Strong Gasoline Fumes May Warrant a Trip to the Dealership

Dear Car Talk:

Every time my wife fills up her 2008 Mercedes CLK 350, the garage reeks of gasoline fumes. We have had our local mechanic (not the Mercedes dealership) check it out and they found nothing.

We stop filling the tank before the nozzle clicks off, to make sure we're not overfilling it, but that doesn't help. The only thing that works is for her to fill up just prior to taking a longish trip, and then there are no fumes in the garage when we return home.

The fumes are very strong, and I am concerned about the possibility of fire. Thank you. -- John

A fire is exactly what you need, John!

Actually, you're right to be concerned. There are four things to check. One is the gas cap, in case it's no longer sealing completely. The second is the filler neck, which can corrode on older cars and cause leaks. The third is the evaporative emissions equipment. And the fourth is the gas tank itself.

If there's a leak in any of those places, though, it should turn on your Check Engine light. The fuel system monitors itself for leaks, and if it can't hold pressure due to a leak anywhere in the system, it'll turn that light on.

In any case, it's not always easy to find a leak like this, so you'll need a competent mechanic who is dedicated to tracking it down for you. You should obviously fill the tank just before dropping off the car. The mechanic will then put it on the lift and use his eyes (to look for a wet spot), his nose (to smell the fumes), and his hands (to feel for liquid gasoline) to try to figure out where gas is seeping out.

If he strikes out under the car, he should also try removing the rear seat and checking the top of the gas tank. That's often overlooked. And these cars can develop a crack on the top side of the gas tank, where it's bolted onto the frame. That's something the Mercedes dealer might know to look for, but your regular mechanic might not.

In the meantime, stop sneaking out to the garage at night to smoke your Cohibas, John. Use the tool shed instead until this is fixed.

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