Owner's Problem Description
Bob's 1996 Chevy Cheyenne experiences intermittent cranking without firing. The vehicle turns over hesitantly and will not start. Bob has replaced the starter and battery, but the issue persists. He reports that the fuel pump operates when the key is turned, and there is fuel pressure at the Schrader valve. Bob seeks assistance to diagnose and resolve the problem.
Professional Technical Analysis
The 1996 Chevy Cheyenne features a 5.7L V8 engine with an estimated mileage of 150,000 miles. The intermittent cranking issue manifests as the engine turns over but fails to start, presenting a classic symptom often linked to ignition or fuel delivery problems. The vehicle shows a consistent cranking sound but hesitates, indicating a potential electrical fault or sensor issue. Given that Bob replaced both the starter and battery, we can rule out these components as the source of the problem. The operational fuel pump suggests that fuel is reaching the engine; however, without a proper spark, combustion will not occur. A common diagnostic step involves checking for spark at the ignition system, particularly from the ignition control module (ICM) and crankshaft position sensor (CPS). If the CPS fails, it can lead to incorrect timing signals, resulting in the engine cranking without firing. The crankshaft sensor, located near the harmonic balancer, can often fail, causing symptoms such as those experienced. Current vehicle conditions show no warning lights active, but a thorough inspection of the ignition and fuel systems is necessary to ensure proper functionality. Emphasizing the interconnectedness of these systems is vital for identifying the root cause of the issue, as a malfunction in the CPS can lead to timing issues that prevent ignition.
Possible Causes
Most common causes (ordered by frequency):
- Crankshaft Position Sensor (CPS) Failure: The CPS is crucial for relaying engine position information to the Powertrain Control Module (PCM), which adjusts ignition timing. A faulty CPS can cause the timing to advance incorrectly, leading to poor ignition and intermittent starting issues. Typically, CPS failures can occur around 100,000 miles due to heat cycling and exposure to engine conditions. The CPS should have a resistance of approximately 200-800 ohms when tested, and a failure may result in open or short circuits. - Cranking without firing, possible stored codes for crank position sensor, no spark detected at ignition coil.
- Ignition Control Module (ICM) Malfunction: The ICM is responsible for controlling the ignition coil's firing timing. A failing ICM can lead to a no-spark condition, particularly when hot, causing intermittent starting issues. Common symptoms include cranking without firing and erratic spark output. Testing the ICM with an oscilloscope can reveal signal integrity issues. - No spark during cranking, intermittent no-start conditions, possible ICM fault codes.
- Fuel Injector Issues: Although fuel pressure is present at the Schrader valve, it’s essential to confirm that fuel injectors are delivering the correct amount of fuel. Clogged or malfunctioning injectors can prevent proper fuel metering, leading to a no-start condition. Injector resistance should be between 12-16 ohms, and a noid light test can confirm whether injectors are firing. - Fuel pressure present but injector not delivering fuel, potential clogged injectors.
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 Spark Check: Before proceeding with further diagnostics, perform a spark check using a spark tester on one of the ignition coil leads. This will determine if the ignition system is functioning correctly. If no spark is detected, the issue may lie with the ICM or CPS. Document any findings.
- STEP 2 - Crankshaft Position Sensor Inspection: Locate the CPS near the harmonic balancer and inspect the wiring for damage or corrosion. Using a multimeter, measure the resistance across the sensor terminals to confirm it is within the 200-800 ohm range. If out of range, replace the CPS (OEM P/N 10456238).
- STEP 3 - ICM Testing: If the CPS checks out, proceed to test the ICM using an oscilloscope. Verify the output signal and look for any irregularities in the waveform that indicate failure. If the ICM is faulty, replace it with an OEM part (P/N 10456229) and ensure proper installation.
- STEP 4 - Injector Operation Test: If both the CPS and ICM are functioning, perform a fuel injector test using a noid light to check if the injectors are firing. If the injectors are not functioning, inspect for blockages or electrical faults. Replace faulty injectors as necessary.
