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Rangie has a new MoT - yay - and no advisories - double yay.

However, over the past year or so the insides of the tail pipes have become a bit black, not sooty as such but definitely blacker than usual. NanoCom tells me the O2 sensors are working and all the fuel trims are ‘bang on’. The MoT report shows the emissions are also ‘bang on’. No smell of fuel at all from the exhaust. Engine runs beautifully.

So why are the tail pipes black? Could this possibly be the swap over to E10 fuel in some way?

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Could be E10, no idea, but if the emissions are spot on, I wouldn't worry about it.

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As Richard said, I wouldn’t worry about it. But, I’ve read that O2 sensors will slowly change to reading slightly lean over time, so they will tell the ECU to richen the mixture slightly. This could cause your pipes to be showing a little blacker than before.

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Both O2 sensors were replaced two and a half years ago and I did suspect them but they seem to be doing the job properly. I’m not worried, just intrigued.

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I believe you have hit the nail on the head.
Ethanol in the fuel will produce more water vapor than without.
The moisture is probably just sort of cleaning out the small amount of soot in the pipes and when it gets to the outlet, the water is evaporating and leaving the carbon.
I have seen this at a friend's house where he backs into his finished garage.
When he starts in the morning, black water spatters his clean white Wall.
Tailpipe has the black stains as well.
I would not be to concerned unless you are using coolant too?