For the last few weeks I have been noticing a popping noise (sounds like a bad miss) which shakes the whole car while at idle. When it first started doing this, it only did it when cold, but now it does it all the time at idle, and sometimes when cold it will stall after starting. Also it puts out a lot of smoke until warmed up. The other thing is that it's only getting about 20mpg and I am driving on a highway for 3 hours a day, going 70mph the whole time. I had stopped running vegetable oil in the car, and that's when the problems seemed to begin. It's not using any oil, and everything seems normal, especially when it's up to speed. Anyone have any ideas? I am kind of thinking a partially clogged injector, if that's the case, would a diesel purge cure it?

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87 300D - Running on Veggie oil; 260,000 plus miles; Original #14 head