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.