* Faulty Glow Plug Control Unit (GPG): This is a highly likely culprit. The GPC controls the glow plugs' heating cycle. If it's malfunctioning, the plugs might not be getting enough power or the right timing, rendering them ineffective even if they're new. Testing the GPC's voltage output and relay function is crucial.
* Low Battery Voltage: A weak battery can't supply the current needed for the glow plugs and the starter motor. Test the battery voltage both at rest and while cranking. Low voltage will significantly impede starting.
* Fuel System Problems:
* Blocked Fuel Filter: A clogged fuel filter restricts fuel flow to the injectors, making starting difficult, especially in cold weather.
* Air in the Fuel System: Air in the fuel lines can prevent proper fuel delivery. Bleed the fuel system thoroughly.
* Fuel Pump Issues: A weak fuel pump might not supply enough fuel pressure for starting.
* Injector Problems: Faulty injectors can prevent proper fuel atomization and combustion. This is less likely to manifest only after glow plug replacement.
* Incorrect Glow Plug Installation: Although unlikely if you're mechanically inclined, double-check the installation of the new glow plugs to ensure they're properly seated and connected. A loose connection will dramatically reduce their effectiveness.
* Compression Issues: Low compression in one or more cylinders makes starting hard regardless of the glow plugs. This requires a compression test.
* Timing Belt: A slightly off timing belt can affect combustion and starting. (This is less likely to be a sudden problem after glow plug replacement.)
* Other Electrical Issues: Check all wiring related to the glow plug system for any breaks, loose connections, or corrosion.
Troubleshooting Steps:
1. Check Battery Voltage: This is the easiest and quickest check.
2. Test the Glow Plug Control Unit: This is the most likely culprit if the battery is okay. A mechanic or auto electrician can do this test.
3. Check Fuel Filter and Bleed the Fuel System: This is a relatively easy check.
4. Verify Glow Plug Connections: Make sure all the connections are tight and secure.
5. Consider a Compression Test: If the above steps don't reveal the problem, this will help diagnose whether the engine has adequate compression.
If you're not comfortable performing these checks yourself, take the car to a qualified mechanic specializing in diesel vehicles. They have the diagnostic tools and expertise to identify the precise cause of the starting problem.
