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Motormouth: If high octane gas doesn’t fix spark knock, what will?

Columnist Bob Weber answers all your car questions.

Q: I have a 2015 Chevy Colorado 3.6-liter V-6 with 45,000 miles that is serviced regularly and uses strictly Top Tier regular gas. When accelerating through second and third gears I regularly experience "plug knock," which is very annoying. Recommendations from service professionals of adding octane booster or switching to premium gas have provided no resolution. No system warning lights appear to indicate any operating malfunctions. I could use your expertise and knowledge to help resolve this mystery.

– C.B., Glenview, Ill.

A: What you refer to as plug knock is more accurately called spark knock or ping. It can be due to higher compression than the fuel is designed for. Choosing a higher-octane gasoline often helps. Since you have tried that, we must consider other causes. Carbon buildup on the pistons or cylinder heads is often the cause. Ignition timing that is excessively advanced used to be an issue, but it is now controlled by the engine control module and can't be adjusted. Piston rod knock may cause the noise, but usually not during a given gear change. However, you can disable the spark plugs one by one and if the noise goes away, it is rod knock.

Q: I enjoy your column and tips but have to correct your mention that the air we breathe is "nearly 70 percent nitrogen"; actually it is about 78 percent nitrogen, 20.9 percent oxygen, 1 percent argon and the rest carbon dioxide, neon, methane, other inert gases. Again, thanks for the column.

– J.G., Cedar Rapids, Iowa

A: Would you believe that it was simply a typo? I tried to type 80 percent, but I have always had trouble with the number keys. To be even more precise, air contains 78.09 percent nitrogen, 20.95 percent oxygen and traces of other gases like argon and carbon dioxide. Thanks for keeping us honest.

Q: I've got a 2004 Ford Expedition, and recently when I turned the windshield wipers on they flickered then just stopped. I replaced the switch on the column, but still, nothing. Here's where it gets weird: If I push the washer fluid button in while simultaneously pushing the rear windshield wiper button for a second at a time, the wipers will inch forward with each push of the rear wiper. What's wrong?

– M.D., Spencer, Mass.

A: Did somebody put the rear wipers on the front? (Sorry, I couldn't resist.) It sounds like an electrical problem and could be a bad ground somewhere. Activating the rear wipers may allow the electrical current to find a weak ground through the rear circuit. This will be a fun challenge for a professional tech.

Q: I have a 2015 Dodge Challenger RT with a 5.7-liter Hemi and a six-speed manual transmission. It has not been started in 11 months. I want to know if there is any way to crank the engine to get the oil circulating to the bearings without it starting. Back in the old days you could just pull the coil wire and crank but not sure about now.

– L.M., Mundelein, Ill.

A: Chances are, your engine is fine. When it was shut down, all of the passages and galleys had oil in them. You could simply start the engine. Avoid revving it, though. The perfect alternative is to obtain an engine pre-lube tank and follow the manufacturer's instructions. One word of warning: You will be introducing oil into the engine, so make sure the same amount has been previously drained. Better yet, drain and replace half of the oil and let the pre-luber fill the rest.

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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