For some time, your engine has started, but more difficult, and it does not purr as usual. Sure, there is an ignition problem, and maybe it is from the spark plug wires bundled together. These cables get damaged over time, usually at both ends, towards the spark plug or coil. To replace them, you must first find them in the engine, measure their lengths and then install them correctly.
Method 1 of 3: Identify an ignition wire problem
Step 1. Open the hood of your car
The hood release lever is usually located under the dashboard on the driver's left side. On many cars, you have to put the kickstand in place, on others, the pneumatic hood holds up by itself. Anyway, check that the hood is holding tight so as not to receive it on the head.
Step 2. Locate the ignition harness wires
These slightly thick black wires are very visible, they all arrive on the top of the cylinder head. Each wire in the ignition harness connects the ignition coil (with 4 or 6 wires) to a spark plug.
Step 3. Be aware that spark plug cables can get damaged
When the engine is running, these electric wires are continuously crossed by a high voltage current, so that over time their resistance eventually increases. The current then circulates more difficult, slowed down as it is by the resistance of the copper. The spark plugs are not receiving enough current to properly and completely ignite the mixture in the cylinders. Likewise, if the insulation plastic is damaged and exposed the copper wire, you will have ignition losses. In both cases, the faulty cables must be replaced.
Step 4. See whether or not to change your spark plug cables
It is not so much a question of duration as a degradation of the cables. See if a wire is damaged, if the motor does not "hum" as usual. If, with the engine running, you see small sparks coming out of the spark plug, it is time to replace the faulty cable.
- There are signs of a faulty ignition: the engine starts poorly, has misfires, does not run very smoothly… These problems do not necessarily come from the ignition wires and can have other causes, such as spark plugs running out or a more serious ignition problem. You need to make sure that it is indeed a cable problem.
- If, with the engine running and the hood up, you see little sparks coming out of the spark plugs, it's almost certainly a wire problem. Depending on the degree of wear of these, the sparks will come either from one place or from several.
- Inspect your cables for a fault. The wire can be cut more or less deeply, it can be cracked, or even partially melted. In all these cases, it is necessary to replace the cable (s).
Step 5. Count the number of cables to be replaced
Now that you have the number and type of wires to change, you can go and buy them from your dealer. The seller will ask you, for security, the model and year of the vehicle in order to sell you the right cable or harness.
Step 6. Purchase identical cables
It is possible to buy only one cable, but in general the whole harness is replaced. If your engine has six cylinders, your harness has six cables. A harness is made up of cables of different lengths. Buy one that is exactly the same as the old one. The length of the cables is also not to the millimeter, you have a margin.
- Each car manufacturer provides its customers with spare cables with lengths a little longer than the original ones to suit the other models in their range. However, before you leave the dealership, make sure your cables are the correct length.
- Don't skimp on quality! There are bundles to assemble yourself, they are less expensive, but the quality is often lower and you have to assemble the different parts, which is not within everyone's reach.
- The automobile manufacturers accept no responsibility if you shorten your cables during a repair. Unless you know it, avoid this kind of hacking or you could bite your fingers!
- The complete harness is often sold, but some stores sell the spark plug cables individually.
Method 2 of 3: Remove the spark plug cables
Step 1. Work only with the engine stopped
Never attempt to replace a spark plug wire while the engine is running. The best is even to remove the key from the dashboard. Likewise, do not work on an engine that is still hot.
Step 2. Locate your cables
Once you find the faulty wires, note their length and direction of connection. Indeed, it is a question of having cables of the same lengths and of connecting them in exactly the same direction. If you reconnect your cable to the wrong spark plug, you may simply not be able to start or your engine will make a funny noise. The amateurs put an adhesive tape on the cable with a number which corresponds to the number of the spark plug: the error is then no longer allowed.
Step 3. Be methodical
The spark plug cables are replaced one after the other. Indeed, the ignition of the mixture in the pistons is done in a precise order. So, by replacing only one wire at a time, you won't have any ignition problems. Remove a first cable, replace it with an identical wire, check that it is firmly inserted on both sides, then go to the next one.
