I've been following this post as my F350 PSD has been having similar hard to start problems. I've been trying to figure out what is wrong for the last two winters...I'm pretty sure that my glow plugs will need to be changed out after a lot of trial and error. At one time my truck would start easily at +20F without being plugged in and with good batteries it would start down to 0F, but preferred to be plugged in. Plugged in over night it would start easily at -20F. I recently found out that my block heater was dead, even after being told it was good last winter. I changed it out and after 45 minutes of being plugged in, my truck started easily. My further testing: I left it unplugged on a night that stayed just above freezing all night and the next day at +35F, it was extremely hard to start. Plugged it in for 30 minutes and it fired right up...Last night it dropped to +10F and I had it plugged in, so when I went out this morning it fired up immediately. I've ordered a Stancor GPR as mine is factory original and therefore 23 years old, so I want something new. I'm headed to The North Slope in Alaska to work for a few weeks, but after I return I will be changing out my glow plugs as they too are original. I hope you find all your answers as well.