Dear Car Talk:

We have a 2000 Lexus ES300 with 242,000 miles on it. About eight months ago, we noticed a gas smell at idle when sitting at stoplights. We took it in to our mechanic of 10 years, and, of course, there was no smell. But being a good mechanic, he checked the car thoroughly for us and found nothing. It happens intermittently, and each time we take it in -- of course -- it doesn't smell for our mechanic, nor can he find anything wrong related to the fuel system.

Initially, we noticed it only when stopped and at idle, and then we found that when we turned off the AC system, the smell would go away. Now it doesn't matter if the AC is on or off, and when it does smell, the smell is worse. Nothing is seen on the ground or on the engine, per our mechanic, who checks it for us every time we bring it in for servicing. Otherwise, the car still runs like a Swiss watch, and I was hoping to get another 100,000 miles from it. HELP!

-- Dan

You may remember that some years ago, when you went in for your state emissions test, the mechanic would have you run the car, and he'd stick a wand in the car's tailpipe. That emissions wand was designed to detect minute quantities of unburned hydrocarbons in your exhaust. You were allowed only a certain number of parts per million, or you'd fail the emissions test.

Well, guess what the best-known unburned hydrocarbon is? Gasoline! So nowadays, we use that old emissions wand to find small gas leaks. That's what I suggest your mechanic try.

It's basically a mechanical sniffer, and it works great. So he can open the hood, fire up the wand and poke around the engine with it. If there's any gasoline leaking at all, the meter will go nuts.

I'd look in places where rubber meets metal. The fuel pump sends the fuel from the tank to the fuel rail. Coming out of the fuel rail, six rubber hoses carry fuel to each injector. So for each one of the injectors, there's a place where the metal of the fuel rail connects to rubber, and then rubber connects to an injector. Those are likely places for leaks.

It could be a bad clamp. It could be one or more bad injectors. Or it could be that the rubber is just getting porous after a quarter of a million miles of use.

There are other rubber fuel lines, too, from the fuel pump to the rail, to the fuel filter, and the fuel return line, and it could be any of them. But if there's an active leak, the wand won't miss it.

You wouldn't necessarily see anything on the ground or on the engine. Gasoline has an extremely strong smell, so it only takes a small amount for you to really notice it.

What's happening is that when the car is moving, it dissipates. But when you're stopped, it gets sucked into the fresh-air vent in front of your windshield, and then blown into the passenger compartment through the vents.

To increase your mechanic's chances of success, leave the car with your mechanic overnight one night, and let him test it in the morning with the wand. If it's so intermittent that the the wand doesn't find it, then have him replace every one of the car's rubber fuel lines. That's a cheap way to start before you begin replacing injectors.

And if you're planning to attempt another 100,000 miles with this car, you'd better get used to idea of replacing stuff. Good luck, Dan.