A Big Hurrah for the Backroom - pete t
Just wanted to say many thanks to all the backroomers who have posted about ill-running mondeos in the past. A quick search of previous threads turned up several apparently likely causes for a rather rough 1.8 belonging to the little brother. Top of the list was tracking on the plugs of cylinders 3 and 4. Lo, what do i find when i look at them, nasty black marks, iffy looking HT leads. Anyway, new plugs and a set of leads and problem appears to be solved.

But.......If this is a common problem, what is the cause behind it? It may be straightforward to fix, but the idea of throwing plugs and leads at the car on a regular basis is unappealing. And does anybody have personal experience with any plugs these Zetecs like - I removed a set of Champion (Eon 4?) and replaced them with the old NGK ones that were lying around. The Champion plugs had been in for a few months, the NGK ones presumably for rather longer before that and despite showing signs of tracking (noticably less than the new Champion plugs) seemed to sort out the misfiring. So a new set of the same NGK went in with the leads, and the car is apparently "sweet as a nut". But is it just going to happen again in a couple of months?

Any ideas? Or, even better, any explanations?
A Big Hurrah for the Backroom - Andrew Moorey (Tune-Up)
Hi Pete, Firstly THANKYOU for posting a result onto the site. Now that you have replaced the plugs and leads the problem should not reappear for a good while. Leads are the achilles heel on these motors and providing you have used manufacturers or quality OE parts and replaced the plugs at the same time all should be well. Ford recommend the use of a silicone grease to inhibit moisture.

Happiness is a T70 at full chat!
A Big Hurrah for the Backroom - 547HEW
Hi All,

Remember this one? Didnt see an explanation so here is my threepence worth.

I would venture to suggest that the cause lies with the unusually large 1.3 mm spark plug gap used on this engine. This was apparently to improve combustion stability at idle. The drawback is a significantly higher spark plug gap breakdown voltage than is needed for similar engines with a more common 1.0 mm gap. This had to be supported by the simultaneous introduction of distributor-less ignition (a 4 tower coil pack), since a conventional distributor would probably flash-over after a small mileage, not to mention the spark energy lost, and radio frequency interference generated. The system is reliant on good dielectric strength. To obtain good system dielectric strength it is necessary to block all paths from high potential to ground, with something dielectrically stronger than air! While it is easy to seal the coil end connections of the HT leads for life, the spark plug end presents more of a challenge.

At this voltage requirement the spark plug's upper insulator is not long enough to prevent flash over in free air between terminal and hexagon. The rubber boot of the HT lead is designed to seal tightly against the plug's insulator to prevent an air path. The plug and HT lead connector, situated deep in the centre of the cylinder head, is subjected to heat soak. After a while, this is sufficient to produce a permanent "set" in the rubber boot, such that it does not seal as well on a different spark plug, being changed at the recommended service interval. Note that outside diameters of insulators vary from make to make of spark plug. To make a good seal it is essential to fit either OE (Motorcraft) or another make of plug which has this larger insulator diameter. At this point as Andrew quite rightly suggests, new leads of OE quality should be fitted. Reason for this, in addition to the above, is there could be carbon tracks down the inside of the rubber boot, which will make subsequent tracking out more likely.

Now, on to why are cylinders 3 & 4 more affected?

Cylinders 3 & 4 are sparked with a positive spark, whereas cylinders 1 & 2 receive a negative spark. This is a feature of the 4-tower coil. With a positive spark it can take approximately 20% more voltage to breakdown an asymmetric spark plug gap, that is where the centre electrode area is much smaller than the sidewire area (typical of platinum tipped plugs). Hence tracking out is more likely on cyls 3 & 4.

"Is it going to happen again in a couple of months?" No, if using OE quality parts, it should be possible to get at least 50, 000 miles in before the plugs/leads need changing.


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