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I recently changed the spark plugs and wires on my 2002 Ford Focus SE with Zetec (DOHC) engine. I used NGK TR5IX (7397-4PK) Iridium IX plugs and Motorcraft wires.

When I received the plugs, they were gapped to about 0.03'. The owners manual it says gap should be 1.3mm (0.051 in) and most online sources I found agreed, so I set the gap to 0.051

But now there is a rough idle, and when I first start going it is really sluggish accelerating (it seems to get better once I've been going for a minute or so). But when I stop the car and let it idle, the engine has been dying (it starts right back up no problem, though, and doesn't die while moving).

I am not sure what could be causing the problem. Is it possible that the plugs I got need to be gapped differently from the manufacturers recommendations? Also, there was some rust around the spark plug wells that I tried my best to clean off before removing the old plugs, but a (very) tiny bit of rust dust fell in when I pulled them out. Could that be it?

Anyways, I don't know what to do at this point - any ideas how to diagnose what is wrong? Thanks!

UPDATE: Ugh ... right after writing this, I discovered the problem: Amazon's crappy car part compatibility tool suggested the wrong plugs for me (2nd time: they also sent me the wrong PCV valve ... definitely won't trust it anymore). So I have requested a refund, and will get the new plugs tomorrow from O'Reilly ... but in the meantime, I guess that changes my question to whether or not I could have caused any permanent damage driving around for about an hour today with the wrong plugs?

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    Yeah the NGK site says get the LTR5IX-11 (stock # 4344) for the DOHC Focus (partcat.com/ngk); and they're gapped properly already at 0.052". Check the plugs you put in for mechanical damage from the pistons like dlu says, and I agree with your decision to avoid driving. You can answer your own question btw.
    – Jason C
    Jul 16, 2016 at 6:02

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Probably not. Engines are pretty tough. But pull a plug before you drive anymore and check for mechanical damage. Pistons hitting plugs would not be good.

Also double check your plug wires and make sure you've got the firing order right and that all of the connections feel solid.

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  • Yeah, I'm sure the wires are right but I think I'll just hold off on driving it until I get some new plugs from the parts store tomorrow. I'll check like you said for mechanical damage though. Thanks.
    – J. Taylor
    Jul 16, 2016 at 5:59
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    It should work anyway, and it will not confuse your ECU, only will correct fuel, no worries. As dlu says about mechanical damage, yea check it, maybe just dropped a plug in the hole, bent the tip, etc.. If it is fine, you can try to decrease a gap to about 0.9mm (0.035"), it should work fine. But if it still misfires - don't drive like this. It will give lean fuel on some cylinders.
    – Arturs Bolsunovskis
    Jul 16, 2016 at 19:03
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Don't worry about engine damage. The main difference between these 2 spark plugs is the length, the one you used is shorter. That means 2 things:

  • the spark was not created in the best place, meaning you were not burning all your fuel. This is the lack of power you were feeling.
  • there is no chance your pistons hit your spark plug since the plugs you used are 7 mm shorter than what you need.

Read more about the NGK TR5IX and NGK LTR5IX. Under 'Specs', look at 'Reach'. 18 mm vs 25 mm.

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