faulty new spark plugs

[banana]

I put new spark plugs into my Fiesta 1.1 last saturday. The car had a fault due to a blocked pipe on the air cleaner and this was unblocked ( after a lot of searching to find the fault). The car ran beautifully - for about a minute . Then it started to misfire. After another seaqrch for the cause it was decided that the fault must be the new plugs - even though they hadnt caused any problems during the run-up to fixing the other problem. Only one plug was not working properly and it was taken back to the shop and replaced. Job done. very pleased. Today -after about 10 miles and when the car had been stood for 2 hours the problem is back .Aaarrrrrggghhh.
Can new plugs be faulty. They were put in properly by a mechanic.

[Impret-Sir]

New plugs can be faulty, like any other mechanical device, but it is rare. I think I would suspect the plug leads, when the problem is occurring, whip out the plug thats misfireing and swap it into another cylinder, does the problem move with it or stay in the same pot?

Plug leads do break down internally with age, and can cause these symptoms.

[banana]

Thanks I.S
A new set of plugs have been bought from another shop where the important question was asked - "what year is it? " . Apparently the plugs changed during year the car was made and the wrong plugs can cause problems. Im not sure if it causes the problems we have had so far - but am waiting with baited breath for further developments.
My mechanic is pulling his hair out ( not experienced with this car ) and is dreading something else going wrong.....

[BillyNoMates]

Probably a long shot but worth checking that the plugs were not over-tightened. I've seen this once or twice which causes the outer ceramic to crack and can cause misfiring.

Worth ensuring the correct plugs for year were fitted and that the gaps were set correctly.

[Si_D]

Hi, I put new plugs into my 1997 fiesta ghia, as recommended by the Haynes manual, and my car lost any acceleration it had, the exhaust was coughing and the car was generally in a mess. I bought different plugs (the ones recommended in the Halfords guides) and there were no further problems.

Ones recommended were Bosch, I think I replaced those ones with NXGs.

Hope this helps!