car cranks great when cold,after warm you have to let set for a hour to crank. on 1991 Mercedes-Benz 350SD

glow plugs working right,turns over fast enough,

Asked by for the 1991 Mercedes-Benz 350SD
poss starter is binding up when its hot
turns over fast,new starter
do u have air in system and are glow plugs staying on long enough
glow plugs working right. what if getting air?
yes that could cause your issue insp fuel lines