Q: My 2014 Jeep Grand Cherokee suddenly developed a clunk downshifting at a stop sign. Then, at the next stop sign, even my wife heard it. Took it to my local garage here, who I have great trust in, as they have saved me a lot of money. They used a machine to analyze it and they indeed fixed the clunk. In looking at the invoice, there was labor charge: reprogram PCM/TCM $175. Not in an upset mode, I simply inquired as to what is a “reprogram PCM/TCM $175.” He kinda stammered as if he was unable, yet not unwilling, to answer. The implication I got was that he was not the sure what the machine does, but it simply fixes the clunk, and it did! Can you give me a better idea of what happened?
J.R., Glenview, Illinois
A: Have you ever downloaded an update for one of your phone’s apps? You are quite able to use it without knowing how the app works. In the case of the Jeep, your mechanic downloaded an update for both your powertrain control module (PCM) and your transmission control module (TCM). He knew what to do without needing to know what the update modified.
Q: I have a high-performance 2007 Shelby GT Mustang that has a supercharger, and it requires premium octane (91-94) gasoline, which you know is very expensive. Could I purchase regular octane (87) or midgrade octane (88-90) gas and add a bottle of STP or Lucas octane booster fluid and get the same or similar octane result, rather than paying the high prices at the retail gas pump?
S.M. Rolling Meadows, Illinois
A: A bottle of octane booster in a 20-gallon tank may increase the octane rating a few points. Do the math and you may be surprised.
Q: When electric vehicles become more commonplace than cars powered by gasoline, do you think that the cost of electricity will fluctuate the way that gasoline prices do? Would it affect the home electricity bill beyond the vehicles' share of consumption?
B.K., Riverwoods, Illinois
A: Gas prices fluctuate because gasoline is a commodity. Electricity prices are controlled by state utilities agencies. To increase prices, power providers must usually petition the agency for permission. Fortunately, electricity fluctuations are limited to only occasional brownouts.
Q: I have a 2003 Chevy Trailblazer with 138,000 miles. My wife drove it and now it’s mine. I have always set the headlights so they always are on, and I have burned out two or three modules over the years as well as a few headlights. Should I just put the lights on when I drive at night or in the rain?
C.W. Coral Springs, Florida
A: Roughly one-third of the states require headlights be turned on whenever the wipers are turned on. (Florida is not one of those states.) Nevertheless, it’s a good habit and increases both your safety and the safety of other drivers. Otherwise, you may choose to leave the headlights off. Your daytime running lights are adequate during mild weather.
Q: I know I should wax my car twice a year, but I don’t. That being said, and leaving my car outside of the garage all year, would it be better to wax prior to several months of hot sun during the summer, or snow, ice and freezing temperatures in our Northeastern winter?
J.W., West Hartford, Connecticut
A: Wax your car in the spring and fall. Not only does this prepare it for those rough seasons, the agreeable weather is more favorable for doing the job — preferably with a cold beer nearby.
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