I have a 98 Sable with a Duratec engine. When the car sits for awhile (a week or more) the engine has a harder time starting, it runs on maybe 4 cylinders and makes a knocking noise for a few seconds and slowly goes away? Is that from the lifters or what would make that knocking noise? I was reading somewhere that it could be a bad oil check valve if it is the lifters, how hard would that be to replace? Any ideas would be appreciated. Thanks!