I have installed Bosch +4 plugs in 5 of our family cars, none of them a Mercedes; perhaps I should interject that I have 5 grown kids who still expect daddy to do almost all their car services and repairs for them, and they tend to buy high-mileage used "bargains" with a spirit of "Oh, Daddy can fix that," so I'm now maintaining a fleet of 8 vehicles for 7 drivers.

First, the bottom line: I recommend that +4 plugs only be used in cars for which the original equipment gap specification is over .036" and with all other ignition components in good condition. I first tried +4 plugs in our '86 Subaru wagon, with a specified gap of .042", with excellent results, in spite of a rather tired-looking cap and rotor. Then I tried them on #2 son's '83 Toyota Celica, for which I had just done an engine overhaul (22RE motor, with .032" recommended gap) and installed a high-lift 276 degree cam, etc., etc. Even though all the plug wires and the cap and rotor tested out right on spec. for resistance, they were not new, and I had missing problems until I put on new genuine Toyota wires -- some of this may be the result of my lack of expertise in adjusting the FI system to match the cam, of course. Then came #2 daughter's '90 SAAB 900S (.032" recommended gap): here we had missing problems right from the start, and new wires and cap and rotor didn't help, so I took the plugs back out, measured their factory gap as best I could with a drill rod (more than .070", I'd say), and then used a smaller drill rod carefully centered on the center electrode with magnification to narrow the gap down about 20% -- the car has run beautifully on them ever since (c. 20,000 miles) except that now it needs transmission work, but we can't blame Bosch for that. 4th in line was a '91 Nissan pickup I recently acquired to facilitate my wife's gardening habit (only 148,000 miles, and the plugs that were in it looked like they were originals). The +4 plugs have worked very well in this 4 cyl, 3 valve per cylinder motor (spec. = .035" gap) for several thousand miles since, but then, that ignition system had already proven its ability to fire, reasonably well, stock plugs with a gap larger than .075" between thoroughly rounded electrodes. Yesterday I installed another set in my wife's '95 Mercury Villager (Nissan Maxima drive train with .035" specified gap) and they seem to be working out beautifully in that one.

Next victim, in two weeks, will be #1 son's '91 BMW 318i (only 205,000 miles) -- the plugs have already been bought for several months. But I've got to do the clutch for him that weekend, too, so I may not get it all taken care of.

I'm just an old codger who likes to experiment with new things, but my M-B is exempt -- it's an '85 300TD.