Are plugs reusable, or do you have to replace them?
Spark plugs aren't just essential to making your engine run; they also give you insight into how well it's running. When you pull a plug, its color, the state of the electrode, and the presence or lack of oil all give you clues as to whether your motor is fit or needs some attention, even if it might feel fine when you're driving down the road.
But then comes the conundrum of what to do with the plugs you've wound out of the cylinder head. Is it okay to clean them up and screw them back in, or should you always get a new set? The answer is yes, you can re-use plugs, though there are a few reasons you might not want to.
It's not hard to understand why you would want to clean up your old plugs and throw them back in your engine. A new set of quality plugs on a Dodge Challenger with a Hemi V8 might set you back $350 in parts alone, which is not pocket change to most drivers, not when gas prices are over $4 per gallon. And cleaning them up isn't complicated and doesn't require a load of expensive tools. You just need an ordinary wire brush and a set of feeler gauges.
Plug manufacturer NGK has a few tips to prepare your used plugs for another few thousand miles of sparking. It recommends first wiping the plug body and firing tip with a clean rag, then spraying the firing end with brake cleaner. Wait 10 minutes, then use a wire brush to clean off any deposits before setting the gaps to the automaker's spec. The final bit of advice is to use a fresh gasket if your plug has a flat seat.
Spark plugs can warn you of bigger problems
But while NGK says you can clean and reuse your plugs, it doesn't recommend it. Okay, it's in the business of selling plugs, so of course it would prefer you to buy new ones. But it does have some valid reasons for suggesting you do. The biggest of those is that a cleaned plug will never perform as well as a brand new plug, no matter how long you spend shining it up. That's because electricity discharges more effectively from sharp edges, and the sharp edges of a plug's electrodes naturally wear and become more rounded over time.
But even if you do decide to buy fresh plugs as a matter of course, you should spend some time looking over the condition of the old ones, because they might be trying to tell you that you need a bigger fix elsewhere on your engine. Dry, black deposits can be a sign of an over-rich air-fuel mixture, weak ignition, or simply that you're using a plug with too cold a heat range.
An oily residue could point to worn piston rings or valve guides and valve stem oil seals allowing oil to enter the combustion chamber, or a faulty positive crankcase ventilation (PCV) system. And wet plugs are often a sign of sticking or leaking injectors, or an ignition problem that's leaving unburned fuel on the plug.
A light tan, gray, or gray-brown insulator tip, minimal to no deposits, and clean threads are the signs of a healthy plug, according to NGK. And they could have years of life left in them. Platinum plugs fitted to cars from the 2000s as standard can last over 50,000 miles, and more modern long-life iridium versions are often good for more than 100,000.
