Expert Advice

Making the Grade
Q: My 2003 Toyota 4Runner has the 4.0-liter six-cylinder engine. The owner's manual shows regular unleaded fuel is required, but recommends premium for top performance and fuel economy. Specs for the same engine in the 2005 Tacoma show premium as the correct fuel. Should I be using premium in my 4Runner?

A: According to applicable Toyota service manuals, the 2003 4Runner and 2005 Tacoma, sharing the common 4.0-liter engine, should run on a minimum of 87 octane fuel, 91 octane or higher for added performance. If your primary goals are economic, use the minimum 87 octane. Any additional mpg from a higher-octane fuel isn't going to compensate for the difference in price. If you notice a spark knock under acceleration, change gas stations and/or up the octane until the potentially damaging noise is eliminated. If there's no change, take the 4Runner in for service. Drivers that focus on performance and utmost driveability should spend the extra cash and burn the 91 octane. If you can't make up your mind, split the difference with a midgrade gasoline. Consider it consumer conscientious of Toyota to stipulate choices in fuel.

A/C Compressor Cycling
Q: Why does my 1995 GMC Sierra's A/C compressor switch on and off when I turn it off?

A: General Motors uses a wide variety of air-conditioning control assemblies that operate in slightly different ways. There are several circumstances that may affect your Sierra's air-conditioning compressor operation. One may be the system automatically energizing the compressor clutch when the controls are in Defrost, Defrost/Heater, or A/C Max mode, even with the A/C switch off. Some systems are designed this way because these particular functions are ineffective with the air-conditioning compressor off. The refrigerant needs to be flowing to draw moisture from the air or to provide maximum cooling capacity. The A/C compressor should normally cycle on and off when in use. This prevents a freezing condition at the system's evaporator core. Causes for an A/C clutch to engage when it's not supposed to include a defective A/C control assembly or PCM, a sticking A/C compressor clutch relay, or possible shorted wiring in a number of related circuits. There are also cases of defective compressor clutches not fully disengaging, a symptom of which is a scraping noise.