Dear Tom and Ray:

For the past four to six weeks, when I start my Chevy Trailblazer (2004) first thing in the morning, the smell of gas and sometimes oil comes out of the air-conditioning vents. Once the car has run for five minutes or so, the smell dissipates and does not come back for the rest of the day. We typically keep the car in the garage overnight.

My husband does not think this is a big deal, since I have taken it to the mechanic twice and they didn't find a problem. The first time I took it in, they replaced the thermostat in the coolant system. The second time, they conducted a fuel-pressure test and an evaporative smoke test. The results did not show any problems. I am still driving it with the gas smell in the morning. I drive my two young girls (5 and 2) around daily. Is my husband right that I should not worry? Or should I be concerned, take it to a different mechanic and ask them to do ... what? Thank you.

-- Shannon

RAY: I'd be concerned but not alarmed, Shannon. Let's assume that what you're smelling is gas, rather than oil. What do you need to be concerned about when you smell gas?

TOM: Fire usually is at the top of my list. And to the great thrill of every hungry liability lawyer in the country, I'm going to suggest that the risk of a gasoline fire due to your particular problem, Shannon, is relatively low (not nonexistent, but low). Why? Because it's been examined carefully by mechanics twice, and they've found no leak.

RAY: It takes very little gasoline to make a lot of gasoline smell. And if your gasoline smell is dissipating after five minutes and not coming back for the rest of the day, it's probably being caused by a very small amount of seepage.

TOM: That said, breathing gasoline fumes is not good for the old brain cells -- especially the brain cells of little kids. So, for that reason, I would ignore your husband, and push to get this fixed.

RAY: When you park the car at night, the fuel system is still under tremendous pressure. My guess is that as the engine cools down, some small fitting or hose shrinks a little and allows a little bit of gasoline to seep out.

TOM: And the next day, when you turn on the car, the smell of that gasoline wafts right into the cowl at the bottom of the windshield, and comes into the passenger compartment through your vents. Then, as the engine heats back up, the gasoline evaporates and the leak seals itself up, and everything is fine for the rest of the day.

RAY: The best way to find this kind of small leak is with an old-fashioned emissions wand. Before everything was centralized in the car's data port, we used to stick a wand in the tailpipe to test a car's emissions system. That wand was designed to pick up microscopic amounts of unburned gasoline -- which were supposed to be combusted or catalyzed before they got to the tailpipe.

TOM: So if you can find a mechanic who still has an emissions wand, he can probe around the engine compartment and use it to detect a very small amount of gasoline anywhere.

RAY: What you want to do is leave the car with that mechanic overnight. Try to simulate the exact conditions under which it misbehaves. So drive the car over one evening, park it and leave the key. Then let the mechanic use that very sensitive wand to pinpoint the leak in the morning, before the engine heats up and the leak disappears.

TOM: And in the meantime, keep the 2- and 5-year-old from smoking cigars in the car first thing in the morning, just to be on the safe side. Good luck, Shannon.