* Fuel System Issues: This is a prime suspect. While you haven't mentioned it, problems here are very common and can cause rough starts. Consider:
* Fuel Filter: A clogged or restricted fuel filter can starve the engine of fuel at startup, leading to a rough idle. Replace it even if it's relatively new. It's cheap and easy and often overlooked.
* Fuel Pressure Regulator: This regulates fuel pressure. A faulty regulator can cause inconsistent fuel delivery, especially during startup. Testing fuel pressure is crucial here. You'll need a fuel pressure gauge.
* Fuel Pump (Lift Pump): The lift pump draws fuel from the tank. A weak or failing lift pump might not provide enough fuel pressure at startup. Listen carefully for a whining sound from the tank area during cranking.
* Injector O-rings or Injectors: While you've addressed the IDM (which controls injectors), worn or leaking injector o-rings can cause a rough start by leaking fuel pressure overnight. A leak-down test on the injectors is necessary to diagnose this.
* HPOP (High-Pressure Oil Pump): While less likely given your other replacements, a failing HPOP can struggle to build pressure at startup. A weak HPOP will often lead to hard starts or no-starts, but it could manifest as a rough idle only at the beginning. Measuring HPOP pressure is needed for confirmation.
* Wiring Harness: Although less common, damaged or corroded wiring, especially in the areas connected to the injectors or sensors, could be intermittently disrupting the signal and causing the rough idle. Inspect the wiring carefully for damage, breaks, or corrosion. Pay particular attention to connectors.
* PCM (Powertrain Control Module): This is a less likely culprit, but a faulty PCM could be sending incorrect signals. This is usually diagnosed after other possibilities are eliminated. You'd need diagnostic equipment to check for fault codes and monitor sensor data.
Troubleshooting Steps:
1. Fuel Filter: Replace this immediately. It's the cheapest and easiest check.
2. Fuel Pressure Test: This is crucial. Use a gauge to check fuel pressure during cranking and idle. Compare readings to factory specifications.
3. Listen for unusual noises: Pay close attention to the fuel pump and the injectors during cranking and starting.
4. Visual inspection: Carefully inspect all wiring and connectors for damage or corrosion.
5. Advanced diagnostics: If the above steps don't resolve the issue, you might need a mechanic or professional diagnostics to check HPOP pressure, perform an injector leak-down test, and check the PCM for fault codes.
Remember safety precautions when working on your vehicle. Disconnect the negative battery terminal before starting any work. If you're not comfortable working on your vehicle's fuel system or other potentially dangerous components, it's best to have a qualified mechanic diagnose and repair the issue.
