Q. I’m retired and my 2012 Toyota Camry isn’t driven very much. I’ve been having problems with my air conditioner blowing warm air. The dealer couldn’t find a problem. I normally keep the gas tank about a quarter full. I filled the tank and that seemed to solve the AC issue. Could the performance of the air conditioner 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 who checked your car simply checked the AC compressor operation and pressures – and not air duct temperature – they would have assumed the system was working normally.  

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 also with a 2.0 turbo charged engine that makes 240 horsepower. I find it to be surprisingly quick, and it uses regular gas. The engine cover says Lincoln, but I believe in other applications it’s an Ecoboost. How do they make more horsepower and do it on regular fuel? 

A. More air equals more horsepower. 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. If you add in air scavenging (how quick air enters and leaves the engine), engine timing, and fuel delivery, then compression, horsepower and torque can change dramatically. It is this combination of engine timing, sensors, engine design, and turbocharger design that can allow for higher horsepower while still using 87 octane fuel. 

Q. I have a 2016 Mazda CX-5 with 78,000 miles. Recently, the car suddenly 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 that worn plugs caused the coil to fail. I replaced them as well as the failed coil. Should I be worried that this could 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 needed to be replaced at 75,000 miles. And it is possible that worn plugs can cause ignition coil failure. If the ignition coil failed due to a worn plug, it may 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.

Q. I have a 2009 Volkswagen CC Sport with 65,000 miles. It’s in nice condition with many new parts, is well maintained, and drives well. The check engine light is on, and after being reset by a mechanic, lights again. This issue is causing problems getting my vehicle inspected. The dealer did make some repairs to an oil separator. In addition, the dealer says there is a main seal leak that could cost up to $2,000 to repair. We are seniors and can’t afford that. I need the check engine light off to get the car inspected. Can you help us?

A. The oil leak has nothing to do with the check engine light. At this point I would check the oil every week so you can determine when you need to add oil. The check engine light is on because one of the sensors is reading outside of its limits. The oil separator is a clue. There may be a leak in the system causing a lean running condition. If one of the seals is leaking it will overwhelm the PCV system, cause the car to run lean, and cause the check engine light to come on. Resetting the check engine light without doing additional repairs will guarantee the light will come back on.

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 question to [email protected]. Listen to the Car Doctor podcast at johnfpaul.podbean.com.

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