See more of the story

Q: I My wife and I recently purchased a 2007 Honda Fit with 103,000 miles on it. Unfortunately, the car gets crappy gas mileage: 22 miles per gallon in mostly city driving, 30 on the highway in warmer weather. The car appears to be in good overall condition, starts easily and runs smoothly. I've replaced the air filter and the spark plugs; neither had any effect. The tires are inflated to specs. The coolant temperature was 172 degrees when the car was warmed up. We have twice had it checked out by our mechanic and he finds no codes on the scanner. Its EPA fuel economy is estimated at 35/27 with an average of 30. What could be going on with this car?

A: The low coolant temperature when fully warmed up may be the cause. A reading of 172 degrees F. is below normal operating temperature and may well prevent the engine management system from operating in "closed loop," utilizing input from the oxygen sensors to optimize the fuel/air ratio and ignition timing for maximum fuel mileage and efficiency.

When you first start the cold engine it operates in "open loop" with no O2 sensor input — basically the warm-up mode. With coolant temperature below the thermostat threshold, the system may be staying in open loop. This makes the thermostat and coolant sensor prime suspects. On your Fit the thermostat should begin opening at 176 degrees and be fully open at 203 degrees. It's possible the thermostat is stuck open and failing to regulate the coolant operating temperature.

Although far less likely, it might be a case of, for want of a better descriptor, "overcooling." If you live in an area of severe winter, the coolant may never reach the thermostat threshold, causing the engine management system to remain in open loop. Are either of the radiator cooling fans operating when the coolant temperature is low? They shouldn't be.

Q: Are you aware of any aftermarket kits that would provide daytime running lights for vehicles not originally fitted with them? I've had two recent experiences where I did not see oncoming cars in poor light because they did not have DRLs. I have no idea what costs would be involved but it wouldn't seem like an expensive "fix."

A: It's not. For vehicles not originally equipped with DRLs, owners have two options. First, a complete DRL kit, including lights, can be added to most vehicles. Costs range from $30 to over $100 and the lights are relatively simple to install. These are wired so that the DRLs illuminate when the ignition is switched on. Most of these kits include LED lights that draw relatively little current.

The other option is to add a DRL control module to the vehicle's existing lighting system. These typically operate the low-beam headlights at full brightness anytime the ignition is switch on. These modules are quite inexpensive — some are under $30 — and can be a DIY installation.

Q: I have a 2002 GMC pickup with 6.0-liter engine. The truck has roughly 211,000 miles on it, has been maintained and is still in pretty good shape. This fall the "Check engine" light started flashing when the speed hit 70 to 75 mph. If I slowed down it would stop flashing around 65 mph. There is no noticeable miss or drop in power. Two shops could not pick up any fault codes. The problem happens like clockwork at higher speeds.

A: A flashing "Check engine" light usually means big trouble — a significant misfire, large vacuum leak or a catalytic converter/oxygen sensor/restricted exhaust issue. In most cases driveability will be noticeably affected. Since it's not, have a shop or dealer connect a data recorder to the truck's diagnostic link to record the event as it occurs to determine what's going on.