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Before easing out of the fast-food’s parking lot, you grab a sip of the hot coffee you picked up at the drive-thru. You take your time as you place the cup in its holder in the center console. No hurry. The approaching traffic may be moving fast, but it’s far enough away for you to accelerate up to cruising speed. Good thing, because at about half-throttle you feel the engine misfire. The closing traffic has time to back off and let you sputter up to road speed. But now your engine sputters like a Harley idling at the curb, even at normal traffic speeds. And there’s coffee all over the console from the vibration.

This project was originally published in the November 2000 issue of Popular Mechanics.  You can find more great projects at Popular Mechanics DIY Central.

Step 1: What's Up

A brief underhood inspection confirms that all the vacuum hoses are on and that none of them have been burned open by leaning against the exhaust manifold. You’ve touched every sensor connection to see if
it’s on tight, you’ve even checked for trouble codes. However, not everything calls for high-tech diagnostics and the latest scan tester. Engine misfire often is caused by leaking spark plug wires, so visual inspection could reveal what’s wrong.

The jackets may be damaged from the outside by abrasion caused by engine vibration (particularly at the plug-end boots), a hot engine compartment, spilled fluids or battery acid, multiple disconnections and reconnections for other service, or even a family of nesting rodents. Run the engine in near darkness, and then look and listen. If you see little electrical arcs, or hear a snap-crack, there is high-voltage electrical leakage. Of course, running the engine at idle doesn’t exercise the ignition system very hard, so you can add something to the test by spraying the plug wire ends with clean water from a household spray bottle.

Another test: With a jumper wire, ground the metal shank of a screwdriver that has a well-insulated handle. Then, run the tip of the screwdriver along the length of each wire and all around at the coil and plug boots. This will often produce an arc from the wire to the screwdriver. Now look at the wires under good lighting. If they are damaged by abrasion, oil-soaked, cut, burned from contact with the exhaust, or have a dried-out look with heat cracks, it definitely shouldn’t be a surprise if you see arcs in the dark under some conditions. However, if they look bad but there are no arcs, do a resistance check.

A conventional plug wire has a resistance of 10,000 to 15,000 ohms per foot of length—if it’s measurably higher, the wire probably is bad. An absolutely failed wire will have a hairline break somewhere, and the resistance will be infinity.

Step 2: Out With the Old, in With the New

Once you’ve found a bad wire, the solution should be simple: Replace the plug wires. First, you have to decide whether you want to buy from a car dealer at list price or get a quality aftermarket brand, if you have that choice. Prices for 1980s models may be under $35 for a popular make at the parts store. For a late-model V8, be prepared to pay the dealer up to $200 or more, even for a popular car.

A word of caution: You may not want “high-performance” wires, even if they fit your car (and they may be cheaper than a conventional type). Some high-performance wires are not the resistance type, and although they may do well on racing cars, they can affect the operation of underhood electronics by not containing radio waves. The actual method of removing the old wire is not always “just pull it off.”

Pulling on the wire itself is sure to cause an internal separation. Of course, if you’re replacing a bad wire, it may not matter, but if any one of the old wires is still good, it’s best to keep it as a spare. Grasp the plug wire by the boot at the plug end. If it’s in a recess or is difficult to access for some other reason, use a spark plug wire tool. There are special pliers of all sorts designed to reach in and grasp the boot.

They’re not expensive, and having them could prove useful if you ever have to disconnect wires for other underhood service. Twist the boot if necessary to break the heat seal to the plug, then pull. As you do this take note if you feel some looseness at the connection (it may have been caused by

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Bio: The official instructable for Popular Mechanics magazine, reporting on the DIY world since 1902.
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