Windshield air pockets: Cracking open a mystery

Q: On my 2011 Dodge Charger, about a month ago I noticed at the upper corners of the front windshield what appears to be air pockets under the glass, radiating in a dendritic pattern in an arc (looks like a fern branch). It is about one inch in radius on the driver's side. I mentioned this to the Dodge service guy said he was not aware of any other of the Dodges having this condition. He said to "keep an eye on it". If this continues should I expect a leak? Should something be done at this time to prevent further growth of the air pockets?

— J.H., Downers Grove, Ill.

A: Since the windshield is an important structural part of the car, we contacted Nik Frye, vice president of sales and marketing at Guardian Auto Glass, and here's what he had to say: "It sounds like the lamination in the windshield is separating from the glass. The reference to the fern-like pattern sounds like what happens when the layers of glass separate from the vinyl interlayer and allow air pockets to form as the separation occurs. This is usually a manufacturing defect and if the windshield is the original part it should be covered under the new car warranty. If the windshield has been replaced it should be covered under warranty by the glass company that installed it. This problem is likely to spread and it is possible the glass could be weaker in the effected areas and that safety could be compromised."

Q: I am a little surprised at your answer to R.L. of Geneva, Ill., about over-filling his minivan. The common man is told at every gas pump by a sticker "Do Not Top Off." I would have thought that you would strongly support the recommendation of his mechanic not to top off. How many more driving miles does one get from topping the tank anyway?

— M.M., Orlando, Fla.

A: R.L.'s question was not specifically about topping off, but trying to fill the tank full. You are right, though, that topping off is discouraged. Not only is there the risk of spilling gas on the ground (an environmental no-no), but of having gas returned to the station's tanks if the pumps have Stage 2 vapor recovery systems (a financial no-no).

Q: I recently read your column about the guy's car reading 2 mpg greater than what he calculates. My 2012 Escape does the same thing, fill up after fill up. My guess is the car makers fudge the reading higher; no reason not to since there's no penalty. Only the window sticker is required to be legally accurate.

— M.D. Chicago

A: Onboard fuel economy readings can drift for a variety of reasons including tire wear. The calculations are at least accurate enough to keep you from running out of gas.

Q: I have a 2006 HHR that has a clunking sound coming from underneath, when going over a dip in the road. Strangely it only happens when the car is very cold. Don't hear it in warm weather. Any thoughts?

— G.V., Chicago

A: It sounds like a suspension issue. If the clunk only occurs when the suspension rebounds, the problem could be a spring with a worn insulator. If it only happens during suspension compression, it could be spring coils clashing. But it could also be due to a problem with a shock absorber or jounce bumper. This is why you need to take it to a trained tech who can often narrow it down by the sound it makes then put it on a lift to inspect for problems.

Q: I have a Ford 2001 F-250 diesel pickup truck with 61,000 miles. When I pull my 5th wheel trailer I mostly use 3rd gear and not overdrive as the truck struggles in hills and head winds to maintain speed. In 3rd gear at 60 mph, the rpm is about 2,500. When I am pulling on a flat surface the engine speed drops to 1,500 rpm then goes back up to 2,500 rpm. The truck is not used much except to pull 5th wheel trailer to Florida and back.

— R.R., Chicago

A: The condition you describe is often called "hunting" and it is probably due to the lock-up torque converter — what is usually called overdrive. We suggest leaving overdrive off at all times when towing. Sure, your fuel economy may drop, but it can't get much lower than you are already experiencing pulling that behemoth of a trailer.

Q: My car recently began dropping a small amount of oil. I am considering adding an oil stop leak product but have been advised against it as it might impact other non-leaking seals. Also I have consulted three different service centers with widely different opinions of the source of my seepage. Is there an oil additive with a dye that would help me pinpoint my leak?

— R.F., Middletown, Md.

A: You can safely use one of the major brands of oil stop leak products as long as you follow the instructions and don't overdo it. There are numerous brands of motor oil promoted as being for older cars and they contain such stop leak compounds. Yes, there are ultraviolet tracer dyes that can help pinpoint a leak, but for years we have used aerosol foot power to help us spot the source.

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