strong>Dear Doctor: I am switching to a high-mileage synthetic blend 5-30 oil in my 1999 Ford F-150 with about 145,000 miles. When I need to add a quart of oil between changes can I safely use regular 5-30 oil? I have a few quarts of regular that I hate to waste if they can be used in a limited amount with the synthetic blend. Paul
strong>Dear Paul: The rule of thought about mixing oil is old school. Adding a different brand, viscosity, type regular, synthetic or synthetic blend will not cause any harm on vehicles with high mileage (70,000 miles and higher). On vehicles still under factory warranty, you do have to stay with the factory recommendation for oil viscosity and type. High-mileage oil does have a different makeup with an additive package for engines with wear. I do not recommend the use of high-mileage oil on engines with less than 70,000 miles, unless the engine has internal problems, such as leaks and burns oil.
strong>Dear Doctor: I own a 2005 Cadillac STS, 3.6L V6 with 75,000 miles. I have been the owner for 3 years and purchased it certified used with 14,000 miles. Although the 22-23 mpg average I am getting is acceptable, I would like your take on performance chip products. Are there any pitfalls? Also, the dealer has replaced the timing chains (62,000) in this engine. I have always felt that the shifting in this drivetrain has never been consistently smooth since I bought it. The dealer says that all updates are present and that the car is operating correctly. Donald
strong>Dear Donald: Most aftermarket performance chips bring out the best driving experience for the driver. The performance chip or computer download can reprogram automatic transmission shift points and firmness, ignition timing and lower coolant fan temperature operation. I have downloads in all my vehicles, including my trucks. Other options to consider with the computer performance upgrade would be a low temperature thermostat, performance low restriction exhaust and cold air intake system. The only drawback is premium gas is required in most cases.
strong>Dear Doctor: I own a 2004 Chevy Tahoe Z71 with the 8100 Vortec engine. It's been well maintained and has 55,000 miles on it. Recently I brought it to the dealership after it began making a loud rapping sound coming from the top part of the engine. The dealership diagnosed the noise as coming from the lifters. To repair it would be quite expensive. Do you think the motor could seize on me if I ignore the problem? Are there cheaper alternatives to a major engine repair?
strong>Dear Brendan: At 55,000 miles with the large 8.1-liter V-8 engine, lifter failure is rare. The engine should not fail or seize. I would suggest a second opinion. You can also upgrade to full-synthetic oil that reduces engine moving part friction. To replace the noisy lifter is not a major expense if needed. I would recommend the repair if you plan to keep the vehicle.
strong>Dear Doctor: I own a 2000 Ford Ranger pickup with the 4.0-liter 6-cylinder that had an engine replacement. The replacement engine had 75,000 miles. Since the engine was installed it has been using a quart of oil every 900 miles. There are no leaks or smoke emitting from the tail pipe. My mechanic checked the compression and vacuum system. He said there was one cylinder where the pressure was lower, but it shouldn't be a problem. I just replaced the spark plugs and had 1 spark plug that was black with buildup. Do you have any suggestions? Chuck
strong>Dear Chuck: First, the compression that was low in the cylinder should be checked to determine if it is a valve or piston ring problem. This is done by squirting a small amount of oil in the cylinder. If the compression goes up, then the problem is piston ring sealing. If there is no compression rise, then the problem is a poor sealing valve. The spark plug that was black with buildup is the cylinder with the problem. You can either try going up in the heat range on that 1 cylinder or using an anti-fouler on the spark plug. This is an alternative to major engine repair. Also have the mechanic check the PCV system operation to make sure it is operating.
strong>Dear Doctor: What are your thoughts on the 2011 Audi S4? Everyone I talked to says the S4 is a great car. Steve
strong>Dear Steve: Audi used to have a turbocharger in some high performance models. Our S4 test car had an intercooler supercharger sitting on top of the 3.0-liter 6-cylinder direct injection engine producing 333 silky smooth horsepower. It was coupled to a seven-speed dual clutch automatic transmission with lightning fast shifts and the Quattro (AWD) system keeps you glued to the road under any condition. You can choose from three driving modes. When set in the advanced mode the transmission shifts later and firmer. My surprise was the gas mileage (18 mpg city / 28 mpg highway) from this midsize high-performance vehicle. If I had one complaint it would be for Audi to bring back the simple controls for creature comfort. Base price is $48,000.
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