* Oxygen Sensor(s): A failing oxygen sensor (O2 sensor) prevents the engine's computer from accurately measuring the air/fuel mixture. This leads to a rich (too much fuel) or lean (too little fuel) condition, drastically impacting fuel economy. A faulty O2 sensor often doesn't trigger a check engine light, but it's a prime suspect for a sudden, significant drop in MPG. You likely have multiple O2 sensors (one or more before and after the catalytic converter).
* Mass Airflow Sensor (MAF Sensor): This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor provides inaccurate readings, leading to improper fuel delivery and reduced MPG. Cleaning it (with MAF sensor cleaner, following instructions carefully) is often a first step, but replacement might be necessary.
* Dirty Air Filter: While less likely to cause such a dramatic drop, a severely clogged air filter restricts airflow, forcing the engine to work harder and consume more fuel. Replacing it is cheap and easy.
* Fuel Pressure Regulator: This component regulates the fuel pressure to the injectors. A malfunctioning regulator could lead to consistently over-fueling the engine.
* Spark Plugs and Wires: Worn-out spark plugs and ignition wires can cause misfires, leading to incomplete combustion and reduced fuel efficiency. These should be inspected and possibly replaced at 40,000 miles, especially if they haven't been changed before.
* Tire Pressure: Significantly under-inflated tires increase rolling resistance, reducing gas mileage. Check and inflate your tires to the recommended pressure (found on a sticker inside the driver's side doorjamb or in your owner's manual).
* Transmission Issues (Less Likely): While less probable given the sudden nature of the problem, transmission problems can impact fuel economy. However, you'd likely notice other symptoms like slipping or harsh shifting.
* Driving Habits: While a change in driving habits *could* contribute to some decrease, it's unlikely to cause such a massive drop. Consider if your driving style has changed significantly (more stop-and-go, heavier acceleration, etc.).
What to Do:
1. Check the obvious first: Inspect your air filter, tire pressure, and look for any obvious damage (leaks, etc.).
2. Scan for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to check for any error codes stored in the engine's computer. This is the most important step. Auto parts stores often offer free code scanning.
3. Focus on the O2 Sensor and MAF Sensor: Based on the symptoms, these are the most likely culprits. Replacing them is often relatively inexpensive, and many auto parts stores offer lifetime warranties on these components.
4. Professional Diagnosis: If you can't identify the problem with the above steps, take your Tundra to a qualified mechanic for a proper diagnosis.
Don't delay addressing this issue. Continuing to drive with poor fuel economy can result in further engine damage.
