I'm trying to solve a slight unevenness in the idle on my '89 300CE. I pulled one of the spark plugs and noted that it was NGK TR5GP. The plug wires are NGK and are blue and say NGK Copper. Are these suitable for the M103? I know that this engine is very particular for the correct non resistor plugs and wires.

I've never heard of NGK wires for a 300E that are specifically made for it.

Put in some Bosch H9DC0 plugs from the dealer and a new set of Beru or Bosch wires, and I bet it will improve. (Might want to do the distributor rotor and cap too)

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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)

My best results came from H9DC0's and Beru Wires. The Bosch wires always seemed to look worse after changing them.

No hesitation, no rough idle, no fouling, everything running as it should

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'76 240D 311K
'92 300E 100.001K
'87 300E RIP