Today: four reasons why it might be time to change your spark plugs.

Dear Car Talk

Dear Car Talk | May 01, 2007

Dear Tom and Ray:

How do I know when I should change my spark plugs? I mean, will my car do anything different? I was getting my oil changed, and the guy said I should change my spark plugs. How would he even know? Thanks. -- Maureen

RAY: Well, generally speaking, there are four reasons to change your spark plugs, Maureen. One is, simply, time. A lot of cars now use platinum plugs that are supposed to last 100,000 miles, so you shouldn't need to change them often. But if your car has 110,000 miles on it and you know for a fact that you've never had your plugs changed, your mechanic would be correct in recommending a new set.

TOM: The second reason we suggest new plugs is if we've examined them and found that the electrodes are worn out. When the electrodes wear out, the spark-plug gap (the space that the spark jumps between) grows. That leads to reduced mileage and can lead to a misfire, which is when a cylinder fails to fire.

RAY: We might also notice that the ceramic insulation around the center electrode is cracking. That can cause a plug to fail, too. If we notice either of these signs of impending spark-plug geezerdom (ISPG), we'd probably recommend that you replace all of the plugs.

TOM: The third reason we might change your plugs is if you already have a misfire that we suspect is caused by a bad spark plug.

RAY: And the fourth reason we tell our customers they need new plugs is if we got a special "case price" on them from our supplier, and we need more room in our parts department.

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