Q. I recently had to replace the engine in my Toyota Camry. It had 234,000 miles, and the newer engine has around 50,000 miles. I’ve notice when it’s really cold, the car does not warm up when driving at highway speed. When it’s idle or going slowly, the engine warms up and the heater works OK. What’s up?

A. Start with a check of the engine thermostat. If the engine was sitting in a salvage yard for a while, the thermostat may be opening at too low a temperature. This would be most noticeable at highway speeds.

Q. I love my 1998 Nissan Maxima and plan to drive it as long as it will go; it only has 123,000 miles. I’d like to understand aftermarket parts. Are there various qualities? Should I be asking for a specific manufacturer, given my plan to keep this car as long as possible?

A. The parts made for the vehicle by the manufacturer typically will fit better and, in some cases, may even last longer than some replacement parts. That said, many replacement parts are equal to and, in some cases, may surpass the original equipment parts. Look for name brands from reputable parts stores such as NAPA. As a general rule, if the replacement part is dramatically cheaper than the original equipment part, the quality may not be as good.

Q. I recently purchased a 2003 Pathfinder with only 27,000 miles on it. Lately, I hear a knock from the engine that is fairly apparent until the car warms up (10 minutes) and is worse when under a load cold. I took the car to Nissan and had someone drive and test it. They told me there are no trouble codes and the knocking noise is normal. I don’t recall hearing the noise when I first bought the car, although the weather was warmer then. Does my truck have a problem?

A. It is not unusual to hear a slight knocking noise when the engine is cold, but it shouldn’t last 10 minutes. I would return to the dealer and have them document the noise. In addition, I would check with another Nissan dealer to see if they have any experience with this noise.

Q. I have a 1999 Honda Accord and the “service engine” light came on. The repair shop told me it was the evap system but they couldn't find anything wrong. They said the problem wasn't on the fuel side. They reset the computer and all has been well for almost three weeks. This morning, the light came back on. What exactly is the evap system and should I be concerned about the light?

A. The evap system is the term used for the evaporative emission control system, which prevents gasoline fumes form escaping into the atmosphere and causing additional pollution. Since 1996 most cars have had an onboard test of this system to ensure it doesn’t leak. Although the car’s performance will not change when the “check engine” light is illuminated, your car is causing air pollution. The most common cause of an evaporative failure is a loose or faulty gas cap.

Q. I heard somewhere that if many people share a car with manual transmission, it would not be good for the car, and it would damage the transmission. Is this true?

A. This comes from the old adage that the more people who drive a car, the quicker it wears out. This may be true—but to a point. What I see is that when many people drive a car, no one takes responsibility for the preventive maintenance and minor repairs. Everyone thinks the other person took care of the oil changes, checking the tires and other important issues. There is no reason a car driven by several drivers should have a shorter life as long as someone takes responsibility for maintenance and repairs.

Q. I have a 1998 Honda Accord with 101,000 miles. We have a problem with our horn that I have been trying to diagnose for some time. The horn will work for a while, then will work sporadically and sometimes will not work at all. I have replaced the clock-spring and checked out the relay by switching it with another. I have checked all the wiring and grounds and everything seems ok. Could I have a defective clock spring? If not, I’m looking for any suggestions.

A. I have seen some Honda vehicles that lose the ground circuit due to a faulty steering column bearing. Check the bearing, and I think you will find your problem.