Hello my 2wd saff had the same problem had 3 sets of plugs in and still done the same ran on 3 and stunk of fuel so i loosened the fuel rail off and pulled the injectors out the manifold and on number 1 injector the little cap that go's on the end of the injector had popped off and was sitting in the manifold so i clipped it back on and it was mint again................( I know you not been near the injectors but neither had i)....hope you get it sorted