Owner's Problem Description
The owner of a 1990 Ford Bronco 5.8L with about 100,000 miles reports difficulty starting after the vehicle sits overnight. When the key is turned on, the fuel pump runs continuously for a few minutes before shutting off, allowing the engine to start. The owner has replaced the cold start valve, distributor cap and rotor, and confirmed good spark from the coil. They've also checked the fuel pump relay and ensured grounds are clean. The vehicle runs fine once started.
Professional Technical Analysis
The 1990 Ford Bronco with a 5.8L engine exhibits a cold start issue, common in vehicles of this age. The vehicle has approximately 100,000 miles, which may indicate wear on various components. When the key is turned on, the fuel pump engages and runs for an extended period, which should typically only be a few seconds unless pressure is low. The fuel pressure should be verified at around 40 PSI when the engine is running. The symptoms suggest a potential issue with the fuel system, possibly the fuel pump not maintaining pressure, or a faulty sensor affecting the fuel pump operation. Since the owner has replaced several components related to ignition and fuel delivery, the focus should shift to the fuel system and its control components, including the fuel pump and the PCM. The continuous running of the fuel pump could indicate the relay is not functioning correctly or that the PCM is not receiving accurate readings from sensors, leading to extended activation of the pump. Additionally, the cold start valve replacement may not address the root issue if incorrect fuel pressure or sensor readings are involved. A thorough pressure test during the cold start condition is essential to diagnose the problem accurately.
Possible Causes
Most common causes (ordered by frequency):
- Fuel Pump Failure: The fuel pump may not be reaching the required pressure due to wear or internal failure, causing it to run longer than normal when the key is turned. A proper pressure test should show around 40 PSI when the engine is running. If the pump is unable to deliver adequate pressure, it may need to be replaced (OEM part number F81Z-9350-AB, cost approximately $150-$300). - Extended fuel pump operation upon key activation, required fuel pressure not met during testing.
- Faulty Fuel Pump Relay: The relay may be sticking or malfunctioning, causing the fuel pump to run longer than intended. Testing should confirm relay operation with a good replacement available for comparison (standard relay P/N 1R3Z-14B192-CA, cost around $20). - Relay is not cutting off the pump after initial activation, leading to longer pump operation.
- PCM Malfunction: The Powertrain Control Module may not be properly controlling the fuel pump relay based on sensor inputs. If the PCM is sending incorrect signals, it may lead to extended fuel pump operation. Testing with an OBD-II scanner may reveal fault codes relating to the fuel system or PCM (common codes P0230-P0235). - No diagnostic trouble codes present initially, but symptoms suggest potential PCM issues.
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 Fuel Pressure Test: Start by checking the fuel pressure with a fuel pressure gauge. Connect the gauge to the fuel rail, and turn the key to the 'ON' position without starting the engine. Record the pressure; it should rise to around 40 PSI within seconds. If the pressure does not rise or falls quickly, the fuel pump may be at fault.
- STEP 2 - Relay Function Check: After confirming fuel pressure, check the fuel pump relay. Use a multimeter to check continuity and operation of the relay. Replace it with a known good relay if any issues are detected. The standard relay part number is 1R3Z-14B192-CA.
- STEP 3 - OBD-II Scanner Check: Use an OBD-II scanner to check for any stored or pending trouble codes that might indicate a PCM or sensor issue affecting fuel system operation. Pay particular attention to any codes related to fuel pressure or pump operation.
- STEP 4 - Sensor Function Testing: If no faults are found with the pump and relay, test the coolant temperature sensor and any other relevant sensors that could affect cold starting. A malfunctioning coolant temperature sensor may lead to improper fuel delivery during cold starts.
