Dear Tom and Ray:

I've been driving for more years than I care to say, but this is the first I've heard of something called a "smoke test." Recently I noticed a gas odor coming from somewhere around the back end of my car. I couldn't pinpoint it to any specific location, e.g., around the gas cap, but the odor was unmistakable. No sign of any leaks on the garage floor, and I didn't smell anything inside the car, including when driving. I thought perhaps gas somehow had been splashed on the car, but after leaving it out in the rain to wash it off (it was too cold to get out the hose), the smell persisted. I took it to a chain repair place for a look, and while they also could smell the gas, they couldn't see anything, and said they'd have to start disassembling things to find it. They suggested that a better option would be to have a "smoke test" done, but I'd have to take it to a dealer, as they didn't have that equipment. So, what the heck is a "smoke test," and is that the best option? -- Chris

TOM: We used a primitive smoke test back in the old days, Chris.

RAY: My brother would just blow smoke from his El Productos into the fuel filler opening, and we'd see where it came out. Then one day he dropped a lit ash in there, and that was the end of his beard AND the customer's Vega.

TOM: Yeah. But a smoke test is the right approach for you, Chris. We have a modern smoke machine at our garage now. It plays Deep Purple when you use it.

RAY: Actually, you put baby oil in it, and the oil heats up and makes lots of smoke.

TOM: Then you feed the smoke into the evaporative emissions system (lots of cars even have a test port for it), and it pressurizes the whole system, including the gas tank, with smoke. If you see smoke coming out somewhere, you know that's where the hole is.

RAY: Lots of cars have filler neck tubes that corrode over time and leak gas vapors. But there are lots of valves inside the evaporative emissions system that leak, too. The smoke machine will help you figure out exactly where it's coming from.