OK, some of you may remember recently I was having intermittent trouble with my 91 300E. It was kicking, bucking, and stalling sometimes when idled in drive, especially with the A/C on. I originally thought it was caused by a bad tank of gas, because it started happening right after a fillup. In the last 2 weeks the problem went from occurring very intermittently to almost constantly. It got to the point where I had to warm the car up for about 5 minutes before putting it in drive or it would stall. Also, while stopped at a light I had to put it in neutral and turn off the A/C or it would buck and kick horribly, though it never stalled. When accelerating, I had to give it gas very very gradually (with everyone honking behind me). Otherwise, if I just stepped on the gas as usual, it would stall. Here is the original thread:
Techron = Placebo Effect?
Anyway, thanks to everyone for their help, but I guess the theory was way off. After doing extensive research on this awesome site and mulling the issue over and over again in my head, I decided to try something different. I pulled the sparkplugs (which were newer, by the way - only 15K on them). I lowered the gap to about .030 and put them back in. Problem solved - idle is very smooth now (well, at least about as smooth as an M103 is capable of) and no more stalling or hesitation. I can start the car up cold, put it in R with my foot on the brake, and turn the power steering lock to lock with the A/C and headlights on at the same time. It's just like before - I have my great car back!
The reason I pulled the plugs to begin with is that the erratic behavior just didn't seem like a fuel issue, in that it only occurred at idle or while accelerating from an idle. I know what poor fuel pressure and clogged fuel filters feel like, and this just didn't feel the same. In addition, turning the A/C off reduced the severity of the misfiring by about 50% (I'm guessing due to reduced load). My cap, rotor, wires, and plugs were replaced about 15K ago, so originally I ruled them out as culprits. But something just didn't feel right - the misfiring and weird stumbling really felt like an ignition problem to me. I pulled the cap & rotor first, and while they did have a little more wear than I would have expected for having only 15K, they didn't look ultra bad (in contrast with the cap/rotor on my 380SE which barely had any terminals left and crumbled in my hand when I removed them). So I cleaned rotor as well as the terminals and the inside of the cap with electrical contact cleaner (there was a little carbon around the center electrode), and then reinstalled them. This did not provide any improvement.
Next, I pulled the plugs. They had some light powdery ash deposits on them, but otherwise looked great. The electrodes were not burnt; all corners were still sharp and square. So I put them back in and kept driving the car. I compiled a long list of things to check through my research, including OVP, MAS, ICV, hoses, fuel pump, coolant temp sensor, and many more. But again my gut feeling told me this was a secondary ignition problem. I was getting ready to order a new coil on FastLane. I didn't bother testing the existing coil, since the problem was still somewhat intermittent and I've played the game before where you test a part on the bench and it tests OK but is really bad. I didn't feel like wasting time.
But before spending the money on the coil, I figured I would lower the gap on the plugs. Maybe the coil output was just slightly weak and was having difficulty sending the spark across the gap? Well as you know lowering the gap has resolved the issues. But here's the problem - isn't .030 pretty much a normal gap for this car? I think I read that it should be .032 so I took it a little lower. I think the gap I had in there was close to .040 or even higher. I remember at the time I bought the plugs (O'Reilly Auto Parts) and asked the counter clerk for the gap, when he gave it to me I thought it was kind of high. Furthermore, when I gapped the plugs prior to installation I remember thinking the gap was kind of big. But who was I to question his computer? So I installed the plugs and the car ran just fine - even passed the emissions test with flying colors the very next day. Ran fine and had no problems at all for 15K. So I never thought about the gap again
So my question is: is the factory gap .032 or close to it? If so, would running a .040 or greater gap create a hardship on the coil? Or is my coil really heading south? And if the car ran fine for 7 months and over 15K miles, why did it just suddenly begin kicking and bucking? Wouldn't the degradation have been gradual? Is this indicative of some other problem which is only temporarily being rectified by the smaller gap? Lastly, and I know I'm gonna get it from all of you for this one, but the plugs I installed were NGK V-power. I know after researching this that the plugs should be the Bosch H9DC0. But the counter guy had these NGKs listed as correct replacement, and he said they were not resistor plugs (I know the resistance on MBZ ignitions is in the wire ends). Was he right? Or should I run out and get the Bosch plugs? Also, I noticed when I pulled the plugs the second time around, they had much more of the grayish beige light ash on them, so when I regapped them I also cleaned them with fine sandpaper before installing, just for safe measure. But what is that stuff? I noticed it had gotten to the point where it coated almost all the exposed metal and electrode surfaces.
And my last question, should I be ordering a new coil? Or will going to the Bosch plugs with the .032 gap solve my problems?
I'm liking the car the way it runs now, and could also just leave it alone.
Any thoughts or suggestions? Thanks everyone!