If you are sure your block heater is working, test it for continuity with a meter. If it is good then remove the wire from each of the glow plugs and test for continuity from the spade on each glow plug to engine ground. Replace any of the glow plugs you find open. These are the 2 items that most often effect cold weather starting.
I posted this privately by mistake so I put it here too. Howie is absolutley coorect on his two items above so check those first. I have a 1984 beauville van with 6.2. This vehicle has been a royal pain at times with the cold starting thing over the last 15 years. Two things I have learned are this. In order to start in real cold weather you have to make good starter motor RPM. That means the batteries and connection all have to be in top shape. My experience is that the leads go bad from excessive cranking and heat at the starter lead terminal. Usually the connection bakes and will not pass current. The less informed generally pull the starter and replace it when cleaning up the connection will do. Then you need to check the battery conections. I always keep a can of hotshot (ether) under the front seat. When it is below 30 degress I give a 1-2 second blast up the intake extension to the front grill. This will fire off immediately upon hitting the glow plugs and get the engine rolling along at idle speed. If you get a lot of black smoke, you have some glow plug problems. Be very careful with the ether in warmer weather. It takes a lot less when warmer and once the engine rolls over it will continue to build speed until the ether is consumed. You can over speed a diesel like this because there is not throttle plate like on a gas burner. Once it goes off it is off to the races. So again, be real careful. Ether evaporates fairly slowly at lower temepratures so it is not a real big problem below 20 degrees unless you get real radical with the spray can. The best starting I've had from my diesel is recently when I mounted three batteries in parallel between the front seats. The van is real old and it is used as backup travel. That said both my Chrystler caravans are out of commission so it was nice to have the diesel van. 2000 amps of cranking with no chance of running out of cold cranking amps. Gotta be careful here too because you can melt the armature on the starter motor if you crank it too hard too long. Give the ether a try. A short little burst to start. The key with a diesel is if you can get a single cylinder to fire the gem will come to life and run. Let me know how it turns out for you. We are 13 degrees this am and mine fired right off after breathing some ether. I don't use the block heater anymore and I know I have some bad glow plugs. My also makes puffs of smoke with the piston stroke so I can see some of the glow pluds are bad and some are good. With 230,000 mile and only injectors this gem has been a good vehhicle. Let me say also that use of ether in the summer can give the problem with over speeding but also preignite and crack the heads or rings. I haven't had a problem in about 15 years. I was a licensed sea going engineer and worked on large ship diesels (8,000 hp). Be careful and take it slow and it will work out.
Ditto on Scotts words on the batteries and cables. They have to be top notch. If you have to replace them, make sure you are using quality parts. I have not ever had a set of batteries make it al the way through 3 winters. When the batteries get weak, it kills the starters. I used to keep a set of ac delco brushes, a pair of the bakelite brush holders, a new ac delco drive and bushings in the garage ready to go. I hear the newer gear reduction starters are an improvement as well. Mine has started on the slopes after sitting all day @ 0 degrees.
Scott giving you the straight scoop on glowplugs as well. If you run the GM check on them a few will be out of range. Don't worry unless they are way out. I am still running 4 or 5 of the original glow plugs in mine. I have almost went through as many controllers, however.
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