A low fuel rail pressure condition in your Duramax diesel engine can trigger diagnostic trouble codes (DTCs) P1093 and P0087, often leading to performance issues. P1093 indicates a large leak in the high-pressure fuel system, while P0087 signifies that the fuel rail pressure is too low. This comprehensive guide will walk you through diagnosing and resolving these issues.
Understanding the Problem: P1093 and P0087
The P1093 code, “Large Leak,” points to a significant pressure difference between the commanded and actual fuel rail pressure, exceeding 20 MPa. Importantly, this indicates an internal leak within the high-pressure system, returning fuel to the tank, not an external leak. P0087, “Fuel Rail Pressure Too Low,” triggers when the pressure drops below 22.5 MPa at engine speeds over 600 RPM.
Diagnosing Low Fuel Rail Pressure on a Duramax
Addressing these codes requires a systematic approach. If both P0087 and P1093 are present, prioritize resolving P0087 first. The diagnostic process for both codes shares similarities:
1. Check for Fuel Supply Restrictions:
A restricted fuel supply is a common culprit. Use the J44638 special tool to measure vacuum restriction on the fuel supply line to the high-pressure pump. Acceptable vacuum at wide open throttle (WOT) in park is a maximum of 5 inches Hg. Under hard acceleration, this limit increases to 7-8 inches Hg. Excessive vacuum suggests a clogged fuel filter. Replace the filter and retest. A slightly elevated vacuum (around 2 inches Hg) might indicate air intrusion. Inspect for air bubbles in the fuel lines using clear lines connected to the filter head. Consider a Racor Fuel Filter Head Assembly for easier monitoring.
2. Inspect the Fuel Rail Pressure Sensor:
With the key on and engine off, the fuel rail pressure should read between 1-1.8 MPa (approximately 145-261 PSI). A reading outside this range indicates a faulty fuel rail pressure sensor, requiring replacement.
3. Examine the High-Pressure System for Leaks:
Inspect the high-pressure limit valve and the fuel pressure relief valve for leaks. Leaking valves require replacement. Also, check for leaks around race plugs, even if recently installed, as they can leak under maximum rail pressure.
4. Test Injector Performance:
With the engine at operating temperature, command 21,000 PSI (approximately 145 MPa) of rail pressure using a scan tool. Failure to achieve this pressure at idle, especially accompanied by hard starting, misfires, rough running, excessive smoke, or poor balance rates, points towards faulty injectors. Perform an enhanced injector return test to confirm.
5. Disconnect the Fuel Rail Pressure Sensor:
Disconnecting the sensor should result in a fuel pressure reading exceeding 175 MPa on the scan tool. Lower readings indicate a problem within the high-pressure system.
6. Diagnose Load-Specific Low Pressure:
If low fuel rail pressure occurs only under heavy acceleration or load, prioritize checking for fuel supply issues. If the rail pressure reaches 21,000 PSI at idle but drops under load, the high-pressure pump is often the cause.
Conclusion
Diagnosing low fuel rail pressure in a Duramax involves a multi-step process. By systematically checking for restrictions, leaks, sensor functionality, and injector performance, you can pinpoint the root cause of P1093 and P0087 codes and restore your engine’s performance. Remember that 1 MPa is approximately equal to 145 PSI, and 100 kPa is roughly 14.5 PSI for pressure conversions.
