Q: In a recent column, you responded to a letter regarding a 1994 Mercury Sable that sometimes emits a strong smell of gasoline. I have, over the years, had numerous customers come to me with that same complaint. On a few cases, we found a problem with the vapor canister, but usually the problem was actually with the motorist; they were consistently over-filling their gas tanks. They were in the habit of trying to squeeze too much gas into the tank when they filled up, leaving no room for normal expansion. Unless they immediately drove long enough to consume the extra gas, the car would emit fumes. -- B.M., Lockport

A: B.M., thanks for pointing this out. You’re right. Overfilling the tank could be the cause of those fuel vapors. A small dome at the top of the fuel tank normally is reserved for air and allows for the expansion of the fuel in the tank. If this domed area is filled with fuel, liquid gasoline can enter the evaporative emissions system when the fuel in the tank expands. Components in the evaporative emissions system, like the charcoal canister, can be damaged from liquid gasoline. While it’s okay to round your fuel purchase to the nearest even amount after the pump handle clicks, it’s not a good idea to continue pumping until gasoline runs out of the filler neck.

Q: I have a 2004 Yukon XL with a 5.3-liter engine. It has developed a coolant leak that takes about a quart every two weeks. There is no visible leak and the oil doesn’t show any water bubbles on the dip stick, or any increase on the dip stick. It has 83,000 miles on the odometer. Is it possible that it has a minute head gasket leak and the coolant is being burned in the combustion chamber? Wouldn’t the oxygen sensor be affected and turn on the check engine light as a fault? I would appreciate any input that you can give me, as to what could be the problem. I changed the pressure cap thinking it was coming out of the over flow, but it had no affect on the loss of coolant. If the coolant was going into the combustion chamber wouldn’t it cause the over flow to bubble? -- T.A., e-mail

A: T.A., normally, a small head gasket leak would cause coolant to be burned in the combustion chamber. This wouldn’t necessarily cause the coolant in the overflow to bubble. This would likely foul up the oxygen sensor. However, the best way to eliminate the head gasket as a possible source is to remove all the spark plugs, keeping them in order. There should be a tan or light brown appearance on the spark plug electrodes. Any that appear to be super clean likely indicate the presence of coolant.

Also, with the spark plugs removed, the cooling system should be pressurized to see if any coolant enters the cylinders. It might be difficult to see, even with a small mirror and flashlight. I prefer using a borescope for this. While checking for service bulletins, I did find #06-06-01-019B. This bulletin suggests that some cylinder heads might develop cracks that would allow coolant to leak into the crankcase. But I don’t know if this applies since you mentioned that the oil level doesn’t appear to be rising. At any rate, I would have the cylinder heads closely inspected.

AutoAdviser: Ira Siegel is an Automotive Instructor and ASE Certified Master Auto Technician. If you have auto-related questions for Ira Siegel you can call and leave your questions at the Auto Adviser Hotline at 708-633-6839 or e-mail a question to Autoadvisr@aol.com.