Owner's Problem Description
The owner reports that their 1987 Chevy Blazer was initially not delivering fuel to the engine. After replacing the fuel pump, fuel flow was restored, but the engine dies as soon as it idles, indicating a persistent issue. Previous repairs included a new fuel pump, fuel filter, and oil pressure switch. Despite these efforts, the vehicle still experiences stalling, and further assistance is sought to resolve the problem.
Professional Technical Analysis
The 1987 Chevy Blazer features a 5.0L V8 engine and is typically seen with a mileage of around 150,000 miles. The owner initially experienced a complete lack of fuel delivery to the engine, leading to the replacement of the fuel pump. While fuel flow was restored, the truck now dies at idle, suggesting an issue with fuel pressure maintenance. The fuel delivery system comprises several interconnected components, including the fuel pump, fuel filter, oil pressure switch, and the fuel pump relay. The oil pressure switch is critical as it cuts off power to the fuel pump when oil pressure drops below a certain threshold, which may explain the stalling if the switch is malfunctioning or the oil pressure is genuinely low. The fuel pressure should be maintained between 9-13 PSI at idle, with pressures dropping significantly indicating a potential blockage or a failing fuel pump relay. Given that the fuel filter, oil pressure switch, and fuel pump are all recently replaced, attention must now shift to the fuel pump relay and wiring condition. Corrosion or poor connections could lead to intermittent power loss, causing the fuel pump to shut off unexpectedly. Conducting a thorough inspection of these components is essential to determine the root cause. Safety remains a priority, especially given the importance of maintaining proper fuel delivery for engine performance.
Possible Causes
Most common causes (ordered by frequency):
- Fuel Pump Relay Failure: A common failure point in older vehicles, the fuel pump relay can become corroded or fail to provide consistent power to the fuel pump. If the relay contacts are oxidized, it may not supply voltage, causing the fuel pump to stop functioning. Testing the relay with a multimeter to check for continuity and proper voltage output is essential. A replacement relay can typically cost between $20-$50. - Intermittent fuel delivery, visual inspection showed corrosion on relay terminals, multimeter testing indicated voltage drop across relay.
- Oil Pressure Switch Malfunction: If the oil pressure switch is faulty, it may incorrectly signal low oil pressure, cutting power to the fuel pump. This safety feature is designed to prevent fuel delivery when there is inadequate oil pressure. Testing the oil pressure with a gauge can confirm if the oil pressure is within specs (approximately 10 PSI at idle). Replace the oil pressure switch if it does not function correctly. - Oil pressure gauge reading below normal, visual inspection of switch reveals damage, multimeter shows switch fails to close.
- Clogged Fuel Filter: Even if the fuel filter is new, it may have been contaminated during installation or may be defective. A clogged filter can restrict fuel flow, especially at idle conditions. Checking fuel pressure before and after the filter can help diagnose this issue. Fuel filter replacement costs around $20-$50. - Fuel pressure drops significantly after the filter, visual inspection shows debris on filter inlet, recent installation confirms quality issues.
- Faulty Carburetor: If equipped with a carburetor, it may have internal issues such as stuck float or clogged jets, preventing proper fuel metering. Testing the carburetor's fuel inlet pressure can confirm if it's receiving the correct pressure from the fuel pump. Carburetor repair kits are typically priced between $30-$100. - Poor engine response, visual inspection of carburetor shows signs of wear, pressure testing indicates low fuel inlet pressure.
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 - Initial OBD-II Scan: Use GeekOBD APP to conduct a thorough scan of the vehicle's electronic systems. Retrieve any stored or pending trouble codes that may provide insights into the fuel delivery issue. Pay special attention to codes related to the fuel system, relay failures, or sensor malfunctions.
- STEP 2 - Fuel Pressure Testing: Connect a fuel pressure gauge to the fuel rail and check the pressure while cranking and at idle. The pressure should be maintained between 9-13 PSI. If the pressure drops significantly when the engine idles, further investigation into the fuel pump and filter is required. Document the readings for diagnostic comparison.
- STEP 3 - Relay and Wiring Inspection: Inspect the fuel pump relay for corrosion and test its functionality with a multimeter. Check the wiring for any signs of wear or damage. Ensure all connections are secure and free of corrosion. Replace any corroded components to ensure proper electrical continuity.
- STEP 4 - Oil Pressure Switch Testing: Test the oil pressure switch with a multimeter to ensure it is functioning correctly. If it fails to close at the specified oil pressure, replacement is necessary. This step is crucial as it directly affects fuel pump operation.
