My 2002 Isuzu Axiom was recently seen by a local mechanic for a safety check and maintenance consultation. I drove the car home the same evening; the next day the battery was dead (had to buy a new one) and the fuel mileage went from 18 mpg around town to 13 mpg the following week. Would water placed in the fuel tank be the possible culprit?

Tips to Improve Gas Mileage:

1. Check for Diagnostic Trouble Codes.
2. Replace oxygen sensor. A bad oxygen sensor can radically affect gas mileage, since it controls fuel injector duration.
3. Go over the whole ignition system. Replace, for example, spark plugs (with exact brand and plug number that came in the vehicle), spark plug wires, rotor, and distributor cap. Replace platinum plugs at half the recommended mileage.
4. Replace air filter and fuel filter.
5. Check idle rpm and engine timing.
6. Check MAF sensor, if so equipped.
7. Check tire pressure.
8. Add Shell Regane or Chevron Techron fuel injector cleaner to gas tank every 6 months. This will clean fuel injectors, throttle body, valves, and combustion chamber.
9. Use full synthetic lubricants; e.g. engine oil, manual synchromesh transmission fluid, automatic transmission fluid, and differential fluid. This should increase gas mileage by 8 to 10%.
10. Check for dragging brakes, including emergency brake.
11. Check alignment.