Q. I have is what has always been a very dependable 2002 GMC Envoy. In this truck there is a dial to switch from two-wheel drive to four-wheel drive as well as high and low range. Lately the gears are changing without me touching the controls. The indicator light switches and there is a grinding noise, is my transmission wearing out?

A. The first thing that needs to be determined is if your GMC has any transmission fault codes. A technician will scan the trucks computer looking for trouble codes. These codes will assist the technician in solving the problem with your truck. There are several technical service bulletins that describe similar shifting problems. Technical service bulletin number 02-04-21-002A seems most appropriate and it involves replacing the selector switch.

Q. Can you help me with information about fuel grades? Are there any real differences between grades or are they all basically the same. Also is there any difference in brands of gasoline?

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A. Octane is a term for gasoline’s ability to resist engine knock. Using low octane fuel in an engine that requires premium fuel will result in a bit poorer performance, fuel economy and the possibility of engine damage. Using higher octane fuel in a car that doesn’t require it is just a waste of money. Regarding the question of brand name versus no-name gasoline? The EPA requires all gas must have a minimum amount of additives to help keep your car’s engine clean. Although some brand name, more expensive gasoline will custom blend additional additives, in my opinion if your car runs fine on cheaper price gas, us it.

Q. I will be doing a drain and refill on my 2003 Toyota RAV-4 transmission. When I am refilling the transmission, should the engine be running or turned off?

A. The transmission in your Toyota RAV-4 has an estimated refill of 3.7 quarts of fluid. When refilling, add three quarts with the engine off, start the engine, and shift from park to reverse to drive. Recheck the fluid level and add to the full mark, being careful not to overfill the transmission.

Q. I’m looking for a small fun-to-drive used car and would prefer a convertible. What do you think of the Mini Cooper convertible? Also is a Saturn Sky worth looking at?

A. The Mini convertible is fun to drive but its overall quality has been just average. This could be a problem with so few Mini dealers to service the car. The Saturn Sky and its twin the Pontiac Solstice are fun cars but with the demise of Saturn and Pontiac some parts could be hard to find. The Mazda Miata continues to be one of the best small, fun to drive convertibles and continues to be my first choice in this category.

Q. I changed my oil recently and had leftover oil. I mixed this with another bottle of oil (different brand, but same weight). Is this OK to use in my car?

A. Mixing small quantities of different brands of oil – as long as they are the same grade and weight – shouldn’t cause any harm, if only used to “top off” the oil level. Oil manufacturers suggest that when performing a complete oil change, you should always use the same brand of oil. Oil contains additives and occasionally those additives may not be compatible with one another. Save the leftover oil for your lawnmower

Q. I have 1998 Honda Civic LX sedan and it runs great. I just had a 200,000-mile scheduled maintenance done and replaced the battery. This morning when I started car it didn’t idle well. I started driving and the car seemed to want to stall and didn’t shift as smoothly as normal. I drove for a few more miles and the car seemed OK. Did this happen because the engine was cold or is the car just getting old?

Ron

A. Everything you experienced could have been a result of the battery being replaced. When batteries are replaced in many cars, the computer needs to go through a relearning procedure. With some cars this may take just minutes; others may take a few miles of varied driving conditions. If the car continues to give you trouble, return to the repair shop that did the service and let them have the car overnight for evaluation.