One basic tune-up procedure for just about any car on the road is the replacement of your spark plugs and spark plug wires (where applicable). On the BMW E36 six-cylinder engines, BMW has eliminated the use of spark plug wires by integrating six small spark plug coils that sit on top of each spark plug. While this configuration may be a bit more expensive than the typical single coil, single capacitive discharge box configuration, it makes the car's ignition system more reliable by removing a component that constantly wears out and fails (spark plug wires). It's a pretty cool setup, not commonly found on older cars. As manufacturing components has become increasingly inexpensive, ignition setups like these have become more common. I recommend replacing your spark plugs every 10,000 miles, or about once a year. In reality, you can probably go longer than that, however, you never really quite know how long the plugs are going to last, or you may forget to do it if you don't setup a yearly schedule. Needless to say, replacing your spark plugs is one of the easiest tasks to do on your BMW - provided you have the proper information, which I will provide here.
Begin by prepping the car. The only thing that you really need to do is to make sure that the car is cold. If you try to remove or install spark plugs in a hot car, then you may encounter problems with the spark plugs gumming up or damaging the relatively delicate threads in the aluminum cylinder head. Just make sure that the car is cold, or at the bare minimum, only slightly warm to the touch.
Let's talk about the six cylinder cars first. The first step is to remove the top plastic covers from the engine. These serve no mechanical purpose - they are there only for decoration and to prevent dust and debris from getting into the recesses of the engine. On the six cylinder cars, there are two covers, a long thin one on the top of the car, and a wider one towards the left. Speaking of left, for the purpose of this particular tech article, I will refer to the left side of the engine as being on the left as you are standing in front of the car looking at the engine. The right side would, of course, be opposite to that. For reference, the windshield washer bottle would then be on the left, and the air filter would be on the right.
On the two plastic covers, there will be two small, snap-in plugs on the top. Carefully remove these plugs (don't drop them into the engine) with a small screwdriver, prying them up as you grab them (Figure 1). Underneath you will find a nut that holds the cover onto the top of the engine (Figure 2). Remove the four nuts on these two covers, and they should both simply slide up out of the way. Figure 3 shows the engine with the center cover removed.
Underneath the left cover, you will see the six spark plug coils that sit on top of each of the plugs (Figure 4). You need to remove each of these carefully, in order to gain access to the plugs. Using a screwdriver, release each connector from each coil. There is a metal retaining ring on the rear of each one that fastens it to the coil (Figure 5). Once you lift up on the retaining clip, then the connector should simply slide out of the coil. Carefully remove all of the connectors from each coil (Figure 6), taking care not to bend the wire harness too much. These wires are stiff, and generally don't take well to being bent in multiple directions. Just be gentle with them.
To assist with your maneuvering of the wires, detach the center clip that holds the wires that come from the center channel. This clip is shown in Figure 7. Gently place the wires off to the side and out of the way, without bending them terribly.
With the wires detached and placed slightly out of the way, you can now remove each of the six coils. Each coil is fastened to the valve cover using two screws. On two of the coils, there are two small ground straps that connect the coil to the stud on the cylinder head. Take note of these ground straps - they must be installed properly when you are finished, otherwise your car may encounter problems. These two ground straps are shown marked by the greens arrow in Figure 8 and Figure 9 (coil already removed in this photo).
Remove each of the two nuts that hold each coil to the valve cover. At this point, the coil should be able to be easily pulled right off of the engine (Figure 10). The coil has a small coil pack on one end, and a spring-loaded spark plug connector on the opposite end. Simply remove the coil/plug assembly and place it off to the side. All of the coils are the same, so it doesn't matter which cylinder bank it came off of - unless you are specifically trying to troubleshoot a bad coil fault code that was displayed by the main computer.
With the coil removed, you should be able to look down the hole and see the spark plug hiding in there. Figure 11 shows what the top of a normal looking spark plug looks like. However, as you remove the plugs, you may discover something peculiar. The way that the ignition system is designed on these BMWs, there is the opportunity for the spark plug holes to completely fill up with oil, if you have a leaky seal on your valve cover. When you pull out the spark plug connector / coil combo, you may find that it is completely submerged in engine oil, as shown in Figure 12 and Figure 13. Looking down the hole, you may not even be able to see the spark plug because the entire hole is filled up with oil (Figure 14 and Figure 15). While common sense says that this is not a good thing, the reality is that this is actually quite common, and doesn't seem to affect the performance of the car one bit. If you do find this oil in your spark plug holes, I would suggest that you go one step further and replace the valve cover gasket. This replacement procedure is very simple, once you have the coils removed, and should only take you about 20 minutes more, providing you have the actual gasket on hand. If you find oil in your spark plug holes, then you should definitely replace the gasket.
