* Fuel Delivery Issues: Even with a new ECM, problems within the fuel delivery system can cause a lean condition. This includes:
* Fuel pump: Weak fuel pump delivering insufficient pressure. Check fuel pressure at the carburetor.
* Fuel filter: A clogged fuel filter restricts fuel flow. Replace it.
* Clogged carburetor jets or passages: Despite the system being electronic, the carburetor itself can still clog. A thorough cleaning or even rebuild might be necessary. This is often overlooked.
* Fuel pressure regulator: If equipped, a malfunctioning regulator can lead to insufficient fuel pressure.
* Leaking injectors: While less common on a carbureted system, there might be a small leak somewhere affecting the fuel-air mixture.
* Vacuum Leaks: Vacuum leaks drastically affect the mixture. Inspect all vacuum lines for cracks, leaks, or loose connections. Pay close attention to lines going to the carburetor and the EGR system. A leak can cause a lean condition, especially at higher RPMs.
* Air Intake System: Problems in the air intake system can also cause lean conditions:
* Air leaks: Check for leaks around the air filter housing, throttle body, and any other parts of the intake system.
* Mass Airflow Sensor (MAF) issues: While you probably don't have a MAF sensor on a carbureted system, it's worth checking the air flow passages for blockages or leaks. The system relies on vacuum and airflow sensors that could malfunction.
* Throttle Position Sensor (TPS): The TPS tells the ECM the throttle position. A faulty TPS can cause incorrect fuel delivery, leading to lean conditions. While less common on a carb system than on fuel-injected ones, the related sensors might be malfunctioning.
* ECM Issues (again): While you replaced the ECM, there's a slight chance it's defective or incompatible with your system. Double-check that it's the correct part number for your year and model.
* Wiring: Check all wiring harnesses connected to the ECM, TPS, sensors, and carburetor for damage, corrosion, or loose connections.
Troubleshooting Steps:
1. Verify Fuel Pressure: This is the most important first step. A fuel pressure gauge is essential.
2. Check Fuel Filter: Replace it regardless, it's cheap and easy.
3. Thorough Visual Inspection: Look for vacuum leaks, cracked hoses, loose connections, and signs of fuel leaks. Pay close attention to every hose and connection.
4. Carburetor Cleaning/Rebuild: This is often necessary, even if you haven't touched it yet. Many issues hide in the carburetor's tiny passages.
5. Data Logging: If possible, use a scan tool capable of reading data from the ECM to monitor fuel trims and other relevant parameters. This will help pinpoint the problem area.
Since you've already replaced major components, the issue is likely subtle. A methodical approach and careful inspection of all fuel and vacuum lines are key to success. Consider seeking help from a mechanic experienced with older Corvettes and carburetor systems – they might have encountered this specific problem before. The Rochester E4ME is notorious enough that this should be no surprise.
