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Quick question from an new and non-mechanically minded member if I may?


I have a T reg 1.8 Zetec focus which is up to date on the service schedule, but which yesterday I noticed was throwing out a fine spatter of unburnt fuel from the exhaust. Drives and sounds fine but am I right in suspecting the Cat might need changing?


Is there anything you guys would advise checking/ doing? Got a reasonable journey planned this week and don't want to aggrivate anything.


Thanks for any help in advance




if your not burning all the fuel replacing the cat wont really help, sounds like you might need to replace the sparks if nothing else...


Thanks. Relatively straight forward job I guess or one for the pros??


replacing spark plugs is a DIY job, but its more a case of you need to confirm they are the issue first.


does your exhaust smoke come out transparent, white, or black / grey?


Also can you smell any petrol around the top of the engine block? or can you only smell it at the tailend?

Changing the spark plugs would certainly be a good place to start, and any DIY mechanic can do these with very little tools.

If its throwing out fuel in the exhaust, it sounds to me like its overfuelling so could be down to a faulty sensor somewhere. First suspect would be the pre cat lambda. But before you go replacing any sensors I would have someone with a live data scanner check it out

Thanks guys. No white or grey smoke output as yet (or any more than you might expect on a cold start) - its transparent when warm and idling and I haven't noticed any petrol smell other than at the exhaust.


Sensor makes sense. Will drop it into the garage after the trip (circa 400 miles) unless you beleive it should be done before?

Are you sure it's fuel and not just water vapour?

Pretty sure but will check again. Thank you.


Do you happen to know if water vapour is indicative of any issue? (Apologies, I really am that clueless when it comes to these things).

Water is produced when you burn hydrocarbons, so is perfectly normal.

However, excessive water vapour should be investigated, it could be coolant entering the combustion chamber somewhere

Many thanks Stef.

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