Ok here goes My son bought a S3 second hand and it ran ok until the dreaded light came on it said then p0301 cylinder 1 misfire it was at this point panic set in so we set about diagnosing the issue
we found that the gasket on the exhaust was blowing and found a gasket was the wrong one fitted and a nut missing
we changed the coil packs to uprated ones from awesome GTI and new plugs still nothing so a user locally offered to help and logged the car and noticed issues on the fuel side so we changed the follower on the HPFP and nothing so we changed the pump this is whem we noticed it moved to cylinder 4 on idle from 1 and also it was running better on high end not brilliant we noticed the low pressure sensor on th HPFP looked old so we changed that and Bingo all miss fires had gone Happy days see pic bellow

until after 24 hours of normal running and checks they came back to Number 4 three days later

we have changed just about everything if it was the cam it would of been doing it days after
if it was injectors it would be the same doing it the day after were now looking at the high pressure sensor and the fuel filter but were rely struggling here garages dont want to no and the ones want to change the injectors over but we had two days of it running fine at idle

on other groups were told oh you need to do a carbon clean i think there just trying to sell that were going to put injector cleaner through as that worked on a car i had ( i was told the same thing carbon clean ) so you can tell why im not a fan of that
I no it carnt be the cam as it would of done number 4 from the start and not number one
my fear is we may of found out why he sold this, but my son has had a s3 before and loves them and likes this but we need to sort this out some how, we would take it to any one in the Yorkshire area willing to sort this the guy that has helped so far has been brilliant but he has no idea what this could be
Help needed

 

Update Miss fire has left Number 4 and moved to Number 1 after we changed out a what we thought was a Original Bosch Evap as we had a Evap code pop up now we only get 3 to 4 miss fires on rotation when it resets The Evap was got from what we thought was a retailer on Ebay but we think its a fake unit
The car still has a real issue starting on cold but does settle down