1. Firing Order and Cylinder Identification: Double-check the firing order is correctly matched to the spark plug wires and injectors. A single miswired plug can prevent two cylinders from firing. Verify you're testing compression and spark on the *correct* cylinders. A simple mistake here is easy to overlook.
2. Spark Plug Wires: Inspect the spark plug wires for damage, cracks, or loose connections. Even if they look fine, a weak connection can prevent a consistent spark. Use a spark tester to verify a strong spark at each plug.
3. Injector Operation: Test the injectors (preferably with a noid light or injector pulse tester) to ensure they are activating and spraying fuel. Two injectors could be malfunctioning or not getting a signal.
4. Valve Adjustment (if applicable): If you had the heads off, you may need to adjust the valves. Incorrect valve lash can lead to poor or no combustion in affected cylinders. Consult a repair manual for your specific engine's valve adjustment procedure.
5. Camshaft Position Sensor (CMP) and Crankshaft Position Sensor (CKP): Although you mention checking the CMP sensor, double-check its operation and the CKP sensor as well. These sensors are crucial for proper ignition timing. Faulty signals will result in misfires or no start.
6. Compression Retest (with Throttle Open): Your compression test should ideally be done with the throttle fully open. This allows for a more accurate reading and identifies issues with the intake system that could prevent proper cylinder filling. Retest the compression on all cylinders with the throttle open.
7. Intake Manifold Gasket: A poorly seated or damaged intake manifold gasket could cause a vacuum leak, leading to misfires, especially in certain cylinders.
8. Exhaust Manifold Gasket (less likely): While less likely to cause a no-start if only two cylinders are affected, a leak here could impact exhaust flow and pressure, and indirectly affect combustion.
9. ECU/PCM Issues: Although less probable, a faulty Engine Control Unit (ECU) or PCM could cause misfiring in specific cylinders. This would require diagnostic tools to properly assess.
Troubleshooting Steps:
1. Start with the simplest checks (wiring, connections).
2. Use a systematic approach. Check each system (fuel, spark, compression) individually for each cylinder.
3. Consult a repair manual specific to your 1998 Ford Explorer 4.0L engine. This will provide detailed diagrams, specifications, and troubleshooting guides.
4. Use a multimeter and diagnostic tools if necessary.
If you've checked all these points and are still having trouble, seeking professional help from a mechanic specializing in older vehicles is recommended. They have diagnostic tools and experience to pinpoint the problem more efficiently. Trying to guess at more advanced issues without proper tools and knowledge can lead to further damage or unnecessary parts replacements.
