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.
disconnected, open the hood and look at the passenger side of the engine.
You will see the following components:
Wires (called HT leads in the UK and other parts of the world)
Rotor (under the distributor cap)
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.
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.
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
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.
electrode rounded off
|Oily deposits, Oil
leaking into cylinder, possibly
||Worn valve guides or
|Carbon deposits, ash on
||Rich mixture, poor
ignition, over-use Of fuel and/or oil additives
Overheating, vacuum leaks
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.
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.
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.
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.
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.
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.
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.
Well, there you have it - it's
really not too difficult at all. If you would like
to see more technical articles like this one, please continue
to support Pelican Parts with all your parts needs. If
you like what you see here, then please visit our online BMW
catalog and help support the collection and creating of
new and informative technical articles like this one.
Your continued support directly affects the expansion and
existence of this site and technical articles like this
one. As always, if you have any questions or comments
about this helpful article, please drop us a