Earlier this year I swapped a KA24E from a '91 2wd D21 into a '86 4wd 720. Seems like a bunch of people have done this swap, and there's quite a bit of info on here about it, which was super helpful. But I'm not all that experienced of a mechanic and definitely some things tripped me up, so thought it could be useful to talk about them here for anybody else who might be interested.
Couldn't find a 240sx to get the engine wiring harness, so I ended up using the wiring and everything from the D21. All the engine and ECU wires are mixed in with the main wiring harness in the truck, so it's hard to get the ones you need. But it can be done, takes a long time and not very fun- gotta pull the whole dash out of the truck, unwrap the entire harness, and trace each individual wire, then label and cut them to be able to detangle and pull the harness out in sections. Anyway the easy wires to sort out are the ones that go straight from the ECU to a sensor or something. But then there are a still quite a few weird wires that do other things. I made a diagram that shows most of the weird ones and how I ended up wiring them in the 720. For a bunch of this I think there are other ways that would work also, but this is one way to do it. I didn't show any ground wires on here but I tried to do all the grounding the same way it was on the D21.
There's one wire that's a tach output signal from the ECU that goes to the tachometer in the dash on the D21. I couldn't get that ECU signal to work with the tachometer in the 720, maybe it is possible, but instead I wired the 720 tachometer just like it was with the Z24, there's a wire from the tachometer to a resistor in-line near the ignition coil, then to the ground side of the coil. So just one splice right near the new coil is all that's needed. It was a blue wire on the KA24 coil.
I'll try to keep adding to this in the next few days, lots to remember!
For a fuel pump I used a Walbro GSL392 in-line fuel pump, got it on ebay. Removed the factory fuel pump and filter, replaced the rubber fuel lines and mounted the new pump in the same place as the old one. I think after the photo was taken I ended up replacing the hose clamps w/ new regular kind. The old weird Nissan hose clamps look cool but I had some issues w/ them leaking. I used the filter from the D21 and mounted it in the 720 engine compartment, like it was on the D21. Oh and it turns out if you have the fuel lines reversed, the engine won't be getting fuel and won't run. The high pressure line from the pump should go to the back side of the fuel rail, and the return line should connect to the pressure regulator.
While the KA24 was out of either truck I got rid of some stuff- charcoal canister, AIV system, SCV system, and EGR. I don't know if it was necessary but I pulled the intake manifold and completely took out the butterfly valve thing for the SCV. There's a rod that extends all the way through the manifold for all 4 valves, and it sticks out the back of the manifold. So after you remove it there's a hole in the back of the manifold that I tapped/plugged w/ a bolt. Then there's still 3 small holes connecting the adjacent intake ports that I didn't do anything with, one of them is shown in the photo. Hope it's ok. I removed the EGR plumbing completely. Got a little cover plate thing off ebay for the egr intake manifold port, and a threaded plug for the exhaust manifold port. That plug was a little hard to find- the thread is M24x1.5. The only vacuum lines left on the truck are one for the fuel pressure regulator, and the big one for the brake booster. Plugged off the extra vacuum ports on the throttle body. I guess my main motivation for getting rid of all that is to have more room to work around the engine, and to never have to deal with or troubleshoot any of those systems.
In other news I still have all that KA24 emissions stuff if anybody wants it, except the egr tube which got mangled in the process..
The donor D21 pickup did not have power steering, but the power steering pump, tensioner pully, and brackets from the Z24 worked on the KA24 with a little modification. The pump bracket bolted onto the ka24 head no problem. Originally the pump mounted w/ 2 bolts on that bracket, 1 bolt on the intake manifold, and 1 on the alternator bracket. But on the new engine only the 2 bolts on the bracket work, also the mounting tab for the one on the intake manifold had to be cut off for clearance. Seems ok so far w/ only the 2 mounting points. Had to bend the metal hydraulic lines a bit to get everything to clear. The other bracket, for the tensioner pