GM Metro/Sprint 1985-1993 Repair Guide

Spark Plugs

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Normally, a set of spark plugs requires replacement about every 30,000 miles (50,000 km). Any vehicle which is subjected to severe conditions will need more frequent plug replacement.

Under normal operation, the plug gap increases about 0.001" (0.0254mm) for every 1,000-2,000 miles. As the gap increases, the plug's voltage requirement also increases. It requires a greater voltage to jump the wider gap and about 2-3 times as much voltage to fire a plug at high speeds than at idle.

When you are removing the spark plugs, work on one at a time. Don't start by removing the plug wires all at once, for unless you number them, they may become mixed up. Take a minute before you begin and number the wires with tape. The best location for numbering the wires is at the distributor cap and the spark plug boot.

Once removed, inspect all spark plugs carefully. Worn or dirty spark plugs will provide satisfactory performance at idle speed but under more demanding operating conditions, they will frequently misfire. Misfiring spark plugs should be suspected from a number of symptoms. These symptoms include poor fuel economy, power loss, loss of speed, hard starting and generally poor engine performance. Spark plugs may misfire due to carbon fouling, an excessive spark plug gap, a broken insulator, bridged electrodes or a damaged spark plug wire or boot.

Fouled spark plugs are indicated by black carbon deposits on electrodes. These black deposits are usually the result of slow-speed driving and short runs in which sufficient operating temperature is seldom achieved. Worn pistons, rings, faulty ignition, over-rich air/fuel mixture, and the incorrect spark plug heat range will also result in carbon deposits.

Carbon deposits on the spark plug insulator tip may become conductive and cause the high voltage arc to track along the tip to some point where it arcs to join the spark plug shell. This arc then ignites the air/fuel mixture later than normal which, in effect, retards ignition timing. Heavy carbon deposits may be conductive to the extent that the arc path now becomes a shunt path to the spark plug shell. This condition prevents the high voltage from arcing and igniting the air/fuel mixture. Once arc tracking occurs, the spark plug may be permanently damaged and must be replaced.

Excessive electrode wear, on low mileage spark plugs usually indicates that the engine is operating at speed consistently higher than those for which the engine was designed or that the spark plug's heat range is too high. Electrode wear may also be the result of spark plug overheating caused by combustion gases leaking past the threads. Electrode wear can become excessive to the point that the high voltage no longer arcs across the electrodes.

Heat range is a term used to describe the cooling characteristics of spark plugs. Plugs with longer nosed insulators take a longer time to dissipate heat than plugs with shorter nosed insulators. These are termed "hot" or "cold" plugs, respectively. It is generally advisable to use the factory recommended plugs. However, in conditions of extremely hard use (cross-country driving in summer) going to the next cooler heat range may be advisable. If most driving is done in the city or over short distances, go to the next hotter heat range plug to eliminate fouling.

Broken or cracked insulators are usually the result of improper installation. Broken lower insulators often result from improper gapping and are usually visible immediately. When gapping a spark plug, always make the gap adjustment by bending the ground electrode. Spark plugs with broken insulators must always be replaced.

Damaged spark plug wires and/or boots cause a similar condition to that of a cracked insulator. The high voltage arc flashes through the wire or boot and grounds on the spark plug shell or the engine.

Spark plugs are protected by an insulating boot comprised of heat resistant material which covers the spark plug terminal and extends downward over a portion of the spark plug insulator. These boots prevent the flashover that causes engine misfiring.

REMOVAL & INSTALLATION



See Figures 1 through 7



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Fig. Fig. 1: Spark plug location at the front of the engine



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Fig. Fig. 2: Removing the spark plug wires from the retainer



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Fig. Fig. 3: Removing the spark plug boot from the spark plug



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Fig. Fig. 4: Removing the spark plug with a spark plug socket



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Fig. Fig. 5: Checking spark plug gap



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Fig. Fig. 6: Bending the side electrode to adjust spark plug gap

This engine has aluminum cylinder heads. Allow the engine to cool before removing spark plugs. Removing the plugs from an engine at operating temperature may damage the spark plug threads in the cylinder head.



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Fig. Fig. 7: Ensure the spark plug hole area is clean prior to installing the new spark plug

  1. Disconnect the negative battery cable.
  2.  
  3. Twist the spark plug boot 1 / 2 turn and remove the boot from the plug. DO NOT pull on the wire itself as this will damage wire.
  4.  
  5. Remove the spark plug from the cylinder head using the proper size spark plug socket.
  6.  
  7. If removing the plug is difficult, drip some penetrating oil on the plug threads, allow it to work, then remove the plug. Also, be sure that the socket is straight on the plug.
  8.  

To install:
  1. Inspect spark plugs for electrode wear, carbon deposits and insulator damage.
  2.  
  3. Check the spark plug. The ground electrode (the L-shaped wire connected to the body of the plug) must be parallel to the center electrode and the specified size wire gauge (see Tune-Up Specifications) should pass through the gap with a slight drag.
  4.  

Always check the gap on the new plugs, they are not always set correctly at the factory.

  1. Wire gapping tools usually have a bending tool attached. Use that to adjust the side electrode until the proper distance is obtained. DO NOT use a flat feeler gauge when measuring the gap, because the reading will be inaccurate.
  2.  
  3. Set new spark plug gap to 0.039-0.043 in. (1.0-1.1 mm).
  4.  
  5. Using a drop of oil, install the spark plug in the cylinder head and tighten to 21 ft. lbs. (20 Nm).
  6.  
  7. Using dielectric grease on the spark plug boot, install the ignition wires on their respective plugs. Make sure you feel them click into place.
  8.  
  9. Connect the negative battery cable.
  10.  

 
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