Misfire, No Codes Diagnosis
I was driving home the other day and I started to feel a miss every once in a while. It usually happened under a load, sometimes it would happen while sitting at a light. I will preface this with admitting me transmission is, for the lack of a better term, shot. Typical mechanic car, I know. Every time the shutter happened, my stomach went into my throat. I would say a prayer at every stop sign, hoping it would move when I stepped on the gas.
Keep in mind The MIL never came on. I figured this would be a good time to hook up the scope and look for shorted coil(s) as they are the most common issue. I wanted to try anything before condemning the trans. My Pico was at work, but my MODIS was home with me. I figured this would be a great time to stretch its legs.
I started by removing the covers and exposing the wiring. My go to test for coils is a current ramp. If you hook up to voltage, you can see that the coil is working, but telling how hard the coil is working can be difficult. I found the main ground for the ignition coils. all the current for all the coils flows through this wire.
I next back probe the coil control wire. If I set the trigger on this channel, I can move the back probe from coil to coil and it'll show me the current on that coil without moving my clamp.
Let's start with cylinder 1
That's a little busy. How about we remove the voltage channel.
We can see a nice smooth amperage rise. Looks ok to me, moving on...
Cylinder 2 Ruh-Roh!!
You can see the amperage rise sharply before leveling off and climbing the rest of the way. This is indicative of a shorted coil. Let's move on...
Cylinder 3 While not as bad, I do see a rise here. I will be replacing this coil as well.
I moved down the line, and the rest of the coils looked fine, like number 1.
A test drive confirmed the fix. Remember, this was diagnosed without a scan tool and the light never turned on.
And remember, always
your amp clamp.
thanks for reading
I've had very good luck on these cars catching the misfires with Mode 6. It's amazing how bad the miss has to be sometimes before setting a fault
That's cool Aaron. I wasn't aware that BMW had misfire data in their Mode $06. Do you remember the MID number or where to find that in there?
Great write up Chris, has there been any work on finding the root cause of the failed coils on these? Or has it been chalked up to low quality OE parts? On the 4.0 and 3.5 Nissans I have found that engine block grounds are the culprit to repeat failed ignition coils. I have similar notes written on my PC's about what to update and what NOT to update (Java on my GM SPS partition specifically!)
Im chalking it up to the location of the coils, and the fact that these are completely shielded from air flow. Also this is a multi-strike system putting extra stress on the coil. I've used nothing but OEM dealer bought coils on these. I will never put anything else in them. When I was at the shop, I would turn down the job before I put in an aftermarket coil. BMW has updated these, but they are