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I've had my SEAT Ibiza Mk2 for about three years and it's just failed its MOT on the emissions test - there's fuel in the exhaust on idle, and only on idle. It's in the garage at the moment and they're pretty sure the dodgy exhaust reading is down to a misfire on one cylinder. Thus far they've:

  • Swapped the HT leads, plugs and injectors around and the same cylinder still misfires, with all the others completely fine.
  • Run a compression test. The dodgy cylinder compresses fine, but still misfires.
  • The HT leads, plugs, distributor cap and rotor arm are all about a year old.
  • The misfire disappears as soon as there's anything on the throttle, it's only on idle.

No mention thus far of the lambda or the throttle body, or the cleanliness of the injectors, since presumably all those would produce problems on all cylinders, not just one. I am going to ask about those when I next see them anyway.

They say the next step is to take the head off and check the seating of the valves, but according to the chap it isn't likely to be that since the compression is fine, as mentioned. After that, the engine will have to come out for inspection basically piece by piece as he described it.

The only other point of note I can think of is that it had a tank of super for an hour and a half's worth of driving two weekends ago, since that was all the garage had. I've heard that the detergent in that can't do any cleaning in one shot but can disturb any large buildups of crud just enough to block the injectors. Does this sound like a reasonable hypothesis?

It's a 1999 Mk2 Ibiza S, 1.4. Does anyone with experience of this model have any pearls of wisdom that could save me the 800 quid or so for completely dismantling the engine? Not that I'm paying that much; I'd rather scrap it. If they can't sort it, it's going to the knackers'.

The astute of you will notice that this is not the first time I've posted about misfiring on this car, on this site. Seems like a never-ending worry.

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Ignition coil(s)? How is the timing? For the cylinder to fire properly, you need to get fuel and air in, have spark, and get the exhaust out all at the proper time. –  Mark Johnson Aug 23 '13 at 0:05
    
As I understand it, this model has a single coil since there's a mechanical distributor. Wouldn't a problem with that affect all cylinders? Similarly for the timing although I suppose diagnosing and fixing anything like that will still involve getting the head off. Would something like a timing problem show up as a fault code? If so I figure they would have mentioned it. –  Tom W Aug 23 '13 at 6:29
    
It might be worth replacing the dizzy cap and rotor arm - they might be fairly new, but I've known them to fail at unpredictable intervals... –  Nick C Aug 23 '13 at 8:02
2  
@NickC thanks for prompting me to think about that a bit more - it's a good point and would explain the symptoms, that is that it compresses fine and none of the aforementioned bits make a difference. A single contact in the diz having worn, got dirty or come off would match up with the consistent failure on one and not the others. I assumed that would have been checked but I can definitely ask. –  Tom W Aug 23 '13 at 8:11
    
How did this go Tom? Any update? –  Rory Alsop Nov 6 '13 at 10:58

1 Answer 1

up vote 1 down vote accepted

Sadly there was no headway to be made and I sold it for scrap value. The garage wanted £700+ to dismantle and inspect the engine. The guy said the last time he'd seen that issue, it turned out to be a worn piston ring. I suggested that surely that ought to cause compression failure which was not evident in his inspection; he agreed, but that was what he found in the previous case and not being able to find anything superficial, the next step is inevitably to look inside. In hindsight and with less stress in my mind I might have considered garaging it and trying to take it apart myself despite having almost no knowledge, as a learning experience. The worst I could have done would still only leave me with a lump of unusable scrap.

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