Q: I took my 2010 Buick in because of a rotten-egg smell. They said to change gas stations and use better gas. I think it's something else.

A: I think you're right. First, remember that every motor vehicle sold in the U.S. is covered by a federally mandated eight-year/80,000-mile emissions warranty that specifically covers the engine management computer and catalytic converter.

The sulfur or rotten-egg smell is created by excess heat in the catalytic converter. Too much unburned fuel reaching the converter can cause it to generate excess heat as it catalyzes the fuel. A brief whiff of this odor on a cold start isn't abnormal, but once the engine is up to temperature you shouldn't experience this.

Low engine operating temperature, an overly rich fuel-air mix, a restricted exhaust flow or a failed catalytic converter are possible causes. A scan tool might pinpoint the cause.

Q: I understand that newer cars and newer oils spell reconsideration of the 3,000-mile oil change rule, but what about older cars? Can I let my '96 Buick Park Avenue go 6,000 miles between oil changes?

A: You can, but I wouldn't. Assuming expected mileage for a 15-year-old or older vehicle, intervals of 3,500 to 4,000 miles would ensure that soluble contaminants - fuel, moisture and combustion byproducts - don't reach dangerous levels.

Paul Brand, author of "How to Repair Your Car," is an automotive troubleshooter, driving instructor and former race-car driver. Email your auto questions to him at paulbrand@startribune.com Please explain the problem in detail and include a daytime phone number. It isn't always possible to send a personal reply.