Van has 3.4 V6. Had recent tuneup. Problem just began. Runs fine when cold, but once warmed up, sputters, backfires (blew open muffler as result), then stalls. Can occasionally get it to restart, but idles very rough, and will stall if I try to put in gear. MAF sensor is good, but noticed that when I unplug the cam position sensor whose harness is right on top of engine, there is no change at all. Help please