- A spark plug wire runs from the igniter to a spark plug. It must be disconnected from both sides to replace it.
- The spark plug should trigger a spark when the piston is at its highest, and ignition is sequential. This is why it is important to respect the location of the spark plug wires. Always start on one side (right or left), then do the next cable.
Step 4. Disconnect the cables
Some people recommend using a special tool (wire puller), but the removal is very well done by hand. The cabochon contains a small metal cylinder that fits tightly to the head of the candle, which is then protected from dirt and moisture. Remove the cable by pulling on the lens. If you pull while grasping the cable, you risk separating the cabochon from the cable.
- Some cables adhere a little too much to the head of the spark plug. To disconnect them, you have to take the cap with your full hand and pull firmly. If there is resistance, turn the cabochon back and forth while pulling it toward you.
- Take a look at the candle and inside the cabochon. If you see black marks, there is a problem with the ignition on the spark plug. Disassemble it and see the state of the electrodes.
Method 3 of 3: Install spark plug wires
Step 1. Reassemble the assembly
Install the new cables exactly the same as the old ones. Before putting the cabochon back on the candle, you can put a little copper grease on the spark plug terminal. The cabochon must then be well inserted on the tip of the candle. There is a laying direction for the ignition wire, wind it up exactly as it was. Likewise, the wires must be reassembled in exactly the same way as before, the mixture in the cylinders is ignited in a precise order. If you plug it in in the wrong order, the engine will not start. Be careful that your ignition wires do not cross each other or are near moving or hot parts.
- The spark plug cables are generally attached to loops sealed on the engine block so that they do not melt on contact with heat. This is why when replacing spark plug cables, they must be put back in these loops to avoid having to change them again. After replacement, follow the cable to see if it does not touch the engine block.
- If you replace your cables with higher performance cables (shielded, for example), beware, as they are sometimes of a larger diameter than the original ones. If this was the case, be aware that there are wider brackets which are easy to install as they only hold by a bolt.
Step 2. Lower and lock your hood
Before getting into your car, check that the hood is securely closed by lifting it upwards. If it does not move a millimeter, it is well closed. If you wish to open it, you will have to operate the release lever.
Step 3. Listen to your engine
Once the cables have been replaced, start your engine as usual. It should easily start and run like clockwork. It should be purring like a cat now that you've replaced the cables at the end of their travel. If the engine is not running perfectly, if it misfires, you will have to check everything. Start by seeing if the cables are well secured on the ignition head or on the spark plugs, if there is not one which is a little too tight, if you have connected the correct cylinder to the correct outlet of the ignition head. In short, check everything.
- Never touch an electric cable of a running engine! You could be electrocuted. The ignition system delivers tens of thousands of volts at each turn on each spark plug. If ever one of the cables got loose and you put your fingers in the wrong place, you might have a funny surprise. As you are on the ground and the human human body is a conductor, the current will pass through you, with the damage that one imagines.
- After changing your cables, if you notice that the idle is chaotic or the acceleration in neutral is not linear, there is a good chance that some cables are plugged into the wrong spark plugs.
Step 4. Take a road test
An ignition problem is verified by running the car under or over speed. Either way, the problem, if it exists, will reveal itself in all its glory. On the highway, if you push your engine while accelerating sharply (overspeed), your engine will suffocate. On a slope, if you don't downshift (under-rev), you are going to have a tremendous loss of power.
Step 5. Your car is fixed
You will be able to swallow thousands of kilometers.
- In order not to make connection errors, it is best to replace spark plug wires one after the other in order to avoid connection problems.
- On recent vehicles, there is no longer an ignition harness, but a monobloc ignition manifold that can be changed in one piece.
- Before disconnecting a spark plug wire, note where it connects on one side to the ignition head and on the other side to the spark plug. The proper functioning of the engine depends on it.
- Water and electricity do not mix. To see if you have current losses on a cable, you can spray three or four drops of water on the suspect cable. You should see a small electric arc between the cable and the engine block. Certainly, the cable needs to be changed.