Owner's Problem Description
The owner of a 1985 Oldsmobile Ciera with a 2.5L TBI engine reported that after replacing a cracked cylinder head, the vehicle is experiencing no injector pulse and lacks energy to the IAC. The owner has not yet checked for voltage at the injector plug or tested fuel pressure, and is unsure about the fuel pump's operational status under load. Previous repairs include the head replacement, but no other electrical diagnostics have been performed. The vehicle currently shows signs of cranking but fails to start.
Professional Technical Analysis
The 1985 Oldsmobile Ciera is equipped with a 2.5L TBI engine (Iron Duke), which is known for its simplicity yet can exhibit complex electrical issues. The vehicle has been reported to have no injector pulse and lacks energy to the Idle Air Control (IAC) valve after a cylinder head replacement. This indicates potential issues with the engine control module (ECM) or related wiring. Given the age of the vehicle (approximately 150,000 miles) and common failure points, it's critical to check all relevant electrical connections and fuses. The symptoms suggest that the injector relay or fuse for the ECM may be faulty. Additionally, the owner has not performed a voltage check at the injector plug or the fuel pump connector, which is essential in diagnosing fuel delivery issues. Voltage should ideally be present at the injector terminals while the key is on (should read around 12V). The fuel pump should also be tested under load to ensure proper operation, as pumps can fail to deliver pressure when subjected to engine conditions. The absence of injector pulse can often be traced back to faulty sensors or the ECM itself, particularly if trouble codes have not been checked. The vehicle should be diagnosed systematically, starting with the power supply to critical components, including relays and fuses, and progressing towards the ECM and its output signals to the injectors.
Possible Causes
Most common causes (ordered by frequency):
- Engine Control Module (ECM) failure: A common issue in older vehicles like the 1985 Ciera, the ECM may fail due to internal component degradation or poor connections. If the ECM is not receiving adequate power or ground, it will fail to send an injector pulse. Voltage at the injector plug should be approximately 12V with the ignition in the 'on' position, which should be tested to confirm ECM functionality. - No injector pulse detected at the injector connector, lack of IAC energy, potential stored codes indicating ECM issues.
- Faulty injector relay or fuse: The injector relay may not be activating due to a blown fuse or relay failure. This can prevent the injector from receiving power. It is important to check the continuity of the fuse and the relay operation to ensure they are functioning properly. The injector relay should provide power to the injectors when the ignition is on, and continuity should be checked using a multimeter. - Visual inspection shows potential blown fuses, relay operation not confirmed, continuity tests needed.
- Fuel pump issues: If the fuel pump is not delivering fuel, it may not run under load due to internal wear or electrical faults. Fuel pressure should be tested to confirm proper operation, ideally maintaining 9-13 PSI for TBI systems. A no-load test may not reveal pump issues, thus requiring a check under operational conditions. - Fuel pressure gauge readings show low or zero pressure, potential fuel pump failure under load conditions.
- Wiring issues or poor connections: Corrosion or damage to wiring harnesses and connectors can lead to poor electrical contacts, causing loss of power to injectors or ECM. Visual inspection and continuity testing of relevant wiring should be performed. Any damaged or corroded connections should be repaired or replaced. - Visual inspection reveals corroded connectors or damaged wires, continuity tests show inconsistencies in power supply.
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 Electrical Checks: Begin with a visual inspection of fuses related to the injector circuit and ECM. Use a multimeter to test for continuity and voltage at the injector connector with the ignition on. Confirm that there is approximately 12V present. If voltage is absent, proceed to check the injector relay and relevant fuses to ensure they are functioning correctly.
- STEP 2 - Fuel System Pressure Test: Install a fuel pressure gauge at the test port to measure fuel pressure while cranking the engine. The pressure should maintain between 9-13 PSI for the TBI system. If the pressure is low or absent, further investigation of the fuel pump operation and wiring integrity is needed.
- STEP 3 - Injector and IAC Testing: Check the injector connector for power and ground while cranking the engine. Use a noid light to confirm injector pulse. Test the IAC connector for voltage to ensure it is receiving power as well. Any absence of voltage indicates issues with the ECM or relay.
- STEP 4 - Wiring and Ground Checks: Inspect wiring harnesses for any signs of damage or corrosion. Perform continuity tests on ground connections to the ECM and injector circuits. Repair any damaged wires or connections found during inspection.
