CAR TALK: Don’t stress about mixing octanes

DEAR CAR TALK: The owner's manual of my car – an Audi A6 – says to use 91-octane fuel. But the pump near me has 87, 89 or 94. Since then, I've noticed that a lot of cars require 91-octane.

Is this a case of where they sell hot dogs in packs of 10 and hot dog buns in packs of 8? Am I just out of luck? Or can I mix some 89-octane with some 94-octane and drive happily? – Bill

DEAR BILL: Drive happily. You can absolutely mix octanes, and they mix linearly. So, if you could fill half your tank with 89 and the other half with 93, you'll end up with a tank of exactly 91-octane.

If you go with half 89 and half 94, which is what's available near you, you'll end up with 91.5-octane. Which is close enough, Bill.

And the truth is, you don't have to stress about it. The engine has a computer management system that will adjust for lower-than-recommended octane fuel. So, if the computer senses pre-ignition (aka pinging or knocking, which can result from too low an octane), it will adjust the timing to correct for that.

You'll have a little less power – probably not enough of a difference to notice – but your engine will be protected against any damage from fuel that ignites too early.

So rough math is good enough here, Bill. You don't need to go to the Sunoco station with measuring spoons.

DEAR CAR TALK: In a recent column, Phyllis, with a 2015 Camry, wrote that she hears a pop when she drives it. I have the same problem with my 2015 Camry and thought it was something in the suspension.

I took it to the dealer and they said it was a known problem. Evidently it is with the rails for the sliding moon-roof. To fix it, they would have to remove the liner and replace the rails to a tune of $2,300. I will live with it. A bummer since the problem started three months after my warranty expired. – Tom

DEAR TOM: This is a good argument for going to the dealer if you have an unusual or hard to diagnose problem.

Sometimes, the place that works on 2015 Camrys every day will know exactly what it is and save you the time and money you'd spend having someone else figure it out.

It's true, you might end up with an estimate that makes you say, "ah, never mind, I'll live with the pop." But at least you now know that it's not dangerous, Tom. So now you can wait until the sunroof starts leaking or until an anvil falls from a tenth-story window onto the car, and then have the rails fixed at the same time. Thanks for the info, Tom.

DEAR CAR TALK: After five years, I'm finally used to the auto start-stop feature on my 2017 Cadillac XTS. When I brought him (yep, a him) home after a long stay at a body shop (sad story), the auto start-stop didn't engage for two weeks. All of a sudden, it worked ... periodically. Now, it works for a couple of days, then not for a few days.

How could a body shop have changed something like that? They didn't work under the hood – to my knowledge. What's going on?

I love your informative and entertaining column! Thanks! – Toni

DEAR TONI: I don't think they changed anything.

My guess is the car sat for a long time in the body shop without being run. That ran down the battery. By the time they repainted it and extracted all the chocolate chips from the muffin truck you crashed into, perhaps it wouldn't even start, and they had to jump it.

And if the battery is not fully charged, the auto stop-start system won't engage. It'll keep the engine running and the alternator turning at stoplights until the battery charges up completely.

If your battery is also old and weak (like from 2017, for instance), it could require more charging than a newer battery and that would lead to the engine running even more of the time.

The computer also will consider things like the engine temperature – it won't shut off if it's still warming up -- and whether there's a heavy demand on the engine – like if you're running the air conditioner.

So, lots of short trips tend to keep the auto start-stop from engaging because the battery hasn't had time to fully charge, and the engine may not be warm.

I'm guessing that nothing's wrong with your XTS, Toni. But have your battery and charging system checked and replace your battery if it's near the end. And then, as long as you don't get distracted again while a muffin truck is in front of you, you should be all set.

Ray Magliozzi dispenses advice about cars in Car Talk every Saturday. Email him by visiting cartalk.com

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