Spark plugs - what wears out?

[Clifford Pope]

Classicswede and others have constantly urged the importance of using good plugs (ie NGK) and changing them frequently if using LPG.

I have just confirmed the wisdom of doing this. My 240 developed a hesitation and occasional cutting out when slowing up, in the early part of a journey. I fitted new plugs and the problem instantly vanished.

But what had actually happened inside the plugs?
They had done 10,000 miles, and looked in perfect condition. No soot, not blistered, nice grey/brown colour with no deposits, electrodes looked still new, gaps still spot on 25thou.
The screw-on things were tight, and the connectors clean.

So something must have deteriorated inside. What actually happens to a plug on LPG but not on petrol?

[classicswede]

Very good question and one I would like to know the answer to.

With platinum plugs the tips burn away making the gap far to big to create a strong spark.

With a basic plug like the 240 that is not the case. I asume it must be the insulation breaking down but there is never any visable fault.

Also bear in mind the rest of the system needs to be upto scratch so leads, cap and rotor are a good idea to replace on a regular basis.

[capt jack]

What a good question! You're right, the bog standard copper-cored plugs I take out of my LPG V70 every 10k miles don't look very different from the new ones that go in, but I've certainly had plugs fail and cause misfires, which new plugs have instantly cured!

The only thing that I can think, apart from the insulation problems already mentioned, is that the higher combustion temperatures of LPG in some way affects the electrical resistance properties of the core metal, which in turn alters the performance? Could this happen without altering the outward appearance of the electrodes? Any expert metallurgists out there?

If I'm reading the Wikipedia article correctly, metals (which are essential of low resistivity) do exhibit an increase in resistivity with higher temperatures. A permanent increase in the core metal's relatively low resistivity would presumably have a detrimental affect on it's ability to conduct the impulse for the spark - essentially weakening the spark.

Cheers

Jack

[capt jack]

Found this answer on a physics forum:

As the temperature increases, the atoms that make up the lattice structure of conductors start to vibrate more vigorously [over greater distances]. As a result of this, mobile electrons in the conductor may collide more often with vibrating ions. Hence an increase in electrical resistance. Picture this, it is easier to walk in a street where people are only standing around, but if its busy [like rush hour in Manhattan], you'll find it harder to make your way through the crowd.

Not my words at all, but the poster sounds like he knows his stuff - reckon it pretty much answers the question!

Cheers

Jack

[Joe Harding]

A copper core plug will have a through resistance of milliohms. The HT leads will have a resistance of about 1500 Ohm (suppression resistance)

The 25kV spark, (assuming a distributor) will travel from the HT coil, along a HT lead, to the central electrode of the rotor arm then actually jump a small gap before embarking on the resistance of the HT lead and finally appearing at the electrode of the spark plug.

Do you really think a deterioration of a few ohms in the plug will be significant?

I'm not saying don't change the plugs, but I am saying not for the above reasons.

If you have low speed rough running, check the distributor. A weaker mixture of LPG vapour and air will need the best spark going to reliably ignite it.

[capt jack]

Originally Posted by Joe Harding

A copper core plug will have a through resistance of milliohms. The HT leads will have a resistance of about 1500 Ohm (suppression resistance)

The 25kV spark, (assuming a distributor) will travel from the HT coil, along a HT lead, to the central electrode of the rotor arm then actually jump a small gap before embarking on the resistance of the HT lead and finally appearing at the electrode of the spark plug.

Do you really think a deterioration of a few ohms in the plug will be significant?

I'm not saying don't change the plugs, but I am saying not for the above reasons.

If you have low speed rough running, check the distributor. A weaker mixture of LPG vapour and air will need the best spark going to reliably ignite it.

Not having any expertise in materials and milliohms I can't argue the figures, but what I do know for a fact is that if I let the spark plugs in my LPG-converted V70 do more than about 10,000 miles I will get a misfire at around 3000rpm under load.

I've used plugs from all the major manufacturers, including Volvo, and I've found with absolutely 100% certainty that in my car only Denso standard copper-core single pin plugs will go the distance. Volvo and Champion are the worst, with NGK and Bosch not much better. I've had some plugs fail after just 2 or 3 thousand miles.

And when I have had a misfire new plugs have invariably cured it.

My V70 has now done 250,000 miles, 200,000 of those on LPG and so this is very much based on my first hand experience.

I do also keep the rotor and distributor contacts clean, and I've changed the plug leads, rotor and dissy cap twice in all that time. But I've changed the plugs just about 20 times!

Cheers

Jack

[Joe Harding]

I agree, but I don't know the reasons why spark plugs fail. As has been said, they come out looking A1

[brodgar]

thx captAIN JACK

AND THX JOE HARDING.


Again it shows the merits of expertise and enthousiam investigation

of the members of this forum

[david philips]

back in the days of noals ark when there was a champion plug tester a sort of box that you put the plug into and connected the lead up to and tested the plug(one at a time) you could turn up the air pressure and watch untill the plug failed and new ones where allways better than old ones ,so like people they get old and less efficient and can die.most just faid away.

[Clifford Pope]

Originally Posted by capt jack

only Denso standard copper-core single pin plugs will go the distance. Volvo and Champion are the worst, with NGK and Bosch not much better.

I'd always been led to believe NGK were the best. I'll give Denso a try next time.