My Fiesta IV -99 (Zetec 1.25) has been running a little choppy for a while. Not very much, but I thought it was time to do something about it.

So I replaced the fuel filter and ran a bottle of fuel cleaner through the system. I also put in new spark plugs. I noticed that one of the old plugs was in much worse condition than the other three.

Ok, time to start it up and see how it runs... Much worse than before! The engine fires up just fine, but it runs really choppy now. I drove it for a day, I could barely get it up to 110 km/h, and the fuel consumption is almost doubled.

The next day I pulled out the new plugs, and one of them (same "problem" cylinder as before) was blacker than midnight. After only one day! Black and dry. I replaced it, and also put on new ignition cables. The car seemed ok for the first 0,5 km, but then the problem was back.

Yesterday I hooked up an OBD2 scanner, but it didn't show any errors.

Any suggestions?

Oh, forgot to mention that I have cleaned the air filter and checked the oil. If that matters in this case.

Cheers!

sounds like you have a cylinder running very rich causing the blackened plug {take it that when you said plug was dry, it wont indicate oil problem)

most likely cause for just one cylinder is a fault with the injector,

ditto, car is running to rich..... did you say air filter was changed? check your map sensor first thing.... clean your maf sensor and any air feed pipes....... also check gaps (fila gauge) in the plugs before you re-fit, as some can be tight or gap to big.... total time 30-40 mins

Thanks guys, I'll try this tomorrow!

I would also suspect an injector, over fuelling for some reason.

I'd doubt the fault is with the map or maf sensor as this would make the black plugs common to all 4 cylinders and not just one.

Hi, A sooty spark plug can also be caused by a bad HT lead.

Hi!

I installed new HT leads, didn't help.

I'll give the injector a try this weekend! Can I just grab an old injector at the scrap yard and put it in my car, or is there more to it?