I don't know, but everytime I have had these mysterious starting problems come up I have always ended up finding some glow plugs that don't glow anymore.
Pull them out, clamp them in a vise and then ground the vise to 12 v and touch the end of the gp with the positive lead. If the gp has failed it will not heat no matter what you do. If it is good you will see a spark and the end of the plug will imediately heat up.