There are NO glow plugs on a Yanmar.
Does he (the mechanic) know what he is talking about? But if the climate is cold, as it is in NY, and it has been, then it still may be cold fuel to run the engine. As soon as it begins to burn the cold fuel and gets to a warmmer bit of fuel from the tank, it will straighten out after the fuel warms up. Remember, only a bit of thr furl burns, then the rest is returned to the tank via the return line. Only the fuel in the fuel lines will be so cold. Does it smoke, too? If so, what color smoke? If it smokes grey, then I would think that that is your problem. Take heed from the other posters here, also. Try as they do!