Q We are having problems with our '95 Saturn SL2. When you start the car it runs for about 25 minutes, then shuts off. We can't afford to bring it to a dealer or mechanic, and we can't afford to buy another car, so we need to fix it ourselves. We have replaced the fuel pump, fuel filter and fuel pressure regulator along with the coil packs and ignition module. We cleaned the exhaust gas recirculation (EGR) valve and replaced the powertrain control module. Still, it shuts off after about 25 minutes. We then replaced the catalytic converter, but this still didn't fix the problem. The car starts and runs fine, but after about 25 minutes it shuts itself down. When we try to start it again and it stumbles and shuts off.

A At this point, I can't help but wonder if a professional shop and its sophisticated testing equipment might have been a good choice.

That said, the "stumbles and shuts off" description of your restart attempts may be a clue. The "stumble" means its got spark and some fuel. Installing a fuel pressure gauge and test driving the car until it stalls would tell you if a lack of fuel pressure is the problem. If so, try a couple of simple fixes. Carry a gallon of water in the car. and when it stalls for lack of fuel pressure, pour the water slowly over the fuel rail, pressure regulator and intake manifold to cool them. If the engine restarts and runs, it could be vapor lock -- fuel boiling in the fuel rail causing a loss of fuel pressure. And even though you've replaced the catalytic converter, make sure there is no exhaust restriction that could lead to an overheated manifold and cylinder head.

Two other possible causes come to mind. A problem with the evaporative emissions system and/or fuel tank vent system or solenoid might not be allowing air into the tank as the fuel level drops, ultimately creating enough vacuum in the tank after 25 minutes of driving to overcome the fuel pump. Try opening the fuel filler cap after it stalls to see if excess vacuum is released, then try restarting the engine. Even though you've changed the pump and filter, accumulated sediment in the tank itself might be slowly blocking the sock filter on the fuel pickup, starving the pump for fuel.

And finally, plugging in a scan tool to check for fault codes might identify the precise cause. Some auto parts stores will do this at no cost.

Q I took my 2010 Buick in for a smell of rotten eggs. They said to change gas stations and use better gas. I think it's some thing 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 temperature 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 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.