Spark Plugs - First Generation Cars
| 01 December 2005
When the engine in your Mustang is running without skipping, chances are you don’t think much about the spark plugs. But if it starts to skip, your first thought could easily be, “It’s time to change the plugs.” To be sure, skipping can be caused by other maladies but bad spark plugs are always a good place to start, especially if the engine has been running well up to that point.
With a basic understanding of spark plugs and how to install them, it’s a relatively easy job to change them and, at the same time, to evaluate what’s going on in the combustion chamber. You’ll find the job very easy on the 6s but with fancy contortions required on some of the V8 installations, especially the big blocks.
The first step should be to remove the old plugs and look at their condition. They can tell you a lot about the condition of your engine, at least the top end. The combustion chamber can be a very unfriendly place to live and certain telltale evidence on the plugs can indicate if their life has been good or if problems are looming.
To remove the plug, first remove the spark plug wire from where it slips onto the connector at the top of the plug. Don’t pull on the wire; you may damage it. Twist the boot to break it free from the plug insulator and then grasp the boot to pull it off the plug. You can buy a tool that will help you pull the boot off if you can’t get it by hand. Blow out any dirt from around the base of the spark plug where it screws into the cylinder head. Use a 6-point spark plug socket attached to a ratchet or small breaker bar to loosen the plugs. On some of the V8s, you might have to get creative with your tools to access and loosen one or more of the plugs in a confined space. Take out all of the plugs and keep them in rotation so you can relate individual conditions to a specific cylinder.
In a perfect world, the business end of the spark plug should have electrodes that aren’t eroded, it should be dry and relatively clean, and it should have an off-white to light gray or tan color on the ceramic tip around the
center electrode. If your plugs have that appearance, life in the combustion chamber has been good. Go ahead and replace them anyway but recognize that you may not have found the source of the skipping. If they don’t have the “good life” look, their condition can help you understand what’s been going on.
Plugs with a dry black fluffy coating are carbon fouled because they’ve been running too cold. A number of things can cause that condition including a plug with the wrong heat range. When you buy plugs, ask for them by application rather than by plug number just in case the manufacturer has changed specifications. Other causes for cold plugs can include a rich mixture, excessive idling, dirty air cleaner element, and an improperly operating heat riser.
Plugs with small black, gray, or brown spots on the ceramic insulator around the center electrode have been running too hot. Usually, the electrodes are a bluish burnt color also. Causes include wrong heat range, overheating engine, loose spark plug, and incorrect ignition timing.
Plugs with melted electrodes and possibly with blisters on the ceramic around the center electrode are the victims of pre-ignition. This is a serious condition that can cause engine damage as well as spark plug damage. Possible causes include wrong type of fuel, incorrect timing, incorrect advance curve, wrong spark plug heat range, burnt valves, or overheating engine.
Plugs with wet black deposits are oil fouled from excessive oil entering the combustion chamber. Worn valve guides or worn rings could be the cause. The engine will need to be repaired in the near future but going to a slightly hotter plug in the short term should help correct the fouling.
Plugs that have heavily eroded electrodes but otherwise have good color and minimal deposits are worn. If this is the condition you find, replacing them should take care of your skipping problem while indicating that the top end of your engine is in generally good shape.
Ok, now that removal and diagnosis are behind us, let’s put in the new plugs. The first step is to gap them to specification; i.e., set the air gap between the center electrode and the grounding electrode. You can use a feeler gauge but a wire gauge designed specifically for spark plugs is more accurate. Another advantage of the wire gauge is that it has a built in tool to bend the grounding electrode to achieve the correct gap. You have the proper gap if the gauge has a light drag when it slides between the electrodes.
Screw the new plugs finger tight into the cylinder head. Installation by hand will assure that they aren’t cross threaded. If you have an aluminum head on your engine, put a light coating of anti-seize compound on the plug threads before installing them. Get out your torque wrench and tighten the plugs to specification, typically 15-20 ft-lb in a cast iron head. They should be torqued to manufacturer’s spec if the head is aluminum. Proper tightening in all installations is an important step. They must be correctly tightened not only to hold them in place but also to seal the combustion chamber and to provide good thread-to-thread contact for conduction of heat away from the spark plug body.
With the plugs installed, all that’s left’s to do is to install the spark plug wires. Push each boot into place over the plug until the connector-clip snaps into place. Be careful to connect the correct wire to each plug so that proper firing order is maintained.
If you were wondering how a hot plug differs from a cold plug, it has to do with controlling how fast spark plug heat is conducted away to the cylinder head. It has nothing to do with engine temperature, just the operating temperature of the plug. Manufacturers control this feature by changing the length of the ceramic insulator nose that surrounds the center electrode. The nose is a poor conductor of heat so the more ceramic mass there is, the hotter the plug will run. A hot plug has a long insulator nose extending high up into the cavity of the plug before it contacts the metal shell. Where it makes contact, the shell will conduct heat away from the nose and out to the cylinder head. A cold plug has a short nose which contacts the shell at a shallower position. Heat is conducted away sooner and from a smaller mass which makes the plug run colder. Because different engines have different requirements, there are many intermediate heat ranges in the progression from a very cold plug to a very hot plug. One of them is right for your engine.