Vehicle: 2018 Chevy Silverado 1500, 4WD, V8-5.3L, Automatic Transmission/Transaxle

Mileage: 132,878 Miles

Problem: The owner’s concern was that the malfunction indicator light (MIL) was on and the vehicle would start and barely idle (about 300 rpm).

Case Details: The technician connected a scan tool and only retrieved one diagnostic trouble code (DTC): P0506—Idle Control System RPM Lower Than Expected. He checked fuel pressure with the scan tool to find that the low-pressure fuel sensor readings were too low —going as low as 26 psi for a second when accelerating. He tried to reset the idle speed but was unsuccessful. Long-term fuel trims were -25 at idle but short-term fuel trims were 0.

He also noticed that the low pressure reading with the key on, engine off (KOEO), would often drop off slowly. At this point, he called an ALLDATA Tech-Assist consultant.

The consultant gave him the following diagnostic advice. To begin, these vehicles are known for high-pressure (H/P) fuel pump issues. Many fail at roughly 120,000-150,000 miles. A failing H/P pump can allow fuel to enter the engine oil, which will drive fuel trims excessively negative (rich condition). Based on the low-side fuel pressure readings, the technician recommended dead-heading the low-pressure fuel pump to verify its output.

The technician also recommended monitoring the fuel rail pressure data. There should be PIDs for both actual and desired pressure. If actual fuel pressure is close to low-side pressure, then the H/P pump is not producing sufficient pressure. By watching the low-pressure reading KOEO, more than likely the pressure caused by leaks through the high-pressure pump will slowly drop. 

The Tech-Assist consultant additionally warned that high alcohol content in the fuel could push fuel trims positive, typically in the 14–20% range, and recommended the tech check the alcohol content.

The technician performed the tests as suggested. He found the alcohol level at 10%, which was good. The dead head pressure was only 83psi.

Confirmed Repair: He replaced the high-pressure fuel pump, cleaned the throttle body, relearned the idle speed, and cleared the DTC. After the repairs, the vehicle ran perfectly, idled correctly and the MIL remained off. The technician remarked that he would have replaced the fuel pump in the tank and wasted a lot of time and money. He thanked the consultant for his guidance. Problem solved!