I am having a problem starting my 83 sd. I noticed last week it was getting more difficult because of the cold. I have started using the block heater. Some mornings it has been down to 25F. The glow plug light is on. With the heater on for 2-3 hrs it starts easily. I checked the glow plugs for resistance and they all looked fine. They are Bosch and about a year old. Today it is warmer 32-34F and I was able to start w/o the heater but with some difficulty. I just went out at lunch time ( now 43F) and had a hard time to start it. The glow plug light is out now. I stopped and checked. Now all the glow plugs are showing high resistance. I checked the glow plug fuse and it is OK. It seems odd that all the glow plugs would fail at once. I am going to remove them and replace them tonight. I will also test them once they are out. Should I check anything else? Maybe they were marginal last week and they just finally died.
I am also due to do a valve adjustment. It has been 15-18K. I am sure this would help starting as well.