* Glow Plug (or Glow Plug Circuit): A faulty glow plug in cylinder #4, or a problem with the wiring/circuit to that specific glow plug, can cause a misfire, especially when the engine is cold or under load. A weak glow plug might not fully ignite the fuel, leading to incomplete combustion. Check the resistance of the glow plug itself and inspect the wiring for damage.
* Compression: Low compression in cylinder #4 is a major cause of misfires. A compression test is essential. Low compression could be due to:
* Worn piston rings: Allowing compression leakage past the pistons.
* Valve problems: Burnt or sticking valves preventing proper sealing.
* Head gasket leak: Between cylinder #4 and another cylinder or the coolant jacket.
* Fuel Delivery (beyond the injector): While the injector itself was tested, there could still be issues with:
* Fuel lines: A blockage or leak in the fuel line leading to cylinder #4.
* Fuel pump: Weak fuel pressure could prevent sufficient fuel delivery.
* Air intake: A leak in the air intake system before or after the turbocharger (if equipped) can cause a lean mixture and misfire in one cylinder. Check all hoses and connections.
* Wiring: Problems with the wiring harness to cylinder #4's components (glow plug, injector, crankshaft sensor, etc.) Check for frayed wires, poor connections, or corrosion.
* Crankshaft Position Sensor (CKP): A faulty CKP sensor can cause misfires, especially if its signal is intermittent. This sensor is crucial for fuel injection timing.
* EGR Valve: A faulty EGR valve might cause poor combustion, especially under specific load conditions. Check its operation.
Troubleshooting Steps:
1. Compression Test: This is the most crucial step. It will quickly pinpoint if a mechanical issue is the problem.
2. Visual Inspection: Carefully examine the wiring harness, fuel lines, and air intake system for damage.
3. Glow Plug Test: Check the resistance of the glow plug.
4. Fuel Pressure Test: Measure the fuel pressure to ensure it's within specifications.
5. Diagnostic Scan: Use a suitable OBD-II scanner to read fault codes from the engine control unit (ECU). This might provide clues about the problem.
It's best to take the vehicle to a mechanic specializing in diesel engines, especially if you're not comfortable performing these tests yourself. A proper diagnosis needs more than just testing one component. They have the tools and expertise to pinpoint the exact problem.
