Why is my truck’s gas mileage declining?

John Paul, aka “the Car Doctor,’’ answers readers’ car-related questions.
John Paul, aka “the Car Doctor,’’ answers readers’ car-related questions.

Q. I have a 2004 Nissan Xterra, it has a six-cylinder engine with a manual transmission and it has about 160,000 miles on it. The vehicle runs great with usual and as needed maintenance, the most significant item of the past year being to replace a leaking oil pan. The vehicle’s gas mileage has consistently been 13-15 miles per gallon in the city and 18-20 miles per gallon on the highway. About a month or two go the mileage started dropping noticeably, to 10-12 MPG in the city and 15-17 MPG on the highway. My mechanic replaced a couple of misfiring spark plugs that he identified as not in the best condition and checked the fuel line but couldn’t find any particular problem. The mileage continues to decline, now possibly to below 10 MPG in the city. Any ideas on what might be the problem?

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A. I would want you repair shop to pay particular attention to sensor readings. The oxygen sensor or coolant temperature sensors may be just on the outer edge of the normal limit, but not bad enough to trigger a check engine light. The other issue could be one of the brakes is starting to hang up. When this happens it can put just enough drag on the vehicle that mileage starts to drop off.

Q. I’m looking for a full sized truck and the more I looked the more I get confused. At one time it was easy it was Ford or Chevy. Now it is Ford, Chevy, Ram, Toyota and Nissan. I don’t need a heavy duty truck but I do tow a 4000 pound boat in the summer and snowmobiles in the winter. Although realistically the truck will mostly be used just to drive back and forth to work. Can you recommend one truck of the other?

A. At one time I would rank the trucks as Chevrolet/GMC as number one followed by Ford, Ram, Toyota and Nissan. They are all good trucks, but for your type of use the Ford may be the winner. The EcoBoost turbocharged engines provide decent mileage when empty and plenty of power when you need it. The Chevrolet has a slight edge in ride and handling but some people are bothered by the cylinder deactivation system, when the engine drops to four-cylinder mode to save fuel. The Ram with the Eco-Diesel is quite nice and certainly worth a look. As good as Toyota products are the Tundra is showing its age. The Nissan Titan is all new for 2016 and I have yet to fully evaluate it, but early reports are quite good.

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Q. I have a 2001 Subaru Forester that I love; it was purchased used at 32,000 miles and now just turned 100,000 miles. The body is a bit beat up because it’s lived on the streets in the city for most of its life. It has always been serviced by the dealer because of a near-death experience (the car not mine) due to an independent shops repair failure. I haven’t had to put much money into my car over the years — brakes, clutch, exhaust, basic stuff, and I have never (and don’t now) feel ripped off. The dealer now is recommending rear struts. From what I understand, it will make the ride smoother but is not truly a safety issue. I’m not particularly interested in a smoother ride, but I am interested in safety. My other consideration is whether it makes sense to put $1,000 into a 2001 car, even if it only has 100,000 miles. I would appreciate any thoughts!

A. As struts/shocks start to wear it can affect the ride, handling and tire wear. If you have ever seen a car hit a bump on the highway and the tire bounce like a basketball this is typical of a completely worn out strut. The average labor repair time to replace rear struts is about 3.5 hours. The costs of the struts can vary from $175 each for factory/Subaru units and cheaper prices for brand name aftermarket replacements. Although you didn’t have a great experience at an independent shop you may find there is some saving using aftermarket parts and taking advantage of the sometimes lower labor rates at a smaller shop. If this were my 15 year old car I would put off the repair unless the struts are badly worn. One note of caution is Subaru engines of this vintage can suffer from expensive engine head gasket and catalytic convertor failures that can cost several thousands of dollars to repair. The typical value of a 2001 Forester is $4200 putting $3000 (potentially) into repair may not be money well spent.

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