In this tech article I will go over the incredibly simple steps needed to tune –up your vehicles ignition system. While this article is specific to my 325is, it applies to all BMW models in general.
That said let’s begin. First, disconnect the battery. This is important as we will be working around electricity, and you don’t want that random voltage spike damaging sensitive components on your car. Even worse, you don’t want a surge of power coursing through your body.
Once disconnected, open the hood and look at the passenger side of the engine. You will see the following components:
- Ignition Wires (called HT leads in the UK and other parts of the world)
- Spark Plugs
- Distributor Cap
- Distributor Rotor (under the distributor cap)
During a normal ignition tune-up all of these items are replaced. It’s recommended that you carry out this procedure at 80,000 miles, however problems such as fouled pugs and worn rotors may prompt you to carry this procedure out earlier.
The first step is to look at the spark plug wiring loom that runs parallel to the valve cover. You will see two 10mm nuts holding onto the valve cover. Remove these nuts and pull the loom clear of the studs. Next, disconnect the wire going to the coil, which is mounted on the inner passenger fender.
Now pull each wire off each spark plug. They should come right off, however if they seem stuck, just give the connector a twist and it should free it up. Next, look at the distributor at the front of the engine. Remove the plastic cover over the distributor by unclipping it. Now, use a small 10mm open-end wrench to remove the three small bolts that hold the distributor cap in place. Remove the wires and cap together. Now look at the inside of the cap. You should be able to see the small contact points inside. They should appear slightly worn. If they show signs of pitting and/or burning, you will need to replace the cap. This wear is normal. It is the by-product of the voltage coming from the coil, then being distributed out to each individual spark plug. This corrosion and pitting is normal for a used cap. Sometimes, in a pinch, you can sand down the contacts and this will suffice, the same applies to the distributor rotor as well. Now, remove the three 10mm bolts holding the distributor rotor to the engine. Take a look at the contacting edge of the rotor. As before, if it shows any pitting or burning, it must be replaced.
Now, we will remove the spark plugs. I’ve found that on the M20 engine, all that is required is a spark plug socket, 4 inch extension and a ratchet. Now, remove the spark plugs and inspect them. A normal, well-used set of plugs should appear to have a tan/grey appearance. If they appear any other way, it’s a likely indicator of possible engine problems. Here’s a small list of what to look for on your plugs.
|Plug Condition||Possible Causes|
|Grey/Tan color, electrode rounded off ||Normal Wear|
|Oily deposits, Oil leaking into cylinder, possibly ||Worn valve guides or piston rings|
|Carbon deposits, ash on electrodes||Rich mixture, poor ignition, over-use Of fuel and/or oil additives |
|Blistered electrode, white appearance||Lean mixture, Overheating, vacuumleaks|
Once you have inspected the plugs, make a note of whatever mechanical repairs or adjustments must me made to the car to correct these problems. Keep in mind that spark plugs are usually a great indicator of how well your engine is running. Many times, I have made mixture and timing adjustments on older cars just by looking at the plugs.
Before we install the new plugs, check the ignition gap. Most plugs nowadays come “pre-gapped” however; I always check the ignition gap. There’s always that one time you get the spark plug that missed the eye of the quality inspector on the line. Most ignition gapers are available at your local auto parts store for usually under a buck. Check the gap by sliding the tool in between the contact arm and the electrode, and slide it around until it drags slightly, now read the mark on the side. This will give you the gap. The factory gap for the 325 6 cylinder is 0.7mm or 0.027 inches. Use the gapping tool to carefully bend the contact arm either closer to or away from the electrode to adjust the gap.
Now, put a small dab of anti-seize on the threads of the spark plugs and carefully thread them back into the cylinder head. Be very careful while doing this, as the cylinder head is aluminum and it’s very easy to strip the threads. Once seated, tighten the plugs down snug but do not over tighten, or you will be soon learning how to install a Heli-coil.
Take the new distributor cap and lay it out next to the old one with the wires still attached. We will now want to open up the new wire holder and determine which connection goes to where based on the old cap/wires. It is crucial you have these wires oriented correctly. This is what is commonly referred to as the vehicle’s firing order. In this case, the firing order on the 325 is 1-5-3-6-2-4 What this means is the voltage from the coil will travel through the distributor, down through the rotor, and when the rotor turns to the number 1 cylinder contact on the distributor cap, it sends the voltage down through the spark plug wire to the spark plug, which in turn, ignites the fuel/air mixture in the cylinder, causing the engine to run. The rotor will now turn in sequence to the number 5 contact, then repeat the procedure, then rotate to the number three cylinder contact, then 6 and so on, til it rotates back to number 1.
To make sure it is correct, lay out the old wires next to the new wires and cap. Each cylinder on the engine has a specific length, so all wee need to do is compare the lengths of new wire to the old, and connect them to the new cap as they are installed on the old cap. Once you have the new wires on the new cap, place the wires back in the holder.
Now, take the new distributor rotor and bolt it back on to the distributor shaft using the three 10mm bolts. Make sure that the dust shield is correctly in place and place the new distributor cap/wires back onto the housing. It is indexed so that it will only go on one way. Once in place, re-install the 10mm bolts that hold it on. Now place the distributor cover back in place and clip it on.
Next, take the wire holder and place the two brackets over their corresponding studs on the valve cover. Once on, re-install the two 10mm nuts and tighten them down. Once the wire holder is secured, take each individual wire connector and slip it on each spark plug. You should be able to feel the connector seating on the plug as you push it on. Don’t forget to install the coil wire as well.
The last step is to re-connect the battery and start the car. You should notice that the car has a little more pickup and runs a bit smoother.
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