* EGR Valve (Exhaust Gas Recirculation): You mentioned this, and it's a good place to start. A malfunctioning EGR valve can significantly impact fuel economy and cause hesitation, particularly at lower RPMs and under load. Check for proper operation and clean or replace if necessary.
* Oxygen Sensor (O2 Sensor): The O2 sensor is crucial for the fuel injection system. A faulty sensor provides inaccurate information to the computer, leading to a rich or lean fuel mixture. This will directly impact gas mileage and can cause hesitation.
* Mass Air Flow Sensor (MAF Sensor): This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor will give incorrect readings, resulting in poor fuel economy and drivability issues. Cleaning it (carefully, following instructions specific to your sensor) is often the first step; replacement might be needed.
* Fuel Injectors: Clogged or malfunctioning fuel injectors can cause a lean mixture in some cylinders and a rich mixture in others. This leads to poor fuel efficiency and hesitation. Testing fuel injector spray pattern is important.
* Throttle Position Sensor (TPS): This sensor tells the computer the throttle position. A faulty TPS can cause erratic idle, poor acceleration, and poor gas mileage.
* Fuel Pressure Regulator: Ensures proper fuel pressure to the injectors. A failing regulator can cause low fuel pressure, leading to poor performance and fuel economy.
* Vacuum Leaks: Small vacuum leaks can disrupt the engine's air/fuel mixture, leading to poor performance and fuel economy. Inspect all vacuum lines and connections for cracks or damage.
* Catalytic Converter: A clogged catalytic converter will restrict exhaust flow, impacting engine performance and fuel economy. You might notice reduced power and a strange exhaust smell.
* Transmission Issues (Automatic): If it's an automatic transmission, problems within the transmission itself (low fluid, worn components) could cause hesitation, especially under load, and impact fuel economy.
Troubleshooting Steps:
1. Start with the easiest and cheapest: Clean the MAF sensor, check for vacuum leaks, and inspect the EGR valve.
2. Check engine codes: Use an OBD-I scanner (your Geo is likely OBD-I, not OBD-II) to read any diagnostic trouble codes (DTCs) stored in the computer. These codes can point directly to the problem.
3. Systematic approach: Address the components listed above one by one, checking for proper operation before moving to the next item. This is a more time-consuming but thorough approach.
Because you've already done a basic tune-up, the most likely culprits are the O2 sensor, MAF sensor, EGR valve, fuel injectors, or a vacuum leak. The fuel pressure regulator and catalytic converter are less likely but still possible. A professional mechanic with a diagnostic scanner can help identify the exact problem more quickly and efficiently.
