How to Replace Spark Plugs and Wires

In this Instruct-able, I will go through the process of changing the spark plugs and spark plug wires using a 1996 Mustang GT as an example. You will need a few thing before you get started:

1. Adjustable wrench

2. 3/8 in Drive ratchet

3. 3/8 in Drive 6 in extension

4. 3/4 Spark plug Socket 3/8 in Drive

5. 8 New Spark Plugs

6. 8 Spark plug Wires

7. 2 Clean Rags

8. 1 tube of Dielectric Compound

Step 1: Battery Disconnect

First things first, What you need to do before attempting any work on a vehicle is to remove the positive terminal cable, so that you are not at risk of an accidental shock. This can be done using an adjustable wrench to match the nut. Next step is to label each of your spark plug wires. Any number pattern is acceptable, as long as it allows you to remember where each wire came from.

Step 2: Intake Removal

Once each wire has been labeled, your next step is to remove the intake tubing for the air filter and mass airflow sensor in order to access the rear of the passenger side of the engine. This saves you some busted knuckles trying to turn a ratchet in a tight space.This can be done with a flat head screw driver and some patience. Be sure to put a clean rag inside the throttle space in order to keep the intake clean.

Step 3: Wire and Plug Removal/Replacement

The next step in the process is to remove one wire and plug at a time and replace them as you go. Doing this saves you from having to use the firing order diagram that determines the where each wire goes. The wires in the image are highlighted red.

Step 4: Wire Removal

In order to remove a wire on a 1996-98 model mustang as shown, first grip the top of the plug boot, closest to the head in a firm grip. Then with a slight twisting motion and upward pulling motion the plug boot should unseat from the plug intact. Severely damaged or old plug boots may rip. You will need to make sure you remove all of the material from the plug well, to ensure you don't accidentally put any rubber inside the engine.

Step 5: Spark Plug Removal

Next step is to remove the Spark plug from its well. You will need a 3/8 inch ratchet and a 6 inch extension as well as a 3/4 in spark plug socket. Spark plug sockets have a small rubber insert in order to grip the plug as you remove it. The spark plug should be fairly tight as you start. Once you get it started it should come out freely. If you experience any resistance when removing immediately stop and check to see if you are going in the correct direction. if you are then check to see if you are straight on the plug as you can damage the threads within the head of the engine.

Step 6: Spark Plug Gap

once the plug has been removed, you can then take your new plug and gap them to the factory specification. The gap determines how fuel efficient the engine is. The standard gap is approximately .045 in for Ford 4.6 V8s. Once you have gapped your plug to the correct specifications, go ahead and insert it into the spark plug well, and start the threads by hand. NEVER START THREADS WITH THE WRENCH!! If you start threads with the wrench you run the risk of cross threading the head, which is a catastrophic failure of the engine.

Step 7: Wire Replacement

Moving on, the spark plug has been replaced, so the wire and boot must now be replaced or re-installed. Before you re-install the wire be sure to use Di-Electric Compound on both plug ends to both protect the wire from the elements and to prolong the efficiency of the equipment. It also allows for much easier removal of plug wires, as it is a Silicone based lubricant.

Step 8: Repeat

Repeat steps 4 through 7 until you have replaced all of your wires. In a V8 there will be 8 wires and plugs. Be sure to count them if you are unsure of your progress. One tip is to buy spark plug wires that are of a different color than the original ones, so that they are easily distinguished.

Step 9: Battery Reconnection

The battery can now be reconnected at this point in time. Once again, the positive terminal can be reattached with an adjustable wrench.

Step 10: Intake Manifold Install

Re-install the intake manifold, keep in mind that you need to remove the rag that was placed inside the throttle before reinstalling the tubing.

Step 11: Test Run

Now that all of the plugs and wires have been replaced, and the battery and intake have been reconnected, now is the time to see if your work was successful. Keep in mind that when you removed the battery terminal, you will have to reset all of your presets in your radio, and if your car had the digital clock, you must reset that as well.

Step 12: Troubleshoot Stage 1

If the car fails to start, First check the battery terminal for a good connection. If it is corroded, a good way to clean the acid would be to mix some baking soda into some distilled water. Use a tooth brush to apply it and scrub the corrosion off before reconnecting the terminal. be sure to clean all residue off before reconnecting.

Step 13: Troublshoot Stage 2

If the engine fails to start or is running but has a rough idle or is having issue running, the next step is to check the plug connections and the plug boots to see if they are seated. Once ensuring the plug boots are seated and your connections are correct based on the diagram provided. Attempt to start the engine again.

Step 14: Troublshoot Stage 3

If the engine still fails to start after the first two attempts, check to see that there are not any fuel injectors unplugged. The final check would be to make sure your spark plugs are torqued to specification within the head in order to avoid compression loss.

Step 15: Final Step!!!

If your car started on the first try, Congratulations! You got lucky. Its good practice to double check your work to ensure you don't create an issue away from home, such as a melted wire on exhaust. Make sure to clean up any di-electric compound you may have spilled, and be sure to secure any loose cardboard from the packaging of your replacement parts. Enjoy your drive!