Thanks DVDBURN & ArubaDave. I strongly suspect this is my problem but as the service bulletin indicates this is a severe problem requiring removing the plenum, timing belt, all the way down to the exhaust cam removal and replacement of the indexing peg with the JWT equivalent. I don't even want to attempt this without having an awful lot of confidence that indeed this is my trouble. That's why I'm going through the "easy" work (not really) and checking out the signal with an O-scope. If I determine this is my problem I'm not sure what I'll do. Any recommendations?
