The purpose of your vehicle’s spark plugs is to ignite the fuel and air available in the combustion chamber. They do this because of a signal received from the computer module or distributor cap.
If the ignition cable, or spark plug wire, carrying this signal fails, the operation of the engine becomes mistimed and under-powered. One or more cylinders will may misfire or weaken. Another result of the fuel and air not combusting completely is a buildup of gases and residue in your injectors or cylinders.
The symptoms of failing ignition cables include rough idling, an illuminated Check Engine Light, and a general lack of engine among others. This all can be very easily avoided with a few simple steps.
Part 1 of 1: Replacing the ignition cables
Socket set and ratchet
Spark plug wire grease (optional)
Tip: When buying replacements, make sure your replacement ignition cables are the correct length. Use your old cables for reference if necessary. The placement of the cable on each cylinder will determine the length of the cable from the distributor or module.
Step 1: Disconnect the battery. Disconnect the negative battery cable to cut power to the ignition cables.
Disconnect the bolt holding the cable to the terminal using a socket or a wrench.
- Warning: Set negative battery cable aside so that it will not make contact with anything metal, or a connection may be made restoring power to the cables.
Step 2: Locate the ignition cables. The cables will be routed from the spark plugs on the top of the cylinders to the distributor cap or module that powers them.
Step 3: Replace the cables. One by one, remove and replace the spark plug wires.
By doing them one at a time, you will not have to worry about accidentally switching the wires.
To remove an old cable, pull directly upward on the boot of the cable on the spark plug end, and then pull upward on the boot connected to the distributor cap or module. Be sure to only pull on the boot; DO NOT pull on the cable itself.
- Tip: If you are sure you are not going to use the old spark plug cables again, you can use a pair of pliers to help you remove them. The pliers will most likely damage the boot and terminal of the old wires, so it is not recommended that you use pliers on any ignition cables you plan to use again. Otherwise, you can do it by hand or with a spark plug removal tool.
Again, make sure that the length of the cable you disconnected is the same as the length of the new cable. Extra wire is unnecessary and your engine may not have room to compensate.
What is even more imperative than matching cable length is that you do not mix up the order of the ignition cables. The spark signals are sent in a particular order to each cylinder to time when the piston is at top dead center (at the very top of the cylinder). Improper installation of these cables can result in poor combustion or misfiring of the cylinder, which will cause drivability issues and potentially severe damage to the engine.
- Tip: If the order of the wires ever gets confused, look up the firing order for your exact vehicle for reference.
When removing the spark plug or ignition cables, inspect them for signs of other issues with your vehicle. The most easily detected are signs of burning carbon or oil. This would indicate a failing valve cover gasket and/or failing o-rings around the old spark plugs.
To install a new cable, push the new ignition cable boot on one end to the module, and then push the next end onto the spark plug.
- Tip: If you wish to use spark plug wire grease (dielectric grease), insert a small drop into the new boot before placing it onto the spark plug.
Move on to the next cable and repeat this step.
Step 4: Reconnect the battery. Reattach the negative battery cable to the terminal to restore power.
Hand tighten the retaining bolt and snug it down with a wrench or socket.
Close the hood of the vehicle once you are finished with this step.
Step 5: Test drive the vehicle. With the car in park, start the vehicle. If the idle is holding steady between 600 and 1,000 RPMs, go for a test drive and observe your vehicle’s drivability.
- Note: Listen for stuttering, rough idling, and misfires, and feel for any sluggishness.
The ignition system on your vehicle plays a very large role in its operation. Issues with the ignition system are going to slow down and depower your engine. Extended use under these conditions will cause various damages and wear out other parts involved. If you follow the steps laid out in this guide, you can fix these issues and avoid further damage. However, if you’d prefer to have a professional perform this repair, you can always count one of the certified technicians at YourMechanic to properly replace your ignition cables for you at home or at the office.
This article originally appeared on YourMechanic.com as How to Replace Ignition Cables (Spark Plug Wires) in Your Car and was authored by Jessica Howe.