Incorrect Diagnosis - £358 out of pocket.

dipsomaniac

Sorry you're right I thought he said x2 in a previous post

AdrianC

dipsomaniac said:

Sorry you're right I thought he said x2 in a previous post

Two coil packs. Each pack does two plugs.

Four plugs.
Set of four leads.

facade

dipsomaniac said:

So how does modern used plugs stress new coil packs?

"Modern" stuff is designed to be "just about good enough for normal use".

If the old plugs are in 100% tippety top condition, then there is no need to change them.

However, one reason that coilpacks fail is because when the plug gap wears even a teenie weenie bit larger, the voltage at the coilpack increases to make the spark, which over stresses the insulation and the pack fails.

If this is the case, then if new plugs are not fitted, the new coilpack will quickly go the way of the last one.

(and if it is a KA/Fiat 500, it often takes the ecu with it)

dipsomaniac

I am genuinely interested in this. How does the coil pack know that the plug gap is a 'teenie weenie' bit larger?

angrycrow

dipsomaniac said:

I am genuinely interested in this. How does the coil pack know that the plug gap is a 'teenie weenie' bit larger?

It gets out its feeler gauge and if the plug is anything but perfect it sits in the corner sulking and refuses to play. 

On a serious note if you are changing expensive pencil coils it really is a false economy to not change the plugs. 

George_Michael

dipsomaniac said:

I am genuinely interested in this. How does the coil pack know that the plug gap is a 'teenie weenie' bit larger?

The larger the gap between the contacts, the higher the resistance and the higher the resistance, the more voltage that is needed to jump the gap. The coil packs are fed from the ECU so this computer could well sense the voltage out of the coil packs.

If the voltage produced is higher than intended, this can lead to the insulation in the coil pack breaking down over time.

dipsomaniac

Clever things these new 'expensive' coil packs

AdrianC

dipsomaniac said:

Clever things these new 'expensive' coil packs

Hardly new.

A pencil coil is just a perfectly normal coil...  but smaller, so it can sit on top of the plug.
Double-ended coils and distributorless ignition have been in mass production cars since at least the 40s.
A single coil pack containing one coil-on-plug per cylinder, which also uses spark ionisation for knock-detection? Production since the early 90s.

facade

dipsomaniac said:

I am genuinely interested in this. How does the coil pack know that the plug gap is a 'teenie weenie' bit larger?

It doesn't.

The voltage rises until it is high enough to form a spark. The wider the gap, the higher the voltage.  The voltage is limited by the physical properties of the airgap in the plug.

Expensive modern plugs have a platinum centre electrode, and the spark polarity is arranged so that the larger outer electrode erodes.  A quick calculation of how many sparks required to wear it to its service limit- the maximum allowed gap, gives how many miles it lasts and sets the replacement interval. (yes, you could drive only in first gear and wear them out faster)

The higher voltages caused by wider gaps put more stress on the coil insulation and the electronics within the coilpack, this and the high temperatures at the top of the engine can cause failure.

cymruchris

Wonder if the op will be back to update us on how they got on?