What do most people do with their spark plugs nowadays? When I was a lad, you'd just clean them up a bit with wet-or-dry, close the gap up a bit, and put them back for a few more years. Haynes tells me to replace them every two years. Several posts on the internet say modern plugs last for 100,000 miles. I'm just wondering what people here think. Probably a question that's been done to death.
Replace them.
Modern engines don't foul them up, so they don't need cleaning. The electrodes slowly erode, and in addition to increasing the gap, this changes the shape detrimentally.
They should be replaced at the manufacturer's stated interval. This is typically 40,000 miles as a minimum. Failure to change them may result in a misfire, and this can cause other damage.
Chris
In message , Chris Whelan writes
My Mondeo is due for 40,000 mile service, and it was suggested here to replace plugs _and_ leads, Mr C, I think. I plan to have that done, there has been a slight fall-off in mpg over the last few months.
Yep, important to change plugs and leads at 40K on Zetec-engined Fords to avoid costly coil failure.
Chris
Thanks...
How would an old lead affect the coil pack?
Presumably because the resistance varies during the duration of the spark, causing voltage surges in the coil which eventually lead to the winding insulation failing.
What usually happens is that either old leads, knackered plugs, or both, cause a slight misfire under load. If you replace the plugs and leads quickly enough, the coil will be OK. If you continue to use it in that condition, replacing the plugs and leads will reduce the misfire, but it will still be there until the coil is changed.
I saw a 1.6 Focus on 72K that still had the original plugs, even though it had a full dealer service history. (Plugs are not part of the standard service schedule, oddly.) It still started and ran well, except for a very slight misfire when cold. Plugs and leads didn't cure it; the coil had to be changed also.
It's very well documented for Zetec engines going back to the 1990's; possibly the coil specification was always marginal?
Chris
The standard of design and construction of modern coil packs is so tight and so designed for finite life that you, the customer, are doomed to renew them. Unlike when things were made to a reasonable standard, the art/science of planned obsolescence has really made great strides. In short, you buy cheap, you buy more.
massive standard plug gaps and high compression need very high voltage sparks, around double the old points set up, not surprisingly everything has to be upgraded. If the plugs and leads are changed on time there is no real problem, but leave it or get some tracking going on and the coil may go, which may also take the ECU with it.
Not convinced. I suspect it might be that the lead goes slightly high resistance which then gives the spark nowhere to go so it tracks all over the place (or summit like that)
Your suspicions are somewhat awry.
The lead is already more than slightly high resistance when it's new. If the resistance went considerably higher than specification, the plug would fail to fire, so there would be no current flow.
The system doesn't 'contain' a spark that has to go somewhere; there will be no 'tracking', not anywhere, unless there was an insulation failure. That's not the failure mode of the Ford wasted spark system under discussion.
In spite of the way basic electrical theory is often taught, the similarity to fluid flow is somewhat overstated.
Chris
Which then means the voltage exceeds spec and things arc over. Where things include your coil.
Yep, and in the case of the Zetec coils this occurs internally causing shorted turns in the coil windings, reducing its efficiency and making the misfire gradually worse. Eventually, the insulation between windings may fail and kill the ECU.
IME, during the course of this sequence of events, no 'magic sparks' are released however.
Chris
Maybe but that doesn't make them wrong.
Tick!
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