Ford Focus 2002 misfire

[JasonF]

Hello all
I've had a slight misfire, low revs, for about a year, more of a stutter. This happens when engine is hot. I've changed the coil and ht leads and still got the same issue. When I rev it on the drive engine sounds great, so any ideas what it could be. Thanks for any help

 

[Burnerman]

A difficult one.....if the coil pack has failed before then this can damage part of the ECU.
However, pop the plugs out and see if they are white.....an over sensitive air mass meter can give a very lean mixture on these.
Also check for water in the spark plug wells - another common issue.
I guess the engine management lamp isn't lit?
John

 

[JasonF]

Cheers Burnerman for your reply, after 12 months of messing about swapping coil packs, leads etc, yesterday I noticed how dirty the air filter was, so I took it out and today I went for a long run in it, I couldn't believe it the engine didn't miss a beat completely a different car. Although I have had changed the oil filter and oil every year, I have never changed the air filter, the one I took out was 17 years old I feel like a right **** now. Thanks for your time to reply.

 

[Burnerman]

17 years?? Ye gods that must be a record!
Maybe treat it to a fuel filter too.....the car might want to get it's own back
Pleased its sorted.
John