i bought a set of ngk iridium plugs for my car about 6 months ago[eb4.0] and 3 weeks ago it developed a bad miss mainly when cold i put another set of plugs in and it runs fine now but its only 1 faulty plug is there a way on testing it out of the car .

test what the spark or spark plug?
make sure ur gap is set to the specs, check leads, dissy, coil. sometimes a plug can fail like anything.

These have been well known to be gapped way big...
Nearly 2mm

i too bought iridiums about 4 months ago and also have an intermittent miss. have had fault codes pulled at ford dealer and comes up clear so not coil pack. while it was at dealer they also pulled out plugs and said platinum tips were all fine. surely they would notice if gaps are too wide. leads are new. i was told u cant/shouldn't adjust gap as could damage electrode??? one way to test plug is to swap one at a time driving it round for a couple of days in between.

the NGK iridiums will get stuffed if you try changing the gap, there pre gapped/.

Maybe something else is fouling the plug or plugs with carbon.
Unless you've already ruled that out..

Are you sure?

Pre gapped doesnt mean correct gap...

Depends how far out they are... if its too big a gap, you'll screw the 'squareness' of the angle... i dunno about you guys, but i never put in 'pre gapped' plugs without verifying gap... prolly more important on boosted engines tho...

IS the plug visibly damaged?

According to NGK the Iridum plugs have an extremely fine cetner electrode and it'll get screwed when the gap is altered.

So you pay big $$ for a long lasting plug, but for that 100.000 odd KM, you have to put up with the wrong gapping?

Blegh.

I must add that a few days after I had put NGK Iridiums in my car that I seem to be getting a really bad miss as well, car stalling at times. No idea what the problem is.. Find out in a few months. Really, pay that much for plugs and have them screw up? Grr

The problem may not have anything to do with the gap.
Using a multimeter test the resistance on each plug, between the centre electrode and the outer terminal and see if they are consistant, if one plug is signifancantly higher than the others than that is the likely cause.

I got mine through work for not myuch more than normal plugs, and the gaps were fine. the best thing to do i guess is to check the gap and if it's wrong return it for one with the correct pre gap.

the leads dizzy cap rotor everything is new all replaced at the same time i checked the gap it pretty close to ford settings

I'm going to relate an experience I had about 2 years ago - this is simply my experience, no guarantee it's the same problem anyone else is having but might help someone.

My car developed a miss - particularly evident under load and over a period got worse at high rpm. Also the car had been blowing very pale blue / almost white smoke for a short while after first start of the day (very early morning or on a cloudy day it could almost look grey) - this would vary from a little bit to s**t and was happening for weeks before the miss appeared - I had pretty much decided it was a guide seal.

Anyhoo, I pulled the plugs and found number 3 plug looked noticeably dirtier than the others - so I cleaned it up and refitted it and the miss stopped (smoke didn't tho). After a few weeks the miss started again and again the number 3 plug was dirty - so I cleaned it up again and again the miss went away - but after only a week the miss was back so at this stage I replaced all the plugs and the full set of leads - the miss disappeared and hasn't come back (like I said this was all 2 years ago) - PLUS the smoke also stopped when I replaced the leads (OBVIOUSLY a cylinder not firing well will get a bit oily and then that will burn off in the exhaust when everything heats up).

I'm fairly certain number 3 plug lead was stuffed - and don't think that new leads rules them out - that set WAS virtually new (had been in the car about 2 months).