There is no way a M103 will do 20MPG routinely in a W126. No way no way. My W124 with a M103 averaged 16-17Mpg doing mostly city miles, and that was a lighter, more aerodynamic car. And that was when the engine WAS running well, which it often was not....when not it was more like 12-14mpg.
My 420 averages about 18-19 city if driven gently, 15 or so driven like a maniac, and about 23 highway on cruise at 75mph with no A/C....about 21-22mpg with A/C.
It seems like the Ignition and CIS on the M102/M103 is much more finicky than on the M116/M117 engines too.
The small old gassers aren't worth the trouble, not enough power and bad fuel economy. The V8's and diesels are much more fun. The only smaller gasser I am a fan of so far is the M112 V6...its a beast. Mine has 132k on it and is bone dry and runs beautifully. Far superior to the M102/M103.
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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)