Spark plugs should be replaced every 10,000mi or once a year. It's a
fairly simple process and takes less than a half hour to do. It is
however important to take your time and pay attention to detail because
a mistake here can cost you time or money. Since the plugs will be
removed, I also performed a compression test, which you can skip over if
it doesnít interest you. This article features a 6cyl 325, a 328 will be
almost identical, and a 318 will be even easier (more on the 4cyl
Ideally, you should change the plugs while the car is cold in order to
prevent binding at the threads and possibly distorting the holes. But
Iím also doing a compression test which requires the engine to be
slightly warm, so I ran it at idle for about 3-4min. Begin by removing
the plastic engine covers. Take off the oil filler cap. Pry up the
decorative caps with a screwdriver to access the 10mm nuts or bolts that
hold them down. 318 owners will find a cool little blue tool under the
cover. This is for pulling the wires off the spark plugs (since the 4cyl
does not have ignition coils sitting directly on top of the plug). Put
the oil filler cap back on once the covers are removed.
This next step applies if you are also doing a compression test. If not,
you can skip it. Remove the fuel pump fuse (#18), the fuel pump relay,
and the main DME relay. Start the car and let it die as it burns off the
remaining fuel in the lines. Crank it one more time to make sure there
is no more residual gas.
With the coils exposed, pry up the metal retaining clips with a flathead
screwdriver, and pull the wire plug straight back off the coil. Then
remove the 10mm nuts to release the coil packs. Just pull them straight
up and out. The coils are interchangeable, but I kept them in order
anyway as I pulled them out. For 318ís simply use the blue tool to pull
the wires off.
Photo below shows the first coil pulled out. In case youíre wondering
how deep that plug hole is, itís a little over 5Ē, so a 6Ē socket
extension will do nicely when you remove the spark plugs. Some of you
will see oil in there, and itís not unusual to find the well filled up.
Unbelievably, the condition does not affect engine performance and the
spark plug fires normally. The leak is caused by worn valve cover
gaskets located near the top of the hole (photo inset 2). You can see
from my engine how the oil comes out and travels down. Iíll have to
replace mine, but that will be for another day. As you remove the coil
packs, thread the 10mm nuts back on the studs so you donít lose them.
Note the 2 ground straps indicated by the blue arrows in the photo. Make
a mental note to put them back on later, or slip them back over the
studs now before threading the nut on so you donít forget. To aid in
removing the coils, you can pry out the two main wire harness clips off
the metal brackets with a flathead screwdriver (photo inset 3).
Make sure the holes are clean before removing the spark plugs. Use a
shop vac or compressed air. If thereís oil, use paper towels to soak it
up, suck it up with the shop vac with a straw attached, or use a turkey
baster. Also clean around the top of the valve cover to be sure nothing
drops into the holes- the last thing you want is crud in your cylinders.
Since the spark plugs are all the way down in the holes, itís best to
use a standard 5/8Ē spark plug socket with a 6Ē extension. These sockets
have a rubber washer inside to grip the end of the plug as you pull it
out. A regular 5/8Ē deep well socket will work, but youíll need needle
nose pliers to grab the plug. When removing the plug, nice and easy is
the word! Use a longer breaker bar or slip a pipe over your ratchet
handle. This provides better control and prevents any sudden jerky
movements as the threads break free. Once they do, pull the
ratchet/breaker bar off the extension and turn the rest of the way with
your fingers so you donít accidentally cross thread. Keep the plugs in
order as you remove them, so you can diagnose any potential problems
Photo of my plugs. Not too bad, I think the color is just about right.
Youíre looking for a tannish brown or a medium gray color. Iím not gonna
go into reading your spark plugs, there are too many websites out there
that do this. Do a Google and youíll pull up dozens of them.
Testing engine compression.
Now is a good time to do this, since you have all the spark plugs
removed. Itís also easier on your battery and starter when theyíre all
off, as opposed to removing one at a time.
If you just had the car running (in case you werenít following the spark
plug removal process above), wait about 5min. This minimizes the risk of
damaging the threads on the head when you pull the plugs out. Next is
very important: remove the fuel pump fuse (#18) as well as the fuel pump
relay and the main DME relay, if you havenít already done this (relay
locations are depicted in 2nd photo at the beginning of this article).
Then start the car and let it die. Crank it once more to make sure
thereís no residual fuel. Now you can remove the plugs.
The average compression gauge comes with the hose and a fitting that
screws into the spark plug hole. The gauge itself has a pressure relief
valve on the side to let air out once youíve taken your measurement.
This set in the photo was $25 at Pep Boys. If your set has a separate
fitting like this one, make sure you tighten it very well onto the hose,
or youíll end up leaving it behind in the spark plug hole when you
unscrew the hose. These fittings are not easy to remove on their own,
since the 18mm socket required will barely fit into the plug hole.
You can do this on your own, but you really want a helper to be at the
driverís seat. Screw the hose into the spark plug hole and tighten just
enough to get a good seal, then connect the gauge at the other end. Have
your helper put the gas pedal to the floor as he/she cranks the engine.
Flooring the gas pedal allows the maximum amount of air to enter the
cylinder. The needle on the gauge will rise up higher and higher with
each stroke, and then level off to a point where cranking no longer has
any effect on it. This typically takes about 4-8 compression strokes.
Record the psi number, release the air through the valve, unscrew the
hose and move on to the next cylinder.
My results were 230-230-230-223-224-232, pretty good for 146k miles on
this engine. The most valuable information you can derive from a
compression test is really just how close the cylinders come to each
other. An acceptable result would be that they fall to within at least
85% of the highest reading. For instance, my highest was 232psi, so the
minimum for the others should be at least 197psi. But more ideally, the
figures should be within 5-10psi of each other. If you have a cylinder
that is drastically off compared to the others, retest it to make sure
you didnít get a bad reading. And while the minimum pressure for an E36
is 142-156psi, low numbers themselves do not necessarily mean engine
trouble. A lot of factors affect gauge readings: the gauge itself,
incorrectly adjusted valves, premature camshaft wear, altitude,
temperature, and even the condition of your battery can retard the
readings. Those who run aggressive cams will typically produce lower
than average numbers as well. The important thing is that whatever the
actual numbers are, they shouldnít vary too greatly from each other.
Installing new spark plugs
Now for the home stretch. Pretty simple and straightforward, but first a
word on anti-seize compound. There have been discussions regarding the
use of it on the plug threads. According Wayne Dempsey of Pelican Parts,
Porsche does not recommend the use of it, citing that it acts to raise
the electrical resistance between the plug and the cylinder head. But
the thought of a plug getting stuck was too scary for me, so under
advice from fellow member (thanks c1apton), Iíd say just a dab couldnít
Stick the plug into your 5/8Ē socket and extension, and thread it into
the hole by hand . . take care not to cross thread it. Then use a torque
wrench and tighten it to 18ft-lb.
Installation of the remaining parts is the reverse order of removal.
Donít forget to put the ground straps back over ignition coils 3 and 6.
I have found no place on BMW torque spec sheets, the Bentley manual, or
online for the torque specs on the ignition coil nuts. I just cranked
them hand tight, but judging from their size, Iíd say a 10mm nut
shouldnít require more than 15ft-lb of torque. Put your fuses back in,
clear all your tools, and give the car a go.
Last and final step: grab a brewsky, dudesky- youíre done!