I have a 1990 IDI with a manual trans, I was driving down the highway yesterday and started going down a hill. While doing this I slightly let off the throttle, the truck then started running rough, I looked in my mirror and saw blue smoke coming out of the pipe doing around 70 mph. Immediately I let off the throttle and this stopped, when I re-applied throttle it ran normal. After this I noticed when taking off when I would let off the throttle to shift gears a small puff of blue smoke would appear. I am fully aware of the internals of an engine and how they work. This is the same shade of smoke that comes out when I fire it up on a cold day and have one or more bad glow plugs, it doesn't effect this much but all the glow plugs are functioning properly. Also it was relatively cold when this happened, 35 or less. All I can figure is that I have a bad injector, they were replaced years ago and have about 150000 miles on them. At that time the injector pump was rebuilt as well. Please help as this is my DD and I might get seen driving my girlfriends little car around if the problem doesn't get solved quickly....