Northstar Engines and System Technical Discussion Discussion, Idle Problems in Cadillac Engine Discussion; My 94 Concours 4.6 is running rough. When in park it feels like its missing, and it use to really ...
My 94 Concours 4.6 is running rough. When in park it feels like its missing, and it use to really run rough when going about 45-45 mph, but now its doing it at 50-55 mph (not so bad at 40-45 now). I recently replaceed the EEGR valve, and will clean the throttle body tomorrow. Should I go ahead and replace the O2 sensor? Some times I smell fuel that has not burned when I am backing out in the morning. I have no current trouble codes listed (just history). My gas mileage has gone down about 100 miles per tank. l.
If you're concerned about smelling unburnt fuel in the exhaust, check it when the engine is hot and has been running at least 15 minutes. The O2 sensors aren't effective until they reach full temperature. The computer senses this and starts adjusting things based on their input (closed-loop operation). So smelling a little fuel when you first start it in the morning isn't worth worrying about. If you still smell unburnt fuel in the exhaust when you get to work, though, that's another story. That could be an O2 sensor.
An O2 sensor, though, is not going to cause rough idle or weird vibrations at highway speeds. The fuel pressure regulator mentioned above could be at fault, or it may just be time for a complete tune-up. When the EGR valve was replaced, were the passages cleaned? Air filter, fuel filter, PCV valve, etc. in good shape? I don't know if the pre-96 4.6's had coil packs or distributors, but has that been checked?