Spark plugs and wire--when to change


The worst price I ever saw for a wire set was $300.00 per side on a V-12 BMW 850 and it was common (or as common as it gets on a low production car) Och!



To give a current example of why plugs should not be allowed to remain in an engine for an extended spell consider what I went through this evening on a car that belongs to my daughter. (2005 Mustang, 4.0 V-6, 55k miles)

All of the plugs are easy to access and it took me over 1.5 hours to remove the plugs and install new ones. Four of the 6 plugs were a heartbeat away from being seized in the threads and it took a godawful lot of seesawing and praying to get the plugs out without bringing the threads with them.
Every thread clean out to the last few was groaning like a barn door and a thread chaser had to be run through every hole. A miracle any of the threads were saved.

As to plug longevity, many of you know that I maintain plugs should be changed at the 50-60k miles mark, maximum.
This car ran fine (at least to her) but it had the original factory plugs in it and they were flat fried. They should have been changed about 20k miles back and it amazes me that it ran as well as it did.
She states it’s a different car now so that shows how one becomes acclimated to sub-standard performance.

I kept one of the plugs and will see if I can get a decent pic of it for posting purposes.
(On a side note and not mechanically related, she came out because she found an “odd colored ring on the bathroom floor” this morning that was not there when she went to bed. Being frightened/puzzled she called and believe it or not, it turned out to be a dead snake.
The oddity was not so much that a snake got in the house and that it was dead; it was the cause of death. It apparently was swallowing itself and consumed about 40% of it’s length before buying the farm. Darnedest thing I’ve ever seen. Got a pic of that too.)



“The maintenance schedule suggests 120k, but that seems far too infrequent. I was thinking something like every 50k or so.”

120 is just fine with this engine. The plugs came with a anti-seize plating, they are irridium tipped and they will easily last 120k and they won’t seize.



Kind of interesting that you had this problem. I changed the plugs in my daughters 03 Corolla at 117k miles and didn’t have any problems. I changed the plugs in my wife’s Honda at 155k miles, At 105k, the dealer had changed the plugs I had just previously put in at 90k due to the extended warrantee agreement with the EPA. They used standard plugs and they actually felt like they had not been properly torqued, although Honda only calls for 13 ft lbs of torque.

I really am sold on the Iridium and double platinum plugs and I think the ones made by Denso and NGK will easily last and not seize. I had a high opinion of the first set of Champion double platinum plugs I put in my Saturn, I recently replaced them with another set of Champions and felt the quality went way down on this second set. The first set were plated so I didn’t use any anti-seize on them, but the second set did not claim to be plated, nor did they appear to be, so I used the anti-seize compound on them.

BTW, when my daughter had her Ford, I used Autolite Platinum plugs in it, they did NOT last, but I don’t remember having a problem removing them, but that could be due to the fact that I used anti-seize on the threads and they blew the tips of at less than 30k miles. I used Bosch in that car after that and each set went 50-60 k without any problems. It seems the regular tipped original plugs didn’t last very long either.



Plugs are absolutely past due for replacement at 115k miles. They may still be working OK, but they are due for retirement. As noted, they may be difficult to remove after this many miles. Bump the wrench in the tightening direction before trying to loosen.

Wire life is a function of years and engine compartment temperature more than miles. You did not mention if you have a 4 or 6 cyl engine. If you have a 4 cyl, the wires are probably fine for a few more years. If a 6 cyl, still probably OK but that is a tougher call. New wires are $40-$60 and you can put them on yourself in just a few minutes. I suggest NGK or Bosch wires. Plugs are about $10 each for this car, and I suggest Denso or NGK.

$400 is about twice what you should pay to have someone else replace plugs and wires.



I think your dealer is trying to make some money off those “free” oil changes. Your owners manual should have plug changes somewhere on the maintenance schedule and I would recommend following it.

If it calls for plug changes every 30k, then it came with copper or iron tipped plugs originally. If you change to double platinum tipped or Iridium tipped plugs, you can go 100k or more miles on the replacements. If the schedule is 100k or 120k miles, then it came with double platinum or Iridium tipped plugs. Just make sure that if the plugs don’t have an anti-seize plating on them, then the mechanic must use an anti-seize compound on the threads.

As for the wires, I haven’t seen a modern set of wires go bad on a vehicle, even with several hundred thousand miles on them, but if you do need new wires, you can use any wires that meet OEM specs. Do not fall for any high performance wires or “low resistance” wires, they will cause teh ignition system to not fire properly.



very thorough and informative reply, thanks!