WELCOME TO OUR Changing spark plug/high tension leads PROJECT
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Summary: In this guide you will learn how to identify your cars spark plug/high tension leads and replace them with a new set.
how to change a set of spark plug/high tension leads
Please note that this is only a general guide and is not specific to any one make or model of vehicle. For more detailed information on your own particular car, please consult a workshop manual specific to your own make and model!
Please read this first
If you have recently driven your vehicle, please leave it for at least an hour for the engine and other components to cool down. If you do not you may risk injury!
Required Tools:
1. 1 x flat head and 1 x philips screw drivers
2. 10mm socket + ratchet (if you need to remove an engine cover)
3. Set of new HT leads (check your vehicle user manual for exact part numbers for your particular model)
Step 1 – Open the bonnet
Open the bonnet of your vehicle via the release catch normally located just under the dash board on either the driver or passenger’s side, walk to the front of the car and slide your hand under the bonnet and squeeze the latch. When the latch releases prop the bonnet up using the provided support.
Some vehicles feature a “sound suppression” cover which will cover the entire top of the engine. To gain access to the HT (High Tension) leads this will need to be removed. This is usually secured in several positions.
Once you have removed this cover (if there is one) you can now begin.
Step 2 - Identify each lead
Depending on how you have bought your set of new leads, they usually come in an A4 size, shallow box. On emptying the contents you may notice that there appears to be five leads, but when you look at your cars engine, you can only see four. Relax, the fifth and seemingly spare lead is in fact just the coil to distributor lead and this will need replacing also. For now, just put this lead to one side.
If you lay out your new leads and reference these to the leads already in situ on your engine, each replacement lead should be apparent as they should all be differing sizes (the closer the spark plug to the distributor, the shorter the lead).
Step 3 - Changing the leads
TIP: before you pull all of the HT leads in your engine off, DONT! Each spark plug fires in a specific order and if you mix the leads up with what plug they should go to then your car will miss-fire and not run correctly. The easiest and most stress free method is to do one lead at a time! This way you should not mix them up.
Now that you have identified which lead should replace which, you can go about changing them.
We will start with lead number one (if you are working on a four cylinder, front wheel drive car, this should be the first lead on the top left of the engine. If you are working on an in-line engine, one that runs from front to back, this will be the first lead at the front).
Pull number 1 HT lead off of the top of the plug by grasping it as close to the top of the engine (or base of the plug) as possible. This may need a little persuasion as if they have not been moved for a while the natural heat of the engine may cause the lead to bond to the plug slightly. (If you have a four cylinder engine with twin over-head cams, the lead will feature rather a long rubber sleeve that slots down into the top of the engine and totally covers the entire spark plug and the hole that it is located in).
TIP: It is a good idea not to simply grab the lead and pull it off, as I said previously, grip the lead as close to the engine or as close to the base of the plug as you can. By just grabbing the lead and ripping it off you risk pulling the lead apart when you may be able to re-use it as a spare!
Now that you have removed the lead from the plug-end, trace the lead back to the distributor cap. Using the same "pulling method" mentioned above, remove the lead from the distributor (before you discard the old lead, take a second to make note of the different connecting ends of the H.T. lead eg. which end came off of the plug and which end came off of the distributor).
You have now removed your first lead. Now, go to your new set of leads and select your new lead (make sure that you have selected the correct one. The majority of newer leads should be numbered from 1 - 4 etc and, as mentioned should be differing lengths)
With your new lead selected, firstly connect the distributor end (make sure that you are connecting the right end of the lead). Push the lead down in firmly, but do not force it. There should be a little resistance.
If the lead is rather hard to push in, as you are pushing it try twisting it from clockwise to anti-clockwise until it is securely in place.
Now, get the plug end of the lead. With this in hand push the end over the top of the plug. This end should just click down over the top of the plug. (if you are working on an over-head cam engine, as said, the plug may be located in a hole and if you have plug leads that cover the plug entirely, they may be slightly awkward to push on. If so, use the afore mentioned twisting technique).
Again, there should be a little resistance as you push, but don't force it. If the lead simply wont fit over the plug, pull it back off and assess the situation.
Now that you have changed the first lead, repeat steps 2 and 3 for the rest.
Once you have changed all of the spark plug HT leads you may notice you have one left. This is the coil to distributor lead. As with the other leads grip each end and remove making sure that you pay attention to each end as the connectors may be different. Once the old lead has been removed, replace with the new one.
Step 4 - Testing
With all of your leads replaced, it is now time to test your workmanship. Firstly, remove all tools and unwanted objects from the engine bay. Now start your car. If all is correct it should fire and start first time. If not, the likely hood is that you have got one of the HT leads on the wrong plug and the engine in not firing in the correct order. To remedy this, you will have to research into your engines firing order, correspond the firing order to the points on the distributor cap and set the engine to top dead center and then reposition the leads in the correct order.
You have now successfully completed this project.
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