The Mercedes of that era do not use spark plug wires in the sense that any sane engine does, at least the E series with the 3.2 L straight 6. On the two I had, 1993 300E and 1994 E320 There were only 3 very short High tension leads. There was one coil pack for every 2 cylinders. The coilpack sat directly on top of one plug and a short lead attatched to the second one. The primary side of each coil was hard wired into the wiring harness. Mercedes used a wasted spark system in these engines.
The biodegradable insulation was a problem on the E series all the way back to late 1992. I wound up rebuilding the wiring harnesses on both cars using less problematic insulation because I refused to pay for "new" harnesses with the a manufacturing date between 1993 and 1996 (the problem years). The wire for the repair cost $35.00 for both cars, New Mercedes ECM boots were another $20.00 and the coil connecters were $11.00 and change. In addition to the engine harness, the electronic throttle wiring had the same problem and will cause exactly the problems described in the OP. I wound up rebuilding those as well. The total for both cars was under $100 and one weekend of soldering beat the heck out of paying $1500 each to the dealership.
Because these were among the first cars with a digital CAN and multiple signals running over the same wires, any faults or noisy connections caused signifigant problems, usually leaving you on the side of the road.
In my case, the insulation problem didnt crop up until ~2008 or so and both cars failed close to each other. My kids drove them through college and up until last year when the both bought more modern wheels. Other than the issue with wiring, these cars were built like tanks.
The only wiring in the car that matches your symptoms would be the high voltage ignition wires. Sounds to me like new plugs and spark plug wires would probably have fixed this vehicle. It's really a shame so spend $2000 on a fix and then dump the car when a $200 fix would do, but I guess that's what keeps the American economy humming.
How weird that biodegradable plastics would be used in under-hood applications. Plastics designed for those apps require a very different set of specs from those achievable with biodegradable materials, as DN has reported on several times.
Well based on what my Toyota has done, and a 99 that we bought and sold because it was showing signs of doing the same thing, it would seem they do not take climate into consideration when building cars, or they miss estimated the heat here in Southern California.
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