Before attempting any work on the cylinder head, it is very important to note that the cylinder head is cast aluminum alloy. This means that it is extremely easy to damage threads in the cylinder head. Care must be taken not to cross-thread the spark plugs or any bolts or studs. Never overtighten the spark plugs, bolts, or studs.
To avoid cross-threading the spark plugs, always start the plugs in their threads with your fingers. Never force the plugs into the cylinder head. Do not use a wrench until you are certain that the plug is correctly threaded.
VW spark plugs should be cleaned and regapped every 6,000 miles and replaced every 12,000 miles.
SPARK PLUG HEAT RANGE
See Figures 1 and 2
While spark plug heat range has always seemed to be somewhat of a mystical subject for many people, in reality the entire subject is quite simple. Basically, it boils down to this; the amount of heat the plug absorbs is determined by the length of the lower insulator. The longer the insulator (or the farther it extends into the engine), the hotter the plug will operate; the shorter the insulator the cooler it will operate. A plug that absorbs little heat and remains too cool will quickly accumulate deposits of oil and carbon since it is not hot enough to burn them off. This leads to plug fouling and consequently to misfiring. A plug that absorbs too much heat will have no deposits, but, due to the excessive heat, the electrodes will burn away quickly and in some instances, preignition may result. Preignition takes place when plug tips get so hot that they glow sufficiently to ignite the fuel/air mixture before the actual spark occurs. This early ignition will usually cause a pinging during low speeds and heavy loads. In severe cases, the heat may become high enough to start the fuel/air mixture burning throughout the combustion chamber rather than just to the front of the plug as in normal operation. At this time, the piston is rising in the cylinder making its compression stroke. The burning mass is compressed and an explosion results, forcing the piston back down in the cylinder while it is still trying to go up. Obviously, something must go, and it does-pistons are often damaged.
The general rule of thumb for choosing the correct heat range when picking a spark plug is: if most of your driving is long distance, high speed travel, use a colder plug; if most of your driving is stop and go, use a hotter plug. Factory-installed plugs are, of course, compromise plugs, since the factory has no way of knowing what sort of driving you do. It should be noted that most people never have occasion to change their plugs from the factory-recommended heat range.
REMOVAL & INSTALLATION
See Figures 3 through 11
To remove the spark plugs, remove the spark plug wire from the plug. Grasp the plug connector and twist/pull the spark plug cable off of the spark plug; while removing, do not pull on the wire. Using a 13 / 16 in. spark plug socket, remove the old spark plugs. Examine the threads of the old plugs; if one or more of the plugs have aluminum clogged threads, it will be necessary to rethread the spark plug hole. See the following section for the necessary information.
Obtain the proper heat range and type of new plug. Set the gap by bending the side electrode only. Do not bend the center electrode to adjust the gap. The proper gap is listed in the "Tune-Up Specifications" chart. Lubricate the plug threads.
Start each new plug in its hole using your fingers. Tighten the plug several turns by hand to assure that the plug is not cross-threaded. Using a wrench, tighten the plug just enough to compress the gasket. Do not overtighten the plug. Consult the torque specifications chart.
RETHREADING SPARK PLUG HOLE
See Figure 12
It is possible to repair light damage to spark plug hole threads by using a spark plug hole tap of the proper diameter and thread. Plenty of grease should be used on the tap to catch any metal chips. Exercise caution when using the tap as it is possible to cut a second set of threads instead of straightening the old ones.
If the old threads are beyond repair, then the hole must be drilled and tapped to accept a steel bushing or Heli-Coilreg;. It is not always necessary to remove the cylinder head to rethread the spark plug holes. Bushing kits, Heli-Coilreg; kits, and spark plug hole taps are available at most auto parts stores. Heli-Coilreg; information is contained in the "Engine Rebuilding" section of this guide.
CHECKING & REPLACING SPARK PLUG CABLES
Visually inspect the spark plug cables for burns, cuts, or breaks in the insulation. Check the spark plug boots and the nipples on the distributor cap and coil. Replace any damaged wiring. If no physical damage is obvious, the wires can be checked with an ohmmeter for excessive resistance. Remove the distributor cap and leave the wires connected to the cap. Connect one lead of the ohmmeter to the corresponding electrode inside the cap and the other lead to the spark plug terminal (remove it from the spark plug for the test). Remove the static suppressor boot from the end of the cable by un-screwing it before testing cable. Replace any wire which shows over 50,000 ohms. Generally speaking, however, resistance should not run over 35,000 ohms and 50,000 ohms should be considered the outer limits of acceptability. Test the coil wire by connecting the ohmmeter between the center contact in the cap and either of the primary terminals at the coil. If the total resistance of the coil and cable is more than 25,000 ohms, remove the cable from the coil and check the resistance of the cable alone. If the resistance is higher than 15,000 ohms, replace the cable. It should be remembered that wire resistance is a function of length, and that the longer the cable, the greater the resistance. Thus, if the cables on your car are longer than the factory originals, resistance will be higher and quite possibly outside of these limits. Test the static suppressor boots separately-resistance should not exceed 5,000-10,000 ohms.
When installing a new set of spark plug cables, replace the cables one at a time so there will be no mixup. Start by replacing the longest cable first. Install the boot firmly over the spark plug. Route the wire exactly the same as the original. Insert the nipple firmly into the tower on the distributor cap. Repeat the process for each cable.