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Q I have a 2001 Ford Taurus. In the lower-left corner of the dash, a light that looks like a small faucet has been on for some time, but the car seems to run fine.

A The warning light in the lower left corner of your dash is the "malfunction indicator lamp," known in generic terms as the check-engine light. If on continuously, it means that the PCM (powertrain control module) has recognized and recorded a fault in the engine management system. A bad oxygen sensor can trigger the warning light, but so can dozens of other problems.

You should have the vehicle diagnosed with a scan tool at a dealer or independent shop. Continuing to drive the vehicle with the light on is not necessarily harmful, but the light may indicate that the engine is running in the "limp" mode for safety, which means it's burning more fuel and possibly creating more emissions.

Q I have '96 T100 Toyota with the 3.4-liter V6 engine. I have always changed the timing belt at about 60,000-mile intervals. Newer engines now recommend changing at higher mileage intervals. Is timing belt material better, could I change my belt at higher mileage intervals? By the way, I have 315,000 miles on the truck, and it still runs great.

A According to Alldata, "On 1997 and prior models, under normal service conditions manufacturer does not recommend a specific maintenance interval. On 1997 and prior models, for vehicles operated under conditions of excessive idling and/or low-speed driving for long distances, replace at 60,000 mile intervals. On 1998 and later models, replace every 90,000 miles or 72 months."

I'm not aware of material differences in timing-belt construction, but the above indicates that timing-belt replacement is recommended only under certain low speed/idling operation. Visually inspect your belt. If there's no evidence of wear, damage, oil or deterioration, you might re-evaluate whether you change the timing belt at all.

Q I have a 1988 Ford Bronco II. It has many miles on it, but a new engine and recently a new automatic transmission were installed. Recently it started to lose power, then run like crazy while driving. The check-engine light does not come on every time this "pulsing" occurs, but the gas mileage is terrible now.

A Focus on the poor fuel mileage. Typical causes are internal fuel leaks from bad injectors or a ruptured fuel-pressure regulator. A bad coolant sensor that's confusing the PCM into thinking coolant temperature is lower than it actually is can cause this symptom, as can a clogged or restricted exhaust or catalytic converter.

I would suggest starting with a scan tool to identify what fault code has been triggering the check-engine light. If this doesn't identify the culprit, you could try unplugging the coolant temperature sensor and test drive the vehicle.

Yes, this will trigger the check-engine light. But if the engine runs better, the sensor probably is bad. Check the vacuum line to the fuel-pressure regulator after shutting off the engine; if there's liquid fuel in the line, the pressure regulator is ruptured and allowing raw fuel directly into the induction system. A fuel-pressure leakdown test would confirm this and may identify any leaky injectors as well.

And check the "SPOUT" connector to make sure it wasn't left unplugged after some of the major work. If the "spark output" circuit is left open -- the plug is not in the end of the connector -- the PCM will operate in base timing mode with no ignition timing advance, giving poor performance and economy.