Q. I own a 2016 Subaru Outback 3.6R with 76,000 miles. For the past few months, I’ve occasionally experienced an unpredictable thump while stopped at a red light or in traffic. The sensation feels as though another vehicle lightly bumped me from behind. A Subaru dealer was unable to duplicate the problem and said it was unrelated to the rear wheel bearings they replaced. Because my vehicle was covered by Subaru’s extended CVT warranty, I opened a case with Subaru in case the issue later proves to be transmission-related. More recently, a check engine light led my local mechanic to diagnose a leaking fuel injector and a misfire. He replaced all six spark plugs and changed the oil, but the thumping has continued. I’ve read that the air conditioning system could also cause a similar symptom, but since this is my son’s car and the A/C runs almost constantly during the summer, it has been difficult to test that theory. What are the most likely causes of this intermittent thumping sensation while stopped, and where should I focus my troubleshooting efforts? Would a transmission fluid change help?
A. Based on the symptoms, the CVT transmission remains a possibility, although there are other potential causes. It is possible the variator, the internal variable-drive mechanism, is occasionally hanging up. I doubt a fluid change will solve the problem, but it may be worth trying. Although CVT transmissions have improved over the years, regular fluid changes are extremely important. Changing the fluid every 30,000 to 60,000 miles can help prevent belt slippage, overheating, and premature mechanical failure. Other possible causes include engine or transmission mounts, air-conditioning compressor engagement, or idle-speed fluctuations. Generally, when the bumping sensation becomes regular and predictable, the transmission is often replaced as a complete unit.
Q. Like you, I worked for AAA, although in another state, for 39 years. I’ve now been retired for 10 years, which seems to have happened in the blink of an eye. My question is about maintenance on my 2018 Chevrolet Malibu. It has only 28,000 miles on it, and I’m religious about oil changes. The car is a cream puff. The Chevrolet dealer recently suggested that, based on age, I should consider a transmission fluid service, brake fluid flush, and a few other services. I would really appreciate your thoughts.
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A. If this were my low-mileage, eight-year-old car, I would have the following services performed: a transmission fluid service rather than a flush. A brake fluid exchange every three to five years is also a good idea to help prevent issues with the ABS brake system. Even though GM states the coolant is good for 150,000 miles, I would consider changing it within the next year or so. Other than that, follow normal maintenance procedures, including replacing the cabin air filter and engine air filter as needed. When the car reaches 10 years old, give the belts and hoses a careful inspection. Years ago, fan belts would only last a few years; today, it is common to see them last 10 years or more. If the car still has its original tires, it is time to replace them. If the battery has never been replaced, I would do that as well, since it is now on borrowed time. Also check the brakes. The brake pads may look fine, but over time the caliper slides can become rusty and begin to bind. This model has also been somewhat prone to undercarriage rust, so keep an eye on that. At the same time, inspect for oil leaks. The 1.5L and 2.0L engines have developed leaks as they age, so they are worth monitoring. Continue to enjoy your retirement.
Q. I have read your column every week for many years and really enjoy it. I bought a new 2025 Acura RDX SH-AWD Advance and have owned it since November. It currently has only 2,400 mostly local miles on it. The EPA fuel economy estimates are 21 mpg city, 27 mpg highway, and 23 mpg combined, but I’m only getting 16 mpg using Shell 93-octane gasoline. Could you please explain what I might be doing wrong?
A. Most likely, nothing. With such low mileage, about 300 miles per month, and most of those miles consisting of short local trips, fuel economy will typically fall short of EPA estimates. Also, if you use the remote-start feature, remember that idling gets zero miles per gallon. I suspect that if you took a 500-mile road trip, your fuel economy would improve significantly. I have a small SUV, and although I drive it several times a week, most trips are less than five miles. As a result, fuel economy is in the teens. The same vehicle, when used for my former 50-mile commute each way, consistently returned 30 to 32 mpg. Acura recommends premium fuel for best performance, but using 93-octane gasoline instead of 91-octane fuel is unlikely to improve fuel economy. It simply costs more.
Q. I purchased a new GMC Sierra 1500 in 2017 with a 5.3-liter engine. It now has 52,283 miles on it. I’ve been hearing a lot about the AFM (Active Fuel Management) system, which switches the engine from eight cylinders to four cylinders to save fuel. Owners have reported problems with lifters and camshafts that can lead to engine failure. Some people have purchased AFM-disabling devices that keep the engine operating on all eight cylinders all the time. I’m wondering what causes these failures and whether General Motors is doing anything to address the problem and help its customers.
A. The issues typically begin at 70,000 miles or more, often with a ticking noise caused by a sticking valve lifter. Poor maintenance may contribute to the problem, but AFM failures have also occurred on well-maintained engines. GM has not officially admitted to a widespread defect, although there have been several class-action lawsuits related to AFM-equipped engines. Interestingly, GM appears to have moved away from AFM on some newer models. If this were my truck, I would consider using a bypass device. Readers have told me that the product from Range Technology is among the better options. I have seen other devices online for as little as $40, but the Range Technology product appears to be a more thoroughly engineered solution.
Q. What is the purpose of the low-fuel warning light? I say it is a warning that I will be running out of gas soon. My partner says it is simply a reminder to fill up.
A. Typically, when the low-fuel light comes on, you have enough fuel remaining to travel about 30 to 50 miles before running out, although the exact distance varies by vehicle. More important, the fuel pump is cooled and lubricated by the gasoline in the tank. Regularly driving with the low-fuel warning light illuminated can shorten the life of the fuel pump. So not a reminder to fill up, more of a warning you are going to run out of gas.
Do you have a car question? Email the Car Doctor for a personal reply. jpaul@aaanortheast.com