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Autoblog Maintenance 101: Spark plug replacement

Now that carburators have been in the auto industry's rear-view mirror for nearly two decades, we experience remarkably few problems getting the correct amount of fuel into the cylinders. Lighting that mixture off has also become an event that can be relied upon, but it's still necessary to occassionally pay some attention to one of the remaining wear items in the ignition system - the humble spark plug. Worn plugs can cause reduced fuel economy, poor drivability and increased emissions.

The recommended service interval for spark plugs is usually every 100,000 miles. We'd prefer not to go that long for a few reasons. First, we've seen a few platinum plugs that were rated to go 100K, but which had either experienced excessive wear or had completely shed their precious-metal "pucks". Second, the plugs' appearance is one of the best indicators of engine health, and we like to check in on things more often than every time the odometer's sixth digit changes. Finally, the prospect of successfully removing the plugs dim significantly after sitting in the heads for a few thousand hours of operation. We think it's a good idea to change the plugs perhaps every 50,000-60,000 miles (or sooner, if your vehicle calls for more frequent service).

When selecting new spark plugs, it's important to get three things correct. First, get the plug size right! A plug with the incorrect "reach" can be disasterous, and the wrong threads or sealing surface can cause severe damage to the cylinder head. Second, the plug's heat range must be correct for the engine, as this parameter is of utmost importance to ensure proper combustion. A plug that's even slightly too "hot" or "cold" can cause sub-optimal performance. Finally, the plug's electrode material is important if one expects to maintain the same service interval and performance. The use of platinum plugs in certain ultra-high-performance applications can increase the propensity for detonation, while the use of a copper plug in an application that calls for platinum will require maintenance on a more frequent basis. If there's any doubt whatsoever, just select the exact same plug used by the OEM; they may be more expensive than what's available from the local parts store, but it can be money well spent.

We're making the assumption here that the plug wires or boots and the distributor cap and rotor (if so equipped) are in good condition. In the "good ol' days", it was common to replace all of these items during every plug change, but modern components can go 150,000 miles or more. If in doubt, swap out the wires, plugs and cap, because they're usually inexpensive compared to the increased fuel consumption that results from worn ignition components.

The tools required for plug maintenance will vary depending on the vehicle. In this case, removing and installing the plugs on my '96 GMC K2500 requires the appropriate deep-reach plug socket, a few different lengths of extensions, a universal or "wobble joint", and a long 3/8" ratchet. Also required are feeler gauges for checking the plug gap, some anti-seize compound for the new plugs, and some silicone dielectric compound for the plug wires (or boots, if you have a coil-on-plug system). You may wish to use a torque wrench to re-install the plugs, although it's not always possible to employ a piece of measurement equipment.  

The first thing to do is to let the engine cool. Not only will this allow one to complete the job without blistered skin, but it will greatly reduce the chances of damaging the cylinder head.

The first step is to locate the plugs. Here, it's easy - simply follow the plug wires from the distributor down to the plugs, which on an OHV two-valve engine will typically be located near the exhaust ports. Engines with overhead cams usually have the plugs installed through the cam covers, and they may be covered by decorative trim panels. For those engines with coil-on-plug (COP) systems, there will be no plugs wires - you'll have to identify the individual coil units, and figure out how to remove them to gain plug access.

As you can see above, we can see three of the four plugs on the left cylinder bank (circled in green). The red circle indicates a coolant temp sensor; it's important not to damage it during this job or mistake it for a spark plug.

If you wish to do one plug at a time (our preference) or if your plug wires are numbered, there's no need to mark the individual wires. If you're the type that wants to pull off every wire at once, make sure to mark them with tape and a permanent marker, or use numbered tags that are designed for this purpose (we keep around a couple rolls of the 3M Scotchcode tags in the Autoblog Garage).

Remove the plug wire by grasping the boot (NOT the wire itself!), and giving it a gentle tug. If necessary, the boot may be twisted as you pull. Yanking on the plug wire will almost assuredly require a trip to the parts store. Pliers designed for this purpose are available at parts stores, but frankly I've not found them to be of much use. Your results may vary, of course. As you do this, be aware of the various heat shields that are often found around exhaust manifolds, as they're apparently designed to remove skin from mechanics' knuckles with maximum efficiency.

After removing the plug wire, use an air hose, brush, or shop vacuum to remove any debris that may be present around the plug.

Once the plug wire is removed, select the proper socket and extension length. Here, we're lucky that we can actually get a socket and ratchet on all of the plugs; other vehicles in our garage that have been modified with headers and angled-plug aftermarket heads require the use of a box wrench.

Make sure that the socket is fully engaging the hex on the spark plug, and break loose the plug. Significant force can be required, and we've been known to employ extra leverage when required. If you're lucky, the plug will be free and can be spun out the rest of the way with your fingertips. If there's a bit of corrosion, it may be necessary to back the plug out using the ratchet. Murphy's Law states that the plug that's installed in the tightest location will be the most difficult to remove.

Here's what the spark plug from the #1 cylinder looked like after 85,000 miles. To be honest, this plug could have provided several thousand miles of additional service, but that's impossible to know without removing the plug. And once the plug is out, well, it just doesn't make much sense to reinstall it when new plugs are only a few dollars each.

Determine the proper gap for the new spark plugs. This information can be found in the service manual, or else simply locate the underhood emissions sticker. Keep in mind that the required plug gap can vary from year to year and model to model, so don't rely on the information that you used when working on a different vehicle.

Even if the plugs supposedly come "pre-gapped", we still use a set of feeler gauges to check the plug gap. If the gap is incorrect, use the tool of choice to adjust it; a set of spark plug gapping pliers is the quickest and safest way to perform the job. Those wedge-type tools work, but they often result in a twisted or misaligned electrode.

Next, we apply some anti-sieze compound to the plug threads and tapered sealing surface. Just a little dab will do ya; excessive amounts will only serve to foul the plug or attract contamination.

Guide the plug back to its mounting locating, being sure not to pick up contamination or damage the plug electrode along the way. Start the plugs using your hands only; trying to thread the plug into place with a tool that provides leverage will increase the chance of cross-threading the plug (this will ruin your day). Once the plug is in a few full turns, it's OK to apply the ratchet and snug it into place. If you can put a torque wrench on the plug, you can reduce the chance of under-tightening the plug. If that's not possible, then install the plug until the taper seats into place, and then apply an additional 1/16th of a turn.

Apply some dielectric grease to the plug boot if it seems dry, and then reinstall it with a firm push. Keep going until you feel a snap or click, and then verify that it's seated with a light tug. Don't over-apply grease to the plug boot, or else it may hinder installation.

We always keep track of where each old plug came from, as we like to inspect each of them for signs of problems. In this case, tiny specks of carbon indicate a possible detonation condition - possibly due to carbon build-up, excessive ignition advance (unlikely in this particular situation), or poor fuel quality. As the old poster that hangs in thousands of garages states, "Used Plugs Tell A Story" [PDF warning, eh?]. Thanks to modern engine design and computer management, most of us will never have to face any of those problems, but nothing beats verifying an engine's health first-hand.

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