Hello, please help as I am running out of ideas. My wife drives a 99 SL1 with 72,000k on it. Within the last 6 months it has started to loose it's power and is running rough at idle and throughout the speed range. It also pings when accelerating, but briefly. It will ping for a second or two, then quit. The spark plugs are new delcos, previously, I had Bosch Platinums. The wires are about a year and a half old and test ok for resistance. 14 months ago, I took out the EGR valve(the newer kind, that is purely electrical) and cleaned it. It was not very dirty and seemed to move ok. I have tested the spark voltage and all 4 wires give out 30kv, but the secondary resistance on the coils is 7.9 and 7.6(Chiltons says 8k to 15k). I have not load tested it because I cannot seem to figure this procedure out. The secondary resistance is low, but the output volatge seems very good. I have an odbII interface for my laptop and no trouble codes have been posted. But what I have found interesting is that my spark advance will jump from 11 to 20 irratically and tends to run in the upper end of the range. This seems high and it appears that the engine is compensating for something, but I cannot figure out what. Any help would be appreciated. Thank you
