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Tips for changing spark plugs on E39 M5
I did a change today and there were a couple of things I figured out that made it easier.
1. Get a 1/4" drive stubby flex-head ratchet. The one I picked up is about 4 inches long, and it made the job SO much easier!
2. All of the screws and nuts for the cover, the electrical harness retainers and the ignition coils are 10mm. It's the only size socket you need for that part of the job. You will find that having a deep socket and a regular socket is helpful. If the deep socket is a 3/8 drive with a 3/8 to 1/4 drive adapter, that's even better.
3. A standard "small" spark plug socket works fine. You need one with a rubber insert that will hold a plug and not drop it, and that won't come out when you pull it off the back of the plug after installation.
4. You need to have a long extension for the spark plug socket - I used a 9" and a 12" together. Use the "wobble" kind from Craftsman - you can't get a straight shot at any of the plugs, but the angles you need are small.
5. The existing plugs are tight. Use a long handled ratchet or breaker bar on the long extension to crack them loose. They'll unscrew easily after that.
6. Use a torque wrench to tighten the new plugs. 30 N-M is the spec, and if you use BMW spark plugs, the torque is on the box.
7. Do the driver's side first. It's the most challenging (it's not that hard, just tedious) and so you should get it out of the way first.
Starting on the driver's side, pop the cabin air filter box off (I won't provide instructions, but it's easy - same process you use to gain access to the brake master cylinder). Rotate the cabin air intake air snorkel toward the engine to make a little more room. Don't remove the snorkel, you don't need to.
Undo the two cap nuts that hold the cover on. This will be your first opportunity to use the stubby 1/4 drive ratchet. The rubber bushings around the nuts and the washers are all separate and loose, but they stick together from the heat. Don't drop them when you take it apart. Pull the cover loose from it's gasket and slide it forward and out.
Undo the two nuts that hold the wiring harness for the coils. I cracked withm with the stubby ratched and a socket, and then used the 3/8 drive deep socket as a nut-driver to remove the nuts. Using the socket allows you to turn the nuts without the risk of dropping them.
Pull the harness away from its studs, and then remove all 8 nuts that hold the coils in place. Again, each one will have a different challenge because of the cramped quarters. I used the stubby ratchet and a socket to crack them, and then used the deep socket as a hand tool to remove them.
Once the coils are loose, but still in place, use a screwdriver blade to pop the shiny metal brackets on the coils up. Slip it into the slot below the bracket and twist gently - the brackets lock the connectors, so pull the brackets up as far as they'll come (about 1/2 inch or so) and then gently pull or push the black connector off toward the rear of the car. They're EASY to take apart, so you shouldn't be struggling with them. Starting from the front of the engine (Cylinder 5) undo the connectors and grab the coils under the "ears" that go over the studs, and pull them out - again, you can be gentle because they come out quite easily. I took all four out and set them aside.
At this point, you're looking at the four spark plugs deep (really deep - remember the 21 inch extension?) in their openings in the cylinder head. Using the long extension, remove the old plugs and screw in the new ones. Torque to 30 newton meters. This is the easiest part of the job!
Once the plugs are changed, put it back together in the reverse order to taking it apart. Install the coils from the back of the engine to the front. The way I did it, I slipped the coil back onto its studs and did up the electrical connection. The push down on the bracket to lock the electrical connection also seats the coil onto the top of the plug.
Once all the coils are in and hooked up, put all the nuts back on the coils and tighten them. Then reposition the wiring harness and install the two nuts that hold it in place.
With all the wiring, coils and plugs back where they belong, it's time to put the outer cover back on.
On the driver's side, but not the passenger's side, you will have to wrestle with the rubber sealing boot where the wiring harness enters the outer cover at the back near the firewall. On the drivers side, the brake lines keep the cover close to the head, and you have to coax the rubber boot, which has a slot in it, to pop into the corresponding opening in the cover. The natural tendency is for the "top" to slip in nicely, and for the bottom to firmly resist going back into place. The trick is to push the top of the boot back and seat the "bottom" and then wiggle the top forward into place. That works!
On the passenger side, there's enough room to just pull the cover back and slip it over the boot. No struggling required.
Anyway, once you have the boot properly seated in the cover, put the two cover cap nuts back on and tighten them. They bottom, so torque them gently after they stop turning and you're done.
Now do the other side. There's more room on the passenger's side, so it goes quicker with less contortions.
Total time - 1.5 to 2 hours.