If you find that you have oil in your spark plug holes, I suggest that you take some paper towels and attempt to soak up as much of the oil as possible, before removing the spark plug. If you don't get rid of the excess oil, then it will leak into the cylinder head through the spark plug hole when you remove the spark plug. This will cause your car to run sooty when you first start it up, and it may even foul your brand new spark plugs that you just installed!
Spark plug removal is easy - you just need the right spark plug wrench. I have one that I love - it's a spark plug socket with a rubber insert that catches the plug. In addition, it has a built-in swivel on the attachment end. This is especially useful when trying to remove plugs in hard-to-reach places, as they are always located on Porsche engines (BMW engines aren't really that bad with respect to spark plug access).
Using a breaker bar, grip the plug and turn it counter-clockwise until it is loose. Then pull out your tool and grab the plug. When the plug comes out, you may want to take a close look at it. The spark plug is really the best way to visually 'see' what is going on inside your combustion chamber. You need to pull out all of the spark plugs to replace them, so you might as well take a close look at them while they're out. While today's modern fuels make plug-reading much more difficult, you can still glean a lot of information from looking at them. A good, well-balanced engine will produce a plug that is light brown in color, and dry. If the engine is running too rich, the plug will often be coated with a lot of extra carbon. Keep in mind that the rest of your combustion chamber probably looks the same. An engine running too lean will have a powdery white coating on it, and the outer porcelain ring may have a burned appearance.
When reading spark plugs, pay close attention to the white porcelain ring around the plug. This white area will give you an excellent background to inspect the color of the plug, and to help determine how your combustion chamber looks inside.
If the plug is wet with oil, then that indicates that there is significant leakage into the combustion chamber past either the valve guides or the piston rings. This is generally a bad sign, and an indicator that a future compression test may not yield good results.
Figure 16 shows an unusual spark plug with all four of its electrode eaten away. I would hazard a guess that this plug was improperly plated from the factory, and as it progressed through it's life, the repeated sparking slowly ate away at the electrodes until they were gone. A plug in this condition would misfire often (if at all), and would generate poor performance for this particular cylinder. Surprisingly enough, none of the rest of the spark plugs in this set exhibited this type of damage. This is what leads me to believe it was defective from the manufacturer.
Figure 17 shows a brand new Bosch Platinum spark plug. While I don't have any specific preference for any specific manufacturer of plug, you should definitely make sure to get the proper ones for your car. Spark plugs have varied over the years as engines have been changed slightly due to smog regulations. The important thing to remember is to get the proper ones for your car (they are scaled by electrode type and also by heat range), otherwise you may encounter odd ignition problems. Spark plugs are cheap - I would go with a brand name like Bosch or NGK, choosing to avoid the no-name brands. Make sure that you measure the spark plug gap (if single electrode) with a spark plug gap tool before you install the plugs.
Install your new plugs using a torque wrench to measure the amount of torque applied to the plug (Figure 18). This is very important, as it is easy to over or under-tighten spark plugs. Make sure that the plug is firmly seated in your spark plug socket as it is very easy to insert the plug into the head and have it cross-thread. This means that the threads of the spark plug don't mesh properly with the ones in the head, instead choosing to "cut their own path." This damages the threads on the head, and in extreme cases, may destroy the threads in the cylinder head entirely. Trust me - you do not want this to happen. Proceed carefully and cautiously here.
Install each plug into the cylinder heads without using any anti-seize compound. Torque the spark plugs to 25 Nm (18.4 ft-lbs). While writing "How to Rebuild and Modify Porsche 911 Engines", I discovered that Porsche doesn't recommend the use of anti-seize compound, as detailed in Porsche Technical Bulletin 9102, Group 2, identifier 2870. The bulletin applies retroactively to all Porsche models and the theory is that the anti-seize tends to act as an electrical insulator between the plug and the cylinder head. This could have detrimental effect on the firing of the spark due to the loss of a good, consistent ground connection. Keeping those findings in mind, I would make the same recommendations for the BMW cars.
With the new plugs installed (Figure 19) and properly torqued, you can replace the coils (don't forget the small ground straps shown in Figure 8 and Figure 9) and reattach the coil connectors (Figure 20 and Figure 21). Snap the wires back into their center holders (Figure 7) and replace the top two plastic covers. When you're done, your engine should look back to normal (Figure 22)
Changing plugs on the 318 4-cylinder cars is a bit different and a bit easier. You remove the spark plug cover in a similar manner (Figure 23). There should be a handy little blue spark plug wire pull tool under the cover. Use it to remove the plug wires from the ends of the spark plugs (Figure 24). With the wires disconnected, remove and reinstall the plugs in a similar manner to how I described the procedure for the six cylinder cars. You will also want to replace the spark plug wires every 30,000 miles or if they look cracked and worn out.
Info from http://www.pelicanparts.com
