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1980-1996 Ford F100/F150/F250/F350 Trucks (including 1997 F250HD and F350) and Bronco Spark Plug Replacement

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This page is taken directly from our digital online manual 36058 for the 1980 to 1996 Ford F100, F150, F250, and F350 pickup trucks (including 1997 F250HD and F350), plus Bronco SUV, with 3.8 liter V6, 4.9L/300 cubic inch inline six, and 255, 302, 351, 400 and 460 cubic inch V8 engines. It covers removal and replacement of the spark plugs, and and checking and adjusting the spark plug electrode gap.


Note: Every time a spark plug wire is disconnected from a plug, the distributor cap, or coil, silicone dielectric compound (available at most auto parts stores) should be applied to the inside of the spark plug boot or terminal before reconnecting.
  1. The spark plugs are located on the left side of the engine (4.9L/300ci inline six) or on both sides of all other V6 and V8 engines.
     

  2. (See illustration) In most cases, the tools necessary for spark plug replacement include a dedicated spark plug socket (spark plug sockets are padded inside to prevent damage to the porcelain insulators on the new plugs), a ratchet handle to turn it, extensions to make it easier to reach the pugs, and a gap gauge to check and adjust the gaps on the new plugs (though most come properly gapped today). A torque wrench should be used to properly tighten the new plugs, without overtightening.
    A special plug wire removal tool is available for pulling the wire boots from the spark plugs, but it isn’t absolutely necessary. 


Tools required for changing spark plugs

Tools needed - 1) Spark plug socket, 2) Torque wrench, 3) Ratchet, 4) Extension, 5) Gap tool

1) Spark plug socket - These sockets will have special padding inside to protect the spark plug’s porcelain insulator.

2) Torque wrench - Although not mandatory, using a torque wrench is the best way to ensure the plugs are tightened properly.

3) Ratchet handle - This is the standard handle to turn the spark plug socket. You should never use a torque wrench for loosening bolts or general ratcheting.

4) Socket extension - Depending on engine, model and accessories, you may need special extensions and universal joints to more easily reach some of the plugs.

5) Spark plug gap gauge - These tools to check and adjust the spark plug gap comes in a variety of styles. Make sure the recommended setting for your engine is included.


 

  1. The best approach when replacing the spark plugs is to purchase the new ones in advance, check the gap, and adjust them if needed. Then replace the plugs one at a time so the plug wired do not get mixed up. When buying spark plugs, check the information found on the  Vehicle Emission Control Information (VECI) label located under the hood and in the owner’s manual. If differences exist between the plug number specified on the emissions label and in the owner’s manual, assume that the emissions label is correct.
     
  2. Allow the engine to cool completely before attempting to remove any of the plugs. While you are waiting for the engine to cool, check the new plugs for defects and adjust their gap.
     
  3. (See illustration) The gap is checked by inserting the proper thickness gauge between the electrodes at the tip of the plug. The gap between the electrodes should be the same as the one specified on the VECI label. The wire should just slide between the electrodes with a slight amount of drag. If the gap is incorrect, use the adjuster on the gauge body to bend the curved side electrode slightly until the proper gap is obtained. If the side electrode is not exactly over the center electrode, bend it with the adjuster until it is. Check for cracks in the porcelain insulator (if any are found, the plug cannot not be used).
Use a wire-type gap gauge. If the wire does not slide between the electrodes with a slight drag, adjustment is required

Spark plug manufacturers recommend using a wire-type gauge when checking the gap - if the wire does not slide between the electrodes with a slight drag, adjustment is required

To change gap bend side electrode only, as indicated by the arrows being careful of porcelain

To change the gap, bend the side electrode only, as indicated by the arrows and be very careful not to crack or chip the porcelain insulator surrounding the center electrode

  1. (See illustration) With the engine cool, remove the spark plug wire from one spark plug at a time. Pull only on the boot at the end of the wire to avoid damaging the wire. Do not pull on the wire itself. Spark plug wire removal pliers should be used if available. 
When removing spark plug wires, pull only on the wire boot with a twisting/pulling motion. Special pliers make this easier

When removing the spark plug wires, pull only on the spark plug boot with a twisting/pulling motion. Spark plug wire removal pliers make this easier.

  1. If compressed air is available, use it to blow any dirt or foreign material away from the spark plug hole. A common bicycle pump will also work. The idea here is to eliminate the possibility of debris falling into the cylinder as the spark plug is removed.
     
  2. Place the spark plug socket over the plug and remove it from the engine by turning it in a counterclockwise direction.
     
  3. Compare the spark plug you just removed to those shown in the guide below to get an indication of the general running condition of that cylinder, and the overall health of the engine.
Spark Plug reference chart
  1. (See illustration) A handy trick is to slip a short length of rubber hose over the end of the plug to use as a tool to guide the pulg, and thread it into place. The hose will grip the plug well enough to turn it, but will start to slip if the plug begins to cross-thread in the hole. This will prevent damaged threads and the accompanying large repair costs. Thread one of the new plugs into the hole with your fingers until you can no longer turn it, then tighten it with a torque wrench (if available) or the ratchet.
The hose will allow you to turn the plug, but not cross thread it

A length of snug-fitting rubber hose over the spark plug will save time by helping you guide the plug into tight places and prevent damaged threads when installing the spark plugs. The hose will allow you to turn the plug, but not cross thread it. 

  1.  Before pushing the spark plug wire onto the end of the plug, inspect the wire and boot for cracks, heat damage, abrasion, and other issues, as outlined in Section 27 of the Haynes manual. 
     

  2. Attach the plug wire to the new spark plug, again using a twisting motion on the boot until it is seated on the spark plug.
     

  3. Repeat the procedure for the remaining spark plugs, replacing them one at a time to prevent mixing up the spark plug wires.


This video is a general overview of inspecting, gapping, removing and replacing the spark plugs on the typical vehicle. Much of it applies to the motors installed in Ford trucks, but it is meant merely as a supplement to the above instructions.