In the late 80's and 90's as more computer controlled equipment came along there are quite a bit of low voltage circuits that contol engine management. I recall in 1989 a neighbor of mine had a 1989 Cherokee Chief. His wife had a problem with it every time it rained hard or she went through a carwash it would run really rough and sometimes stall out. Back and forth to the dealer numerous time. Then one morning I was over talking to him, I was standing in front of the Jeep and he started it up. I saw the smallest wisp of steam come from behind the grill. As it turned out, the engine harness was pinched behind the radiator mount and finally chaffed the wiring. Point being, a slight grounding or bad connection can cause problem that may take a long time investigate. Start with the basics, check all electrical connections, wiring harnesses, grounding straps etc.

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Dennis
2006 E320 CDI Black/Black Leather. Nav, HTD Seats HK stereo 32000 miles.
2000 VW Passat 1.8t 300,000 miles as of Dec 09, 2009 original owner
1997 E300d Silver 300,000 miles As of Jan 05, 2010
3rd owner SOLD