* Transmission Issues: This is the most probable cause. A failing transmission, low transmission fluid, a clogged transmission filter, or a slipping torque converter can all cause the engine to bog down under load. Check your transmission fluid level and condition. If it's low, dark, or smells burnt, you likely have a transmission problem.
* Exhaust System Restriction: A severely clogged catalytic converter or muffler can restrict exhaust flow, causing the engine to struggle under load. Listen for unusual noises from the exhaust system.
* Torque Converter Clutch: If the torque converter clutch isn't engaging properly, the engine will rev but the car won't accelerate. This is a more advanced issue often requiring professional diagnosis.
* Differential Problems: Though less likely, problems within the differential (rear-end) can also cause issues with acceleration under load.
* Wiring/Electrical: While less probable given your description, a faulty sensor (like a mass airflow sensor or throttle position sensor) or wiring issue *could* cause a problem only under load. A professional scan tool would help here.
* Ignition System: Though less likely since it runs in neutral and park, a weak spark, particularly under load, could be a factor. But this would usually manifest as misfiring rather than just bogging down.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the absolute first thing to do. Check the level and condition of your automatic transmission fluid. If it's low, add the correct type (check your owner's manual). If it's dark, burnt, or smells bad, you need a transmission service or repair.
2. Listen to the Exhaust: Check for unusual noises (restrictions) in the exhaust system. A clogged catalytic converter will often restrict exhaust flow, especially under load.
3. Professional Diagnosis: Given you've already replaced the fuel pump and cleaned the tank, a professional mechanic with a scan tool is your best bet. They can diagnose electrical issues, check for transmission codes, and assess other potential problems far more efficiently than you can at home. The scan tool will be able to read codes that may point directly to the problem.
Don't continue driving the car extensively if it's bogging down significantly, as this could cause further damage, especially to the transmission. Get it checked by a mechanic sooner rather than later.
