Owner's Problem Description
The owner reports a problem with one cylinder on their 2001 Ford Explorer that has spark and fuel but will not fire. Compression checks out at 150 psi, and the owner has already replaced the coil pack, spark plug, wire, and injector. They have assessed the vacuum lines as good, and checked electrical resistance, yet the cylinder remains non-functional. Seeking advice on the next steps to resolve this issue.
Professional Technical Analysis
This 2001 Ford Explorer with a 4.0L V6 engine has a reported issue of a non-firing cylinder despite having spark, fuel, and good compression at 150 psi. The owner has replaced key ignition components including the coil pack, spark plug, and fuel injector, as well as checked vacuum lines and electrical resistance. Given these factors, we must consider additional diagnostics, particularly timing and sensor functionality. The spark is described as a strong blue, and the plug shows signs of fuel saturation, indicating fuel delivery is adequate. Potential issues could stem from the crankshaft or camshaft position sensors, which significantly influence ignition timing. If either sensor is malfunctioning, it could lead to incorrect timing, preventing combustion even with good compression. Compression is within acceptable limits; thus, further investigation into the timing components and sensor outputs is essential. Proper diagnostic steps include checking the timing with an oscilloscope and ensuring the sensors are providing accurate signals to the PCM (Powertrain Control Module). Safety checks should include ensuring no fault codes are present in the OBD-II system, particularly related to timing or sensor issues.
Possible Causes
Most common causes (ordered by frequency):
- Crankshaft Position Sensor (CPS) Failure: If the CPS is malfunctioning, it can lead to erratic or incorrect timing signals to the PCM, preventing the cylinder from firing. This is crucial for spark timing and fuel injector operation. Testing the CPS with a multimeter should reveal a resistance of 200-300 ohms and a proper square wave output when cranking the engine. - Check engine light potentially indicating CPS issues, lack of timing signal during cranking.
- Camshaft Position Sensor (CMP) Malfunction: Similar to the CPS, the CMP provides critical timing information. A faulty CMP can cause the PCM to misfire the cylinder, despite correct compression and fuel delivery. The CMP should produce a voltage signal between 0.5V to 4.5V depending on engine position. - Possible stored codes related to CMP failure, irregular signal output when tested.
- Timing Chain/Belt Issues: If the timing chain or belt has slipped or is worn, it could lead to incorrect valve timing, resulting in a non-firing cylinder. A visual inspection should check for signs of wear or misalignment, and a timing light should confirm correct timing alignment. - Unusual engine noise, visible wear on timing components, timing light indicates misalignment.
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 - Sensor Testing: Begin by testing the crankshaft and camshaft position sensors. Use a multimeter to measure resistance and ensure both sensors are functioning within specifications. The CPS should read between 200-300 ohms, while the CMP voltage should range from 0.5V to 4.5V. This initial check can identify potential sensor failures that directly affect timing.
- STEP 2 - Timing Inspection: Conduct a thorough inspection of the timing chain or belt for wear, slippage, or misalignment. Use a timing light to verify that the ignition timing aligns with the manufacturer's specifications. If timing is off, re-align the components as necessary to ensure proper engine operation.
- STEP 3 - Live Data Monitoring: Utilize GeekOBD APP to monitor live data during cranking. Look for variations in timing signals and fuel injector operation. This can provide insights into whether the PCM is receiving correct signals from the sensors and if the injector is functioning properly.
- STEP 4 - Visual Inspection of Engine Components: Conduct a visual inspection of all related engine components, including wiring harnesses and connectors. Look for signs of wear, corrosion, or damage that could impact electrical flow to the ignition and fuel systems.
