A rough engine idle indicates a misfire

Junior Damato | Ask the Auto Doc

April 11, 2008

DEAR DOCTOR: I own a 2002 Ford Ranger pickup with a 4.0-liter V-6 engine. The truck has more than 100,000 miles on it. There is a No. 6 misfire, and the engine idles rough. I replaced all the tune-up stuff and checked for a vacuum leak and took a compression test. What am I missing? Ray

DEAR RAY: There is no question that something is missing, but at this point you've done everything you could. Now it's time to get the truck to a qualified technician who will recheck for any vacuum leaks and do a cylinder balance test. A rough idle indicates a definite misfire.

DEAR DOCTOR: The air bag warning lights are illuminated on both my BMW 3 Series and my Jeep. What does the air bag warning mean? Can I just ignore the light? Alice

DEAR ALICE: The air bag warning light illuminates because there is a problem in the air bag system. When the air bag light is on the system is shut down, meaning you have no air bag protection in the event of an impact. The problem could be anything from a seat position indicator to a faulty sensor. In some rare cases, a simple reset of the system clears the codes and the system is fine.

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DEAR DOCTOR: The factory Delco battery in my 2002 Buick Rendezvous died without any indication that the battery was weak. While the battery was being removed, the plastic base on the positive terminal broke and acid spilled out onto the plastic wheel well. Have you seen this problem on other batteries? Dan

DEAR DAN: Yes. Battery failure is common - such as in your situation, with side terminal metal bases falling apart. In some cases, the batteries of today do not work as hard as batteries of yesteryear. Today's alternators can keep even weak batteries charged up, and fuel-injected engines start right up, unlike the old carbureted engines. At my shop, we make it a practice to check batteries on vehicles three years and older.

DEAR DOCTOR: I own a 2005 Mercury Mountaineer V-8 with all-wheel drive and 31,000 miles on it. Over the last 5,000 miles, I noticed that when the SUV sits for a few hours or overnight, there is a delay of gear engagement of about five seconds when I put the transmission in reverse. What do you think is occurring? Adam

DEAR ADAM: The problem occurring within the transmission is a loss of internal transmission fluid pressure. It can be caused by anything from a leaking valve or solenoid to a pump problem. To determine exactly what is going on, a technician will hook up a pressure tester and a scan tool after the vehicle has been sitting for a few hours. He will then monitor the gauge and the scan tool when the engine is started.

DEAR DOCTOR: We own a 1999 Jaguar Vander Plas with only 40,000 miles. One morning the engine turned over but failed to start. The dealer checked it and said the fuel had washed down the cylinders, causing low compression. The dealer extracted the fuel, oiled down the cylinders, cleaned the spark plugs and changed the oil at a cost of $400. They advised not to start the car and make a short trip, but rather to run the car until it gets up to operating temperature. I never heard of such a thing, and I value your opinion. What are your thoughts? Mike

DEAR MIKE: The condition you describe is more common in cold weather winter conditions. Have the technician make sure the engine has no trouble fault codes and the thermostat is getting the engine up to normal operating temperature. An engine will run rich when cold and during warm-up. If there are too many short trips in cold weather, and not enough running highway time, you may get a flooded condition and even saturate the oil with gasoline and possibly damage the catalytic converter.

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, or e-mail info@motormatters.biz