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Motormouth: Don't top off your gas tank

Q: My wife owns a 1999 Subaru Forrester. Recently, we smelled gas after her gas purchase, and, after some conversation, she decided it was best to stop when the nozzle automatically shuts off. No more topping off. The problem is that after her last gasoline purchase, when she drove away, she saw that her tank was only 3/4 full. Do you have any recommendation for being certain her tank gets a true fill-up while not suffering any bad results from topping off?

— M.S., St. Charles, Ill.

A: Squeezing that last drop of gas into the tank is a bad idea. It is a throwback to when most people paid with cash and tried to avoid getting weird change. Check your owner’s manual for the tank capacity. With that you can extrapolate the amount pumped versus the gauge reading after the fill up. The gauge may be off.

Q: In Bob Weber's response to a question about spark plug replacement, he mentioned the problem only occurring on the Ford Navistar engine. I am a Navistar employee and all engines sold to Ford were diesel engines which do not have spark plugs. This response implied that our engine had an issue that could not be true. In a future column, I would appreciate a correction made.

— Deborah Shust, Navistar

A: Right you are. I mistakenly wrote Navistar engine when I meant to say Triton engine. The spark plug removal issue was common with the three-valve Triton V-8 engine in the late 1990s and early 2010s. Good catch.

Q: I enjoy your articles including the one today having to do with poverty inspiring learning about repairing cars. That certainly was the case for me. A couple of weeks ago you discussed a problem having to do with battery voltage and commented that 12.2 volts would indicate a fully charged battery. But a battery at 12.2 volts is at about 50%, while a fully charged battery will show 12.6 volts.

— D.K., Chicago

A: Right again. Automotive batteries have six cells (connected in series) having 6.1 volts per cell when fully charged. By the way, at 11 volts the battery is considered too dead to start the car.

Q: Your answer to the question about the freezing of pure water and pure ethylene glycol is flawed by one word. Your statement said “when mixed they react forming different molecules. This is not what happens. What does happen is that when mixed they interact causing a freezing point decrease and a boiling point increase. Principally this is caused by the individual molecules of water and ethylene glycol being separated from one another. The result is that they cannot easily form the necessary molecular interaction to allow crystal formation. It would surprise me greatly if the explanation from Prestone did not use the term interact rather than react. I suspect that you will hear a similar explanation from chemists around the country. Thank you for your interesting column. As a former “car nut” in high school, I still enjoy stories about cars and especially about engines.

— Paul Holm (chemist and professor, retired)

A: Thanks, Paul. When I read your message, I remembered that mixtures can be separated physically, but compounds chemically. Coolant is a mixture of antifreeze and water. I will be glad to see 2020 fade from my rearview mirror.

Bob Weber is a writer and mechanic who became an ASE-certified Master Automobile Technician in 1976. He maintains this status by seeking certification every five years. Weber's work appears in professional trade magazines and other consumer publications. His writing also appears in automotive trade publications, Consumer Guide and Consumers Digest.


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