Owner's Problem Description
The owner reports an intermittent service engine light on their 1998 GMC 1500 with a 5.7L Vortec engine, approximately 98,000 miles. The vehicle shows a code for a cylinder 5 misfire, which occasionally flashes but usually goes out after driving for a few miles. A full tune-up has been performed, yet the issue persists, sometimes taking over 300 miles to reappear. Additionally, the oil pressure gauge, which typically reads between 30-50 PSI, occasionally drops to 0 but then returns to normal after about 30 seconds. An aftermarket oil pressure sender was replaced, which worked for about 20 minutes before the problem recurred. The owner is unsure if these issues are related.
Professional Technical Analysis
This 1998 GMC 1500, equipped with a 5.7L Vortec engine and approximately 98,000 miles, is presenting two primary symptoms: an intermittent service engine light with a cylinder 5 misfire and erratic oil pressure readings. The misfire issue is confirmed by a stored fault code, which typically occurs every 300 miles or so, indicating a potential problem with the ignition or fuel delivery specific to cylinder 5. A full tune-up was recently performed, including replacement of spark plugs, wires, ignition coil, and fuel filter, aiming to resolve this issue. However, the persistent misfire suggests further investigation is needed, potentially involving the fuel injector for cylinder 5 or an issue with the engine's vacuum system, which could affect fuel delivery. The oil pressure gauge, showing normal values under typical conditions, drops to 0 intermittently, which raises concerns about the engine's lubrication system. This could be linked to an issue with the oil pressure sending unit, especially since an aftermarket sender was installed and failed to resolve the problem. It is crucial to confirm the accuracy of the gauge readings with a mechanical gauge to rule out an electrical fault. Additionally, the absence of abnormal engine noise during pressure drops suggests that the issue may be electrical rather than mechanical, but this should be verified. The interconnected nature of these symptoms suggests a thorough diagnostic approach is warranted, focusing on the ignition system, fuel delivery, and oil pressure monitoring.
Possible Causes
Most common causes (ordered by frequency):
- Cyl. 5 Misfire due to Fuel Injector Failure: The fuel injector for cylinder 5 may be failing to deliver adequate fuel due to clogging or electrical issues. A typical injector resistance should be between 12-16 ohms. If the injector's resistance is out of this range or if electrical connections are loose, it could lead to misfiring. This can be confirmed through an OBD-II scan for specific injector performance codes and using a multimeter to test injector resistance. Replacing the injector with OEM P/N 17113681 is recommended if found faulty, with costs around $100-150 for the part. - Intermittent misfire code for cylinder 5, injector resistance test results, fuel trim readings indicating lean condition.
- Oil Pressure Sending Unit Failure: The aftermarket oil pressure sending unit may be malfunctioning, causing erratic oil pressure readings. This is common with aftermarket parts. Mechanical gauge testing should be performed to verify actual oil pressure, which should read between 30-50 PSI when the engine is at operating temperature. If the mechanical gauge shows consistent pressure, the sending unit should be replaced with OEM P/N 12558359, generally costing around $50-80. - Inconsistent oil pressure gauge readings, aftermarket sender replaced, mechanical gauge results.
- Electrical Connections and Ground Issues: Poor electrical connections, especially ground points for both the fuel injectors and oil sending unit, could lead to erratic signals and misfire codes. Clean and secure all ground connections and inspect wiring harness integrity. Voltage at the injector connector should be around 12V during operation. This may require visual inspection and possibly the use of a multimeter to check for voltage drop across connections. - Loose or corroded electrical connections found during inspection, voltage readings at connectors.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - OBD-II Code Retrieval: Begin with a comprehensive scan of the vehicle's OBD-II system using a professional scan tool or GeekOBD APP. Retrieve stored codes and freeze frame data to assess the conditions under which the misfire occurs. Pay particular attention to any pending codes that may not be currently active but indicate intermittent faults.
- STEP 2 - Fuel Injector Testing: Check the resistance of the fuel injector for cylinder 5 using a multimeter. The expected resistance should be between 12-16 ohms. If the resistance is out of range, replace the injector with OEM P/N 17113681. Additionally, inspect the wiring and connectors for damage or corrosion.
- STEP 3 - Oil Pressure Gauge Verification: Install a mechanical oil pressure gauge to validate oil pressure readings. Ensure the engine is at operating temperature, and verify that the oil pressure remains steady between 30-50 PSI. If the mechanical gauge shows consistent readings, the issue likely lies with the sending unit.
- STEP 4 - Electrical Connection Inspection: Inspect and clean all electrical connections related to the fuel injectors and oil pressure sending unit. Ensure that ground connections are secure and free of corrosion. Use a multimeter to check for voltage at the injector connector during operation, ensuring it is around 12V.
