I have a Focus 1.6 Auto 02 that on idle has always made a poping sound through the tube leading off from the MAF. I had the car looked and for possible air leaks the first thing I replaced was a breather pipe behing the inlet manifold. That didn't change anything so I have just been putting up with it also the car takes 5 - 6 sec to fire up and sometimes smells of fuel through the vents inside. Last month when I was out the car got really juddery and could bearly pull away and almost cutting out a call to the AA and it turned out to be a cracked spark plug The AA man also said it sounded like the inlet manifold was sucking in air on the right side. They got the car running but it was still very juddery. Two days later I had a family mechanic replace the inlet manifold gasket and did a set of new plugs and oil at the same time. After this the car still took 5 - 6 secs to start but it drove fine but still made the poping noise at idle. Once before I had the engine light on and had the code read it said it was the front 02 sensor took this out and cleaned it up no more light. Anyway a few days ago when driving I noticed juddering around 50Mph the next day when trying to pull away it sounds spluttery and then all of a sudden picked up but now this is all the time and juddery through out.
I took the plugs out last night I don't know what they are ment to look like for month old plugs but heres some links. I cleaned them up and put them back in but its still the same could a fulty 02 sensor cause this?