What is the best octane gasoline for my vehicle?

Q. Is it necessary to use premium 91 octane gas or can I safely use regular 87 octane in my 2017 Acura RDX? What about mid-grade 89 octane?

A. Acura recommends 91 octane fuel, and the key word is recommends, not requires. If the car runs normally, and you are happy with the performance and fuel economy with 87 octane gas, use it. Acura also recommends, and I agree, when possible, to use TopTier gasoline. TopTier fuel has more detergents than non-TopTier fuel, which helps keep combustion chambers clean. Over time the combustion chambers (essentially a firebox where a controlled explosion takes place) can accumulate carbon, which can cause engine misfiring.

Q. I listen to your radio program and lately you have reviewed several products, a DIY alignment tool, 12-volt Jump Pack, and a $22 electrical tester that was similar to a much more expensive PowerProbe. Are you obligated to give complimentary reviews?

A. The Gyraline, a DIY alignment tool, is pretty slick, but it won’t replace a $40,000 alignment machine. If you modify or repair your own vehicles, or have a track car, the tool seems handy to have and could save a set of tires. The jump-pack was by NEBO. It tests well and what I like is the design seems quite water resistant. This could be a handy emergency tool, in a boat, side by side, or large motorcycle. The PowerProbe clone was from TopDiag. I purchased this tool more out of curiosity (the low price was appealing) than need. It has been very handy with its built-in voltmeter, 15-foot leads, and the ability to apply power and ground to devices and circuits. Is it PowerProbe? No. Is it still handy? Yes. I have never felt obligated to give a good review, and never have or will accept any payments for reviewing products. 

Q. I have a 2020 Jeep Cherokee, with 35,000 miles and it has never been in a wreck. The check engine light came on so I took it to a local repair shop. They had it for seven hours and told me they could not perform a diagnostic check because their equipment was not up to date. I took it to a second shop where they came up with a code which stated, “lost communication with active grille shutter.” That shop ended up telling me they cannot fix it because my car is too new. What gives? 

A. The shop that read the code may not be confident enough in their diagnostic skills to follow through on the repair. The active grill is nothing more than a type of shutter system that opens and closes to allow more air flow or increase aerodynamics. Since your Jeep is five years old and certainly not too new, it may be time to find a new shop. 

Q. Not so much of a question as a comment: I recently bought a 2025 Subaru Forester. It is so superior in quiet, comfort, and ride to the 2015 Legacy I had until recently, which I never really liked. Unlike some cars, earlier Subarus had skimped on sound deadening, and it always felt tinny.  Not so with the new Forester. It is very, very nice. Not as silkily smooth as the 2017 Toyota Avalon I wrote to you about once, but worth recommending to people looking for a comfortable, quiet car that provides a secure road sense and great visibility. One question is that some people tell me to change the oil in the first 500 miles. Others say wait until the first scheduled oil change. 

A. Thanks for the tip as I have not yet evaluated the 2025 Subaru lineup yet, but Subarus, especially in New England, are always easy to recommend. I think changing oil in the first 500-1,000 miles is a little too early. I would do the first oil change at three months of 3,000 miles and then follow Subaru’s schedule of oil changes every 6,000 miles or six months, whichever comes first. 

Q. I own a 2002 Honda Civic LX with approximately 128,000 miles and an automatic transmission.  Over the past couple of months, I have noticed on the initial cold start of the day, there is a louder than normal one-time “thump/bang” type of noise. I did a cold start in neutral, but there was no difference. My Honda trained mechanic did a thorough inspection, but did not find anything obvious. This included “rocking” the engine through various gears. My concern is at this point I am just waiting for the problem to reveal itself and I will get stuck somewhere. I have friends who had the same vehicle, but they did not recall this issue. Their cars lasted approximately 160,000 miles. Any thoughts would be appreciated.

A. At this point, the best thing you can do is leave it with your mechanic overnight, and if necessary, several nights to replicate the noise. It may take 12 hours or more of cold weather to cause the noise. Some possibilities include motor and transmission mounts, the exhaust system, and the transmission itself. 

Q. I have not been used car shopping in a long time. When I was shopping for a used car from an individual, I would use the Want Advertiser or Auto Trader. These publications were at every convenience store but seem to be long gone. What resources does someone consult to find late model, private party used cars? What about buying tips? 

A. Craigslist, once the most popular, seemingly has been replaced by Facebook Marketplace. Iseecars.com does have private party listings as does AutoTrader.com. Like all private party purchases including used cars, be cautious where you are meeting someone. It is always a good idea to bring a friend. If possible, have the car inspected prior to buying it. With so many flood cars possibly coming to the market, having a VIN check such as CarFax is a good idea. Also, make sure there is a title and the person selling the car is the person who is listed on the title.

John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive business and is an ASE-certified master technician. E-mail your Car Doctor question to [email protected]. Listen to the Car Doctor podcast at johnfpaul.podbean.com.