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My 92 Civic has very old spark plug wires, probably original, but their resistance is in-spec (under 20k ohms), they have no external damage, and the vehicle has no misfire or other symptoms associated with bad wires, aside from having worse gas mileage than it used to. Is there any reason to replace them? Would doing so perhaps get a more optimal spark timing or stronger spark, more-complete combustion, etc.? Or are those claims just made up to sell wires?

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I +1'd Brian's answer for preventative maintenance. My car made it through an unusually cold winter in MI, USA. It got down to -6°F that year, my car started on the first key turn. A few months later, the first real rainy day of spring, it just turned over and wouldn't start. Wet wires were likely the culprit, I'm about 90% sure. I wound up doing a semi-major tune up so I can't be 100%. –  BigHomie Apr 29 '13 at 14:01

2 Answers 2

up vote 7 down vote accepted

Mostly replacement is preventative maintenance. Eventually they'll get cracks that moisture can get into, etc. If they test good, they're probably fine for the moment, however, at their age they could develop problems any time.

On my '91 Toyota, I've been through several sets now, and have to say that in my case, OEM is the best. I've tried other brands (including "performance" brands) and have never seen any measurable improvement in performance. I have, however, had a lot of early failures with those supposedly better (and dramatically more expensive) performance wires.

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If it ain't broke don't fix it. If your wires already check out and you don't have misfires it is probably not worth replacing them just yet.

You could possibly get better performance out of better coils and plugs but even those numbers are pretty minimal. Best thing I can recommend for fuel economy is to use seafoam in the engine and fuel and then change the spark plugs soon afterwards.

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I already used seafoam spray in the intake system and especially the IAC valve and it helped a lot. –  R.. Apr 22 '13 at 22:11
    
Oh, and the spark plugs are new -- I changed them along with replacing the head gasket since I figured the old ones might have corrosion from coolant. :-/ –  R.. Apr 29 '13 at 3:01
    
By the way I like your answer and +1'd it but I'm accepting the other one because Brian's characterization of replacement as "preventative maintenance" and personal experience going through "several sets" of replacement wires is probably the most convincing evidence that I'm best off making use of the (probably much better) factory wires as long as they work. –  R.. Apr 29 '13 at 3:04

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