Possible Causes:
* Low Oil Pressure: Running low on oil can damage the engine's oil pump. If the oil pump is damaged, it won't provide sufficient lubrication, leading to low oil pressure. This can cause the engine to seize or prevent it from starting even if it cranks. Check your oil pressure gauge (if equipped) or have the pressure tested. This is a *serious* issue and needs immediate attention.
* Fuel Pump Issues (despite testing): While you say you've checked the fuel pump relays, there might be more to it than just the relay. The pump itself could be failing (weak output), the fuel filter could be clogged, or there could be a problem with the fuel lines (leaks, blockages).
* Ignition System Problems: Though you mention spark at the coil, ensure the spark is reaching the spark plugs effectively. Worn spark plugs, faulty ignition wires, or a failing distributor cap/rotor (if your 4Runner has a distributor) could all cause a weak or intermittent spark, preventing the engine from firing.
* Crankshaft Position Sensor (CKP): A faulty CKP sensor prevents the engine computer from knowing the crankshaft's position, thus hindering fuel injection and spark timing.
* Cam Shaft Position Sensor (CMP): Similar to the CKP, a malfunctioning CMP prevents proper synchronization of fuel and ignition.
* Mass Airflow Sensor (MAF): A dirty or malfunctioning MAF sensor can give the engine computer inaccurate air/fuel mixture readings, preventing proper combustion.
Troubleshooting Steps:
1. Check the Oil: Add oil to the correct level *immediately*. If you've already run it low, the damage might be done, but adding oil is essential to prevent further harm.
2. Test Oil Pressure: This is crucial. A low reading indicates significant engine problems.
3. Verify Spark at the Plugs: Don't just check the coil; use an inline spark tester at each spark plug to confirm a strong, consistent spark.
4. Fuel Pressure Test: A fuel pressure gauge will accurately measure the fuel pressure at the fuel rail. This is essential for diagnosing fuel delivery problems.
5. Inspect Fuel Filter and Lines: Look for visual signs of damage, leaks, or blockages.
6. Check the CKP and CMP Sensors: These sensors are prone to failure and can prevent starting. Testing them requires a multimeter or specialized tools.
7. Clean or Replace MAF Sensor: A clean MAF sensor can sometimes resolve starting issues.
8. Scan for Codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the engine computer. These codes can provide valuable clues about the problem.
Important Note: If the engine was severely run low on oil, there's a strong possibility of significant internal engine damage. Further diagnosis may require professional assistance and could lead to costly repairs. It's crucial to address the oil issue first and then proceed with further diagnostics based on the oil pressure test results.
