* Fuel Injection Pump Issue: The most likely culprit is a malfunction in the fuel injection pump itself. Diesel pumps don't rely on the ignition system in the same way petrol pumps do. A problem with the pump's internal shut-off mechanism (often a solenoid) could be preventing it from stopping fuel delivery when the ignition is switched off. This could be due to a faulty solenoid, wiring problem, or a mechanical failure within the pump.
* Stop Solenoid Failure: Diesel engines often have a stop solenoid directly on the injection pump. This solenoid is activated when the ignition is switched off, cutting power to the pump and stopping fuel delivery. A failure in this solenoid (it could be stuck or not receiving power) would prevent the engine from shutting down.
* Glow Plug Relay/Timer Issue (less likely but possible): While glow plugs are primarily for starting, a faulty glow plug relay or timer might be sending incorrect signals to the fuel system, potentially affecting the shut-off sequence. This is less likely to be the direct cause but could contribute to the issue, especially if the engine is difficult to start after the event.
* Fuel Leak: A leak in the fuel system after the shut-off valve could lead to a continued supply of fuel to the engine. While less common, a persistent leak would need to be addressed immediately.
* Wiring Problems: Problems in the wiring harness connected to the fuel shut-off solenoid or the injection pump could also prevent proper signal transmission, resulting in the engine's continued operation. Corrosion or damaged wiring is a potential cause.
Troubleshooting Steps (Not for DIY unless you have experience):
Diagnosing this requires some mechanical knowledge and appropriate tools. Attempting these steps without experience could be dangerous. It's best to have a qualified mechanic investigate:
1. Check the stop solenoid: A mechanic can test the solenoid's operation, checking for power and whether it's activating correctly when the ignition is switched off.
2. Inspect the fuel injection pump: This may require specialized tools and expertise to diagnose internal problems.
3. Examine the wiring: Check for damaged, corroded, or loose wires related to the fuel system and the stop solenoid.
4. Fuel system pressure test: A pressure test can check for leaks in the fuel system.
In short: Don't attempt to fix this yourself unless you're very experienced with diesel fuel systems. A continuously running engine is dangerous and could cause serious damage. Get it to a mechanic immediately.
