3.9 vs 4.0?

[SGFMORover]

actively looking for my next rover and in a dilemma my 95 was a 3.9 and I was really very fond of it, I get the impression its less troublesome than the 4.0? my old 3.9 is still in good shape, truck is retired due to dash fire.
I'm looking at a few old discos and wondering if the 3.9 can be put into a truck with an ailing 4.0? any issues with swapping them? I know the ecu would have to be changed with the engine but wiring ancillary items? any issue bolting a 3.9 into a truck that has a 4.0 with a 5 speed??? really puzzled.

[phoenix]

if you plan on installing a 3.9 14 cux where a 4.0 gems used to be, it is almost a straight swap specially if the 3.9 is a serpentine engine.
the 4.0 does not have a distribuitor so its front cover will have to be installed into the 3.9, the 4.0 uses a different style torque converter so you will have to move the flex plate to the 3.9 you cannot retain the 3.9 flex due to the induction pick up, the 4.0 crank uses a dowl pin to locate timing the 3.9 does not so a little ingenuity is in order other wise it will never start... the 4.0 uses two knock sensors in order to control timing advance and pinging/knock.... some late 3.9 do have the tapered fitting for these if yours dont then you cannot retrofit it to run on the 4.0 electronics.. if you do then the plenum and all other ancillaries are just a bolt on... lastly the 4.0 gems is an obd2 platform while the 3.9 is a 14 cux.
two totally different engine management systems, check your local regulations as far as emmisions testing, quite often this is not allowed as you are going from newer technology to older (down grade) but upgrade older to newer is allways allowed.
the 4.0 and 4.6 blocks have crossbolted main caps, the 3.9 does not, not a big deal unless you plan on supercharging.

[1996trek4]

You can find a pretty decent D1 in the 3-5k range... Not sure what you're gaining by buying a truck with a bad motor and then doing all of the work to swap your engine over... I just bought a 96 with bad brakes, ignition switch and horrendously curled dash for 500 bucks-runs great, no smoke, no tick...Throw calipers rotors and pads on the front I already have, used ignition switch and a dash repair trim piece, replace all the fluids and I'll turn it for a few hundred more than I'll have into it just to keep another one on the road. They are dirt cheap right now, just be patient. Most are going to be 4.0L trucks so finding a 3.9 is going to be more difficult. I do not think there are any advantages of one over the other. Personally, I prefer not having a distributor. We got very few 1994 and 1995 trucks with the 3.9. I think the highest sales year for the D1 was 1996 and all of these through 1999 were 4.0L.... 1999 the D1 and D2 were sold side by side for a short time in North America as the D2 actually came out in Europe in 1998.