Cracked heat shield may cause exhaust rattle

Question: I have a 2008 Chevy Malibu with 72,000 miles. Last week I heard a rattle on the driver's side, the same side where the exhaust pipe is located. The rattle doesn't happen above 5 to 10 miles per hour; it happens while stopped or backing up my driveway. The dealer says a piece of metal broke off inside the exhaust muffler. They can't get to the problem without replacing the whole exhaust system for $700. They said it won't be a problem for the car itself. What do you think?

Answer: In my experience, this type of exhaust rattle is often caused by a broken or cracked heat shield surrounding the bottom of the catalytic converter. I've seen the shield repaired by welding or wrapping with a steel shipping strap. I've also seen the entire heat shield removed, although this exposes the extremely hot catalytic converter to potentially combustible materials like leaves and newspaper.

I'd suggest getting a second opinion, perhaps at a muffler shop. Ask the technician to check the converter shield. If it is not broken and there actually is broken metal inside the muffler, I see no reason the muffler itself cannot be replaced.

One more thing. The federally mandated emissions warranty covers the catalytic converter and computer for eight years/80,000 miles. If the rattle is coming from the converter itself, it may be covered by the emissions warranty.

Q: I bought a new 2013 Ford F-150 that now has 6,600 miles on it. From day one, it has clunked when I accelerate from a stop. The dealership installed a shim kit in the rear end, greased the U-joints and said the clunk was gone. It isn't gone. It's worse, if anything. A Ford field representative said this was perfectly normal and was engineered to do this. Do you think this is actually the case? I didn't spend $40K to have to live with an irritating problem that makes the truck feel like an old clunker.

A: That's a tough one. I looked up Ford technical service bulletin No. 14-0090, dated May 2014, in my ALLDATA database. It describes an "intermittent click or snap-type noise from the rear axle on initial light acceleration from a stop while in drive or reverse." Two repairs are outlined. If the first repair - applying a Motorcraft penetrant to each of the U-joint retaining clips - does not stop the clunk, the second step outlines the procedure for removing the pinion gear flange, replacing the oil slinger, adjusting the pinion bearing pre-load and reassembling the flange and U-joint coupling with four new bolts.

It would appear the dealer has performed the second-step repair. Since the vehicle is still under warranty and the repair was not successful in stopping the clunk, I would suggest you encourage the dealer to perform the repair again. Unless Ford develops an updated repair for this issue, your options are limited.

Q: At the gas station, if the car before me was refueled with 87 octane and I select 92 octane, how much 87 octane fuel is pumped into my gas tank before the 92 octane fuel is delivered?

A: Interesting question. My neighbor, who like me uses non-oxy gas in his boat, tells me that as much as half a gallon of the previous octane choice will be pumped before your octane choice begins to fill your tank. I don't know if this is accurate, but even if half a gallon of 87 octane is added to 15 gallons of 93 octane, it won't significantly affect the octane level reaching your engine.