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Ask the AAA Car Doctor: Turbochargers, spark plugs, and more

John Paul

John Paul


John Paul is the AAA Car Doctor and president of the New England Motor Press Association. He’s here to answer your questions, so drop him a line to find out more about your car.

Q. My son purchased a used 2020 Kia Soul Model S with 13,600 miles on it from a local Kia dealer in May of this year. After a couple of days, the engine light on the dashboard came on. He brought the car back to the dealership, and they attached a diagnostic meter to it.  They said there was a problem with the cooling system. They serviced it and said the problem was corrected.  A few days later, the engine light went on again.  We brought it back, and they replaced a faulty thermostat.  It stayed off until he started for a trip down south, and the light came on again. We looked under the hood, no leaks, and the car wasn’t running hot. A neighbor attached a diagnostic meter, and a code came up for the cooling system. He reset the light, which stayed off until two days ago. What problem causes the engine light to go on, and why can’t Kia correct it?

A. Certainly, Kia can fix the problem. At this point, I would go back to any Kia dealer when the engine light is on and have them check for codes. As the second owner, the car is covered by a 60,000-mile 5-year warranty. The limited basic warranty offers bumper-to-bumper coverage for most vehicle components not covered by any powertrain warranty.

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Q. I have what I think is a simple question. I got a flat tire, called AAA, came to help, and put my spare tire on. I think I need to get my tire replaced. Do I go to the car dealer, or can I go to a garage and see if they can replace and repair the tire? Or is it mandatory to go to the dealership for it since it’s the original tire?

A. The dealership or a tire store should be able to match the tire exactly with the original tires on the car. You may need to replace two tires depending on the car and mileage. At this point, I would go to whatever is most convenient and quickest to inspect the flat tire. You may find that once the tire is carefully examined, it can be repaired as good as new.

Q. I have a 2017 Infiniti Q60 with 45,000 miles on it. The Airbag warning light came for the occupant seat. I took my car to a local Nissan dealer because the nearest Infiniti dealer was an hour away. I was told three sensors might have to be replaced. Due to complexity, replacing the first or second one might not fix the problem. I had the first sensor replaced for $1200, but the airbag came back on. The second sensor would cost nearly $8000 because it includes the entire seat. And there is no guarantee that replacing the seat would fix the problem. Do you know of any safety recall or workaround that would fix this issue other than driving this car off a cliff?

A. There are currently no recalls or even any technical service bulletins that I can find in my technical database. I would suggest you contact your Infiniti dealer or Infiniti customer service. The idea that all they are doing is guessing at your expense is just wrong. There are specific diagnostic procedures for airbags systems. In addition, the passenger airbag sensor in the seat is a replaceable part (at $800 plus additional labor). There is also an airbag module and associated wiring.

Q. I have a 2012 Toyota Camry. I am retired, so the car does not get driven very much. The gas tank was about 1/4 full. I took the car in for regular service, and at first, the air conditioner was blowing warm. When I arrived at the dealer-about 10 minutes later-the, A/C felt cool. The service department said it was okay. A few weeks later, with the gas gauge still about 1/4, the air conditioner was blowing hot air for a good 20 minutes. Now something changed. I added fuel to have a full tank, engine off, of course. As soon as I started the engine, with a full tank of gas, the A/C was blowing beautiful cold air. So here is my question: Could the air conditioner’s performance be directly related to the volume of gas in the tank?

A. The two should have nothing to do with each other. I suspect a sticking air blend door that, for whatever reason, was sticking and then closed and allowed cold air into the cabin. If the dealer simply checked the A/C compressor operation and pressures, not the air duct temperature, they would have assumed the system was working properly.

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Q. Back in 2006, I had a Jetta GLI with a surprisingly smooth 2.0 turbo-charged engine that made 200 horsepower, although it did require premium fuel. A few months ago, I bought a low-mileage Lincoln MKZ AWD with a 2.0 turbocharged engine that produces 240HP, which I find surprisingly quick – and it uses regular gas. The engine cover says Lincoln, but I believe it’s an Ecoboost in other applications. How do they make more horsepower and do it on regular fuel?

A. More air equals more horsepower, so if the turbocharger runs at higher boost pressure, it will make more horsepower. Typical VW boost pressure is 7-11 PSI, and the Ford Ecoboost pressure runs up to 17 pounds of boost. Add in air scavenging (how quickly air enters and leaves the engine), engine timing and fuel delivery, compression, and horsepower and torque can change dramatically. This combination of engine timing, sensors, engine design, and turbocharger design can allow for higher horsepower while still using 87 octane fuel.

Q. I have a 2016 Mazda CX-5 with 78,000 miles. Suddenly, the car started shaking, sputtering, and had no power at all. Apparently, the spark plug coil failed. I was told the spark plugs need to be replaced at 75,000 miles, and worn plugs caused the coil to fail. I replaced them as well as the failed coil. Should I be worried this could/ would happen again with the other three original coils? The car is almost impossible to drive when this happens.

A. The dealer is correct. The spark plugs need to be replaced at 75,000 miles. And it is possible that worn plugs can cause ignition coil failure. If the ignition coil fails due to a worn plug, it might be a unique situation. Still, If it were me and even with my frugal (okay cheap) nature and I planned on keeping the vehicle, I would replace the three coils to prevent future problems.

Got a car question? Email the AAA Car Doctor for a personal response.

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