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How do you tell when a spark plug needs to be replaced?

I Purchased a 1990 Mazda Protege and I am not sure when the last time the spark plugs were replaced. What are some easy ways to figure this out?

sparkplugs can last 20 years and sparkplugs can last 2 minutes. If you are fixing up the car so you know where everything is at (IE change the oil now so you know you have to change it in 5000miles or whatever your owners manual suggests) then I would just change them now. Just buy regular sparkplugs. Don't go for the silly expensive irridum, or quad power etc etc. fancy sparkplugs don't increase horse power or anyhting unless you are running some crazy engine. otherwise if you want to keep the same ones you can look at them. make sure the center electrode has nice square edges and don't have a bunch of deposits on them. They should be a light brown color... almost like cinnamon i guess. that tell you that everything is burning good in the combustion chamber.
rickharris4 years ago
Most garages these days are able to plug in a lap top and run engine diagnostics. When they need replacing the car tells them. Otherwise change them anyway if you are in doubt. Modern plugs last a long long time 40,000 miles +
JimJong4 years ago
When they stop sparking?
lemonie4 years ago
If you replaced them and noticed a difference you could say "they needed replacing". But generally speaking it's not a good idea to "fix" things that aren't a problem.
You could remove and inspect them, as per this guide: http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm

L
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