Auto Doctor: Expect gas mileage to drop in the cold

A 2005 Lincoln Town Car equipped with the

A 2005 Lincoln Town Car equipped with the 4.6-liter engine may be getting bad gas mileage because of a faulty engine thermostat that's preventing the engine from getting to its operating temperature. (Credit: Ford Motor Company)

Dear Doctor: I drive a 2005 Lincoln Town Car equipped with the 4.6-liter engine. It has only 58,800 miles and I'm getting terrible gas mileage of 11 miles per gallon when I used to get nearly 18 mpg. I had a mechanic do the following: change the spark plugs, air filter, PCV valve, had the fuel injectors cleaned, and had the front 2 oxygen sensors in front catalytic converter replaced. I use synthetic blend oil every three months and I even add a Chevron gas additive, but nothing helps. What's next? -- Frank

Dear Frank: I recommend that you have the technician make sure the engine thermostat is allowing the engine to get up to operating temperature. You can also have the technician observe the front upstream oxygen sensors to make sure they are operating as designed. One factor related to reduced gas mileage (particularly in the Northeast), is the gas formula is changed for the winter months to make for easier starts in the cold weather. This formula does lessen gas mileage by 1 mile per gallon.

Dear Doctor: I have a 2005 Ford Explorer Eddie Bauer with the 4.0-liter V-6. I recently installed new BF Goodrich Rugged Terrain T/A tires (stock size: 245 65R17). I also installed a Magnaflow cat back exhaust, a K&N air filter, E3 spark plugs and new plug wires, and new gas filter. I change my oil every 3,000 miles with Valvoline full-synthetic 5w30 oil. After all of this work my average fuel mileage went from bad to worse. I went from 15.8 mpg to 12.2 mpg. What should I do? -- Mike

Dear Mike: A mistake many SUV owners make is putting on aggressive tires, such as any all-terrain A/T style tire. The rolling resistance is higher than that of a highway H/T design tire. The larger, wider, and heavy the tire, the more resistance the tire has. And this will reduce gas mileage. The addition of the free flowing exhaust and fresh air intake should improve gas mileage, not lessen it. I would check the odometer in a measured mile for being accurate. Next, fill the tank up, drive 150 miles and refill it at the same pump and divide the miles driven into the gallons used for an accurate mileage reading.

Dear Doctor: I own a 2005 Saturn Ion with 50,000 miles. The battery and starter work fine, but occasionally the car won't start in the morning. When I turn the key there's no sound -- no clicking or engine turning over. What is the problem? -- Richard

Dear Richard: I see a lot of these problems. The best way to check the car is to have your technician connect a 194 test light and socket to the S terminal at the starter motor. He can run the wire up to the windshield and tape it to the bottom of the windshield. When the engine does not start, if the light illuminates when the key is in the start position, then the problem is very likely a faulty starter motor. If there is no light when the key is turned to the start position, the technician will work his way back to the ignition switch at the steering column and check for voltage with the use of the same test light.

Dear Doctor: I have a 2007 Ford Mustang GT with a five-speed manual and only 35,000 miles on it. It is very difficult to shift into second gear when cold. Do you think a fluid change would help? -- Pete

Dear Pete: It's not unusual for some manual transmissions to have some resistance shifting into gear when cold only. Unlike the old days when we used 90 weight gear oil in manual transmissions, today's manual transmissions use either automatic transmission fluid or full-synthetic manual transmission fluid. In some cases, yes, a fluid change can help. You can also try shifting into third gear and then back into second gear -- or just skip second gear until the transmission fluid warms up. You may also want to make sure the clutch operation is working as designed and try bleeding the clutch.

Dear Doctor: I have a five-speed automatic transmission on my 2004 Nissan Maxima that has about 131,000 miles. I've had my transmission replaced three times. I've also had the computer for the transmission changed three times and other things for the transmission replaced. The problem I keep having is the transmission randomly shifts itself out of gear and back into neutral. What would make an automatic tranny kick itself in and out of drive? -- Kenny

Dear Kenny: Your car, like most late model vehicle transmissions, is controlled electronically. The first step is to check for any stored trouble fault codes in memory or pending. A basic check of all power and ground connections needs to be done. The next step is a factory scan tool connected for a road test performance. If there are no codes and the scan tool information is normal, then I'm sorry to say it would be next to impossible to determine the cause of the problem. There is no ongoing pattern of this complaint in any of my database reference sites. -- Junior Damato, Motor Matters

Junior Damato is an ASE-certified Master Technician. Email questions to info@motormatters.biz. Mail questions to: Auto Doctor, 3 Court Circle, Lakeville, MA 02347

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