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My 2007 3.2 S80 has around 55k miles on it and I'm getting a pretty bad knock now and then when running on regular gas. The car was fine for years on the lower grade but something happened recently. Mileage is fine, and power doesn't seem to be going away so it isn't really affecting performance, but I don't want to take the hit on higher grade gas.

I took it to a dealer for a recall and had them check it out but all they did was flash new firmware, which didn't help at all.

I have a 2008 XC90 and my dad has a 2010-11 XC70 with the same engine, but neither has either of these issues despite running on regular as well.

Any thoughts on what could be causing it?
 

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Take a look at the spark plugs. My last car (Prius) developed pretty significant pinging in warm weather at ~90k and 5.5 years of ownership. The telltale was carbon buildup on the plugs, which very likely extended to the entire combustion chamber. I blame it on the previous two years of mostly city driving and shorter trips (vs. mostly highway and longer trips before we moved) resulting in the engine not getting completely up to temperature a lot of the time. I ended up using premium fuel until trading the car for completely unrelated reasons.
 
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