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AUTO DOC: Gas filter change doesn't affect mileage

Junior Damato | Ask the Auto Doc

March 21, 2008

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DEAR DOCTOR: I am the original owner of a four-cylinder 1999 Subaru Legacy Outback with 55,000 miles. I change the oil every three months and take very good care of the car. When they changed the gasoline filter, I started getting poor gas mileage. The dealership cleaned everything that should make me get better gas mileage. They said it wasn't that bad and I should see a slight improvement. Now I am getting horrible mileage. What can I do to get better mileage? I use the 89 gasoline - the regular octane is much worse. Is it just the age of the car or can I do something to improve the mileage?

Anne

DEAR ANNE: The changing of a gas filter will not affect the gas mileage. I suggest hooking up a scan tool and taking a look at all of the computer information for sensor values. A lazy upstream oxygen sensor, leaking fuel pressure regulator, engine not getting up to proper temperature, dragging brakes, low tire pressure and a dirty air filter all contribute to poor mileage. You should also have the technician pull the spark plugs and check their color. If the plug is black, then the engine is running rich, and the tech will know what to do.

DEAR DOCTOR: I saw a magazine advertisement about huge increases in fuel mileage. The ad claims that one can turn any vehicle into a hybrid with a $1,000 fuel-cell modification kit. A 2007 Honda Civic stock got 35 mpg, but with the modification kit it achieved 85 mpg. What is your opinion about this claim?

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Angelo

DEAR ANGELO: Frankly, I put no stock in this kind of claim.

DEAR DOCTOR: I drive a 1999 VW Passat. When it rains I get a puddle in the car, and soon after I get many electrical problems. I've been told the water comes in through the battery compartment and/or the sunroof. I feel that drilling a hole in the base of the battery compartment would allow the water to leave more freely, and closing up the sunroof with some silicone would end that problem. What do you think?

Donna

DEAR DONNA: We see a lot of vehicles with water leaking into the interior. We check for leaks by running a water hose over the roof and body panels and/or use a smoke machine in the interior to locate the leak. You can also use an electric leaf blower to pressurize the interior and look for the source of the leak. I do not recommend drilling holes to let water escape. The source of the leak must be found.

DEAR DOCTOR: The speedometer in my 1995 Honda Accord with 120,000 miles has started acting strangely in the morning, when the car is cold. The speedometer needle does not move; later, after, say, a half mile, it starts working. What's wrong with it, and can I fix it myself?

Keys

DEAR KEYS: A look on our Identifix Web site flags the most common problem with lazy speedometers as faulty speedometer heads. Look to see if the D4 (Drive) light will flash as well.

DEAR DOCTOR: I have been having problems with a 1995 Cadillac Eldorado. In the morning the car starts up fine, but later in the day it takes a while to start. The car stalls at 500-1,000 rpm, but on the highway it drives fine. Could you explain what is causing this problem?

Edwin

DEAR EDWIN: The first step is to check trouble fault codes, followed with a check of fuel pressure and volume. I would also check the fuel filter. The coolant sensor could also be out of range, causing a lean condition and possibly causing carbon buildup in the throttle body.

DEAR DOCTOR: I own a 1991 Dodge Dynasty with 120,000 miles. When I start the vehicle in cold weather, the transmission will not immediately shift. It goes into a limp mode where it stays in low gear and does not go through the other gears. When I stop and turn the car off and on again, everything is fine. What is the problem?

Ken

DEAR KEN: The first thing to do is check the trouble codes stored in the computer. I have seen everything from a low battery to poor ground connections to an overcharging alternator causing similar problems.

Junior Damato is an accredited master auto technician, radio host and writer for Motor Matters who runs a 10-bay garage. Send questions to Junior Damato, c/o Special Sections, Newsday, 235 Pinelawn Rd., Melville, NY 11747-4250.