Over the past couple of weeks, and ever since my last service appointment, the car had developed a nasty tendency to misfire during normal acceleration (full or near full-throttle acceleration was smooth). The result was some mild idle surging and even erratic shifting. My mechanic and I took it out for a test drive but as usual, the car totally changed character and failed to demonstrate the malady.
He asked me if the check engine light had ever come on while driving and I said "no". He volunteered that it might be a bad plug or coil. I then asked if we should just replace them all and he said "I've been down this road too many times...let's wait to see if we can get more information before we start replacing parts".
When we got back to the shop, I asked him if he had any "fix it in a can" for me as a first cut solution, and he handed me a bottle of "BMW Engine Treatment", essentially Chevron's Techron additive relabled. Apparently, this stuff was originally developed for BMW as a cure-all for various gasoline-quality related ills of the 1980's. The stuff is supposed to dissolve carbon deposits in the injectors and serve as a general fuel system detergent. But, the downside to it is that, over time, it eats fuel lines, which is why you don't see it formulated in pump gasoline. Strangely, this stuff is $5+ a bottle in the store under the Techron name, but $2 in BMW garb. The "price was right (read: free)" today, as was my mechanic's precious time, so I walked away that day pretty happy, with an understanding that we'd have to let it get worse before we could attempt to solve the problem. I thanked him for his time and went to work.
Over the next several days, the misfire problem seemed to get better, but it still occurred occasionally when I'd stop at a light and then press on the gas lightly. The misfires seemed to happen up to around 2500RPM and then it would smooth out. Full throttle starts were never a problem. The check engine light never went on, so I just figured I'd wait until the next scheduled service in 3000 miles and sort it out then.
I had just completed the washing ritual one Sunday when I hopped in the car to put it to bed for the night. I turned the key and the car just turned over, but didn't start. I tried again. No dice. I then applied a bit of throttle and it started, so I let off the throttle...and it died again. After some experimenting, I figured out the car would run, but only if I held the throttle at a level that produced an idle -- strangely enough, I could control the RPM to a point far below a normal idle (believe it or not, the car idles smoothly as low as 300 RPM, but I digress...). The second I released the throttle, the engine would die stone cold. Of course, I couldn't drive it this way -- if the engine died, I'd not only lose my power steering and brakes, but since you have to turn the key off before you can turn it to the START position again, I'd very likely lock the wheel and lose my ability to steer. Not good when you're staring down a telephone pole.
So, I called for a flatbed early the next morning and they showed up about an hour later with a fork-type towtruck. I called the company manager and said "WTF, I asked for a flatbed." He then gave me some excuse about "you called before 8AM and the previous shift manager, bla bla bla...and we don't have any flatbeds right now". "Okay", I said. "This guy fucks up my car and you'll be hearing from my attorney".
Fortunately, the driver turned out to have three years of experience in towing (which is like dog years in that business I would imagine), and was pretty knowledgeable about towing BMWs and Mercedes. He said that it's totally safe to tow a BMW with a fork-type towtruck (and in fact it takes a lot less time to prep), but it must pulled backward by the rear wheels. You need to lock the steering wheel and keep it in park. If you try to drag the rear with the tranny in neutral, apparently you can burn out the fricton plates (what friction plates, you ask? don't ask me...I'm not a transmission expert, but they're probably the same plates that can get screwed up if you sit in traffic for extended periods of time with the tranny in neutral or do lots of shifting between N-D-R and back again. Another point -- if they tow with a flatbed, make sure they pull the vehicle up onto the bed with the tow hook...don't let them hook on to any part of the undercarriage. BMWs (particularly the E46) have some soft frame parts that can be damaged by the hook.
In any case, $75 later, my car arrived at my dealer safe and sound.
I told my mechanic about the problem and he suggested it was likely an idle control valve. This is an electromechanical valve that controls the volume of air entering the intake at roughly 20% throttle and below. There are two sets of variable resistance contacts...one that ultimately serves to close the valve, and another to open it. Under normal conditions, the two forces balance each other out and produce a smooth idle. However, if one set fails (they typically go to high resistance), the valve closes (or sticks) and the engine will either run roughly or not at all. Hmmmm...I thought...where have I seen that before...
When he had the car in the bay, he remembered our test drive and checked the DME for fault codes and found that the number 6 cylinder had reported a misfire. I had no idea that they could report a misfire in this way, but apparently they use the crank sensor to measure crankshaft acceleration and can tell by the power pulses (or lack of same) which cylinder misfired. Pretty neat. In any case, he pulled the #6 plug and found the center electrode slightly recessed (and on these platinum plugs they are supposed to be flush with the surrounding insulator). He happend to catch me the next day as I was swapping loaner cars (long story), and he said that it indeed needed an idle valve, and he recommended that we replace all plugs. I told him to go ahead.
I drove the car home today with a smooth idle and no apparent ill effects. Guess we'll have to wait and see. Thanks to my mechanic for saving the parts and taking the time to print a hard copy of the DME fault code report for me...not something he normally does, but he knows I'm interested in the details (and yea, he knows pictures of everything will show up here) :-)
Total Parts, $200, Labor (3.5 hours), $300. Towing, $75, for a grand (almost) total of $575. Total mileage, 68800.