QSome years ago I traveled to England and saw many small, diesel-powered cars. I have read that most of the new cars sold in Europe are diesel. Considering that diesels get significantly better fuel mileage than gasoline-powered cars, why haven't the car manufacturers offered the diesel option over here?

AThey have. VW offers their TDI diesel in several models, as do Audi, Mercedes and BMW. Other carmakers have offered small, diesel-powered cars in the past, but higher diesel costs and stricter diesel emissions have discouraged come carmakers. And unfortunately, Americans haven't fully embraced the diesel concept, remembering the smoke, smell, durability problems and lack of driveability with earlier diesels.

With modern direct-injected diesels, you can tell there's a diesel engine under the hood only by a lower maximum rpm on the tachometer, a slight clatter on cold start-up, no spark plugs and remarkable fuel mileage.

Today's direct-injection diesels offer fuel economy, durability and drivability well-suited to American driving habits. I hope that someday American motorists will agree.

QI recently replaced the battery in my '93 Toyota myself. I now suspect that the computer settings were affected when the battery was disconnected during replacement. The car is now idling a little rough in gear or in neutral when stopped at a light. If the computer is involved, is this a costly fix?

ADisconnecting the battery should not cause problems with the vehicle's computer systems. It will cause a loss of "learned" operational parameters such as idle mixture, climate-control settings, and memory settings for the radio, seats and mirrors, but it will relearn all but the memory settings in a few driving cycles. If the rough idle continues, make sure no electrical or vacuum connections were disturbed during the battery swap.

QI bought a 5.4-liter Ford F-150 new in '04. Three years ago, it began to produce a shrill whistle when held steady at about 1,900 rpm. I can reproduce the noise in both second and first gears at that rpm. It occurs only in the winter, and it's louder and more persistent the colder it gets outside. I can vary the pitch with slight movement on the gas pedal and make it disappear at higher or lower rpm. The Ford dealer didn't know the cause. I suggested a cracked vacuum hose, but they said no. It's loud enough to be annoying.

AMost likely, cold temperatures are causing an induction air leak -- air being pulled through the air intake, air cleaner, mass airflow sensor, throttle body and intake manifold. Since you can generate the noise, why not have someone hold the engine at 1,900 rpm in park while you use an automotive stethoscope to pinpoint the source of the sound? You could even spray aerosol brake cleaner at specific areas of the induction system in the hopes of hitting the air leak and momentarily changing the pitch of the whistle.

Likely causes are wear on the throttle shaft in the throttle body, throttle body base gasket -- or, as you suspect, a vacuum leak. A shop can smoke-test the induction system with a nontoxic smoke to help pinpoint any vacuum leaks.