Like pets, cars have many ways of letting us know when things are not 100 percent. Sometimes, figuring out the best response isn’t easy.

HOW BAD IS IT IF I...

By John Goepel

...drive when the "Check Engine" light is on?
This irksomely uninformative warning light comes on when the car’s computer sees something, somewhere, it doesn’t like—a malfunctioning sensor, the air/fuel ratio, engine temperature. "Check Engine" seldom means a dire emergency beneath the hood—such heavy news is more likely brought to you by the thermometer or oil light. When the "Check Engine" light comes on, take the car to a repairer with all deliberate speed.

...drive when the engine overheats?
Overheating can creep up on an engine or it can be almost instantaneous. There are many possible causes: a poorly maintained cooling system, a leak, mechanical failure (of the fan or water pump or thermostat), electrical failure, overloading the vehicle. The classic cause (and, sometimes, classic symptom) is the burst hose with its picturesque billows of steam.

Cars vary greatly in their resistance to overheating, but as a rule, the length of time between the temperature needle’s reaching the red zone and serious engine damage is fairly short. Hoses can burst, gaskets fail, and in the engines of those who really persist, there is likely to be a tendency for moving parts to weld themselves to one another.

The basic trouble with overheating is that the various metals in an engine expand at different rates when heat is applied. Tolerances among rapidly moving parts are pretty close, and when the engine is operated outside of the heat range it was designed for, these tolerances can be thrown fatally out of whack.

Often, the thermometer needle’s relatively slow rise gives ample warning. To delay, or possibly head off, trouble, turn off the air-conditioning and turn on the heater. You’ll be warmer, but the engine will be cooler. In any case, when your car overheats or makes a serious threat to, pull to the side of the road as soon as it’s safe to do so, open the hood, and let the engine cool off. There’s no need to remind you, of course, but people who try to take the cap off a hot radiator are likely to be sprayed by superheated coolant. So don’t do it.

...drive with the parking brake on?
An impressively high percentage of the population has driven at least a block or two with the parking brake engaged. Fortunately, few people are so oblivious to their car’s newfound sluggishness that they continue to press on.

When you engage the parking brake, you manually set the brakes on one axle, most often the rear. It’s usually the same pair of brakes that come into play when you step on the brake pedal. Driving a block or two with the parking brake on is likely to generate some heat and cause some wear but unlikely to do any real damage.

...top off the gas tank?
You’ve probably noticed the little signs on gas pumps warning that topping off—squeezing into the tank every last drop of gas that will fit—is forbidden. Topping off is unlikely to damage your car, but it can add to air pollution.

Topping off tends to allow more gasoline vapor to escape into the air, especially when those last few ounces of gas dribble down the side of your car. The pollution a little raw gasoline adds to the air when it evaporates is considerably worse than would come from that same volume of gas if it ran through the car’s engine.

Most cars have a vapor recovery system as part of their pollution control equipment. It uses a canister of activated charcoal to capture gasoline vapor that exists in the car’s fuel system. Eventually it sends the vapor to the engine. Topping off can make it difficult for the recovery system todo a thorough job.