After filling up I added Red X additive to the tank and noticed that the fuel guage only showed about half a tank (circa 240 miles). It registered the fuel burn for the journey, but then when re-starting erratically showed miles left to run in the tank at anywhere between 170 and 240 miles (even though by my guessing it had about 350 miles left) until it was down to about 100 miles and was reading properly again. I also got random alerts that there was water in the fuel (though I know there wasn’t) and after refueling again today to full, the same has started to happen.

Has anyone come across this and resolved it?

 

What car and engine?
Hard to guess?
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sorry, thought I’d mentioned. XF 250, 2010 model with the 3.0L engine

 

Is this a diesel by any chance ?