Hi everybody,
I am trying to get rid of a slight rough idle and have already run injector cleaner through the engine with still a tad of roughness left, particularly in wet weather.

I read the archives regarding the resistance of spark plug wires as well as the MB CD.
As per CD, the resistance of the spark plug wires should be between 700 and 1300 ohm for a 1987 420 SEL.

My readings are:
Zyl: Ohm
1 1320
2 1322
3 1302
4 1303
5 1351
6 1370
7 1335
8 1342
The wire has “Bosch 7 mm Silicone High temperature” written on it.
As per my records, the PO had the wires, distributor, rotor and plugs replaced in 2/1999 with 36,735 miles on it. Car has now 48,500 on it.

I guess the original “indestructible” copper wires have been replaced with Bosch silicone wires by the dealer in 1999 and my reference of 700-1300 ohm might not apply to these.

1. Is the resistance for these wires “within spec.” or do I need to replace them (with the copper core) because they are all over 1300 ohm? Could those few ohms make a difference in the idle?
2. Is it likely that distributor and rotor cap need replacement after 12,000 miles or 4 years?

Right now, I am thinking of replacing the spark plugs only with a little wider gap to see whether it improves on the idle.

Thanks for your help

Reinhard Kreutzer