of a CP4.2 pump failure.

The Duramax's CP4.2 pump can be replaced with the more durable CP3 pump

The Duramax's CP4.2 pump has proven to be a cause of catastrophic failures that require more than $8,000 in repairs to replace the entire fuel system. The alternative for performance-upgraded 6.6 Duramax LML engines is to convert to a more durable CP3 pump.

The CP4.2 pump, which flows about 20% lower volume than the stock CP3 used in earlier Duramax engines, fails because it needs to have a certain level of lubrication between the components that pressurize the fuel headed to the fuel rails. The low volume and ultra-low-sulfur diesel prevent the CP4.2 from doing its job.

Over time, the roller tappets on the actuators, which are forced up by the cam-like driveshaft inside the pump, start wearing from a lack of proper lubrication. Whether it's air in the fuel, a lack of fuel, or a lack of lubrication in the fuel, there's typically little or no warning when "wear" turns into a costly catastrophic failure.

A grenaded CP4.2 creates an estimated $8,000 to $12,000 in repairs because the entire fuel system—from tank and lines to injectors—has to be replaced because minute metal debris from the failed pump usually makes its way through the system before the engine shuts down.

The CP3 is used in previous generations of Duramax trucks and all the 6.7L, 24-valve Cummins powerplants. It's proven to be durable and can be modified to flow more than enough fuel to meet the demands of bigger injectors, higher-flowing turbochargers, and, with aggressive ECM programming, has been known to support up to 900 hp.

The CP3 conversion is relatively straightforward (as much as it can be when accessing an injection pump sitting at the very front of a 6.6L Duramax LML engine's valley). The CP3 sits comfortably in the engine valley, ready to meet whatever demands are placed on it.

The CP4.2 to CP3 conversion requires disconnecting the ninth injector because there's no provision in the CP3 for supplying the 80 to 90 psi to feed it. It also requires a custom tune to reprogram the ECM so the CP3 plays well with the piezo injectors.

The Duramax LB7 and LLY engines are susceptible to bent rods when power is doubled

The LB7 and LLY Duramax engines are known for their susceptibility to bent rods when power is doubled. This is mainly due to the combination of their high compression ratio of 17.5:1 and the use of forged-steel, cracked cap connecting rods, which have less material in the bottom of the beam area. This design leads to elevated cylinder pressures and increased stress on the rods, causing them to bend at lower horsepower and torque levels.

To prevent rod failure, it is crucial to limit the amount of torque the rods experience through careful tuning. Upgrading to stronger aftermarket rods, such as those offered by CP Carrillo, Wagler Competition Products, Manley Performance, Brian Crower, and Howards Cams, is also an effective solution.

Additionally, the LLY engine introduced a variable geometry turbocharger, which further contributes to the risk of rod failure. The quicker spool-up and higher compression result in higher peak cylinder pressure and torque, putting more strain on the connecting rods.

Frequently asked questions