261K! There is no problem with that. You must have taken care of it! I've owned them since 1990. Great trucks. I run mine to 200K and then get a new one.
As far as the problem, when you say original parts, do you mean you changed the parts from the original engine and placed them on the new one? Also, there weren't too many changes in the 3.9, but if left the older sensors in the engine when you changed it, the computer may not be reading things correctly.
Let me know about that, and also, make sure the crank sensor is good and not dirty.
Finally, have you checked fuel pump pressure and has the check engine light come on?
Saturday, October 3rd, 2009 AT 9:58 PM