1996 Ford F-150 noises

What is happening? You did a complete tune up and now the idle is low and it sounds like like a diesel, check your work that you did. Especially the wires and plugs. What it sounds like is that there could be two spark wires crossed.

Lets see 1996 you should srtill be able to do it. You say that you did the oild change first then the engine started to sound like a diesel engine, that is when you did the tune up, Right? I don't see where cjanging the oil to a heaver oil would do it. Try resetting the computer. Has the engine light been on? Disconnect the negative side of battery and leave off for a minute or two, then reconnect, to completly reset the computer the truck has to be drive about 50 miles, this does not have to be done all at once. The computer could have picked up on a code that changed the timing a degree or two. I don't know if this will straighten the problem but somehow I feel that the computer picked up on the change in volocity of the oil. Let me know what happens. We have a very good ford moderator and will check with him to see what he says.