Popping at idle, and bad mileage?
 

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?

I think I may have located the cause of the problem. I noticed that a line coming out the bottom of the square solenoid near the trans amp was broken off. I am thinking this controls the wastegate? Is that correct, and would that cause the above symptoms? Also, did I do any damage by driving it this way for so long?

Edit: After doing some research, I figured out this solenoid controls the transmission, so I didn't fix the problem after all. I did put in some injection cleaner, but didn't drive it enough yet to see if it will help.