Dear Car Talk:

I have a very puzzling and scary problem with my 2004 Chevrolet Trailblazer.

About six months ago, it started to have a gas smell in the cabin when the engine was started. It would dissipate in about 10-15 minutes. It didn't happen often, but I mentioned it to my mechanic who said he had never heard of that.

About two months ago, it started happening again and now it happens more frequently. It happens whether the gas tank is full or not, and the fumes are coming from the air conditioner vent.

Sometimes it smells strongly outside as well as inside, as if someone is pouring gas inside the vehicle. I'm actually scared the vehicle could catch fire!

Of course, when I take it back to the mechanic, the smell has dissipated and even leaving it with them for days reveals nothing.

Can you help? — Donna

Well, it's a 2004 Trailblazer, Donna, so the first thing I'd consider is lighting a match. But if you'd like a less messy solution, you need to find a mechanic with an old-fashioned emissions testing wand.

Back before cars tested themselves, and we could just plug into their computer to get the results, we used to test a car's emissions by sticking a wand up its tailpipe. The wand would detect unburned hydrocarbons (i.e. gasoline) in the exhaust, which would tell us whether the emissions system was operating properly or not.

Whenever we have a customer with a gas leak, we still use that wand to sniff it out. It's very sensitive, can detect gasoline in concentrations of parts per million, and pinpoint exactly where a gas leak is coming from.

The reason it seems like a huge leak is because it doesn't take much gasoline to create a big smell. So, if it's just a drop or two of gas that's leaking onto the engine, evaporating right away, and wafting in through the ventilation system, it's unlikely that your mechanic can find it with his naked eye. Or his naked nose, especially after that nose has been bombarded with engine fumes and car-dog odor for decades.

Find a shop with an emissions wand, leave the car overnight, and have them use the wand to sniff around your fuel injectors and fuel rail. You probably have a leaky injector or a leaky seal there.

While not impossible, it's not likely to catch fire if the leak is that small. But I'd wear sneakers when you drive just in case, and get it fixed soon, Donna.

Got a question about cars? Write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.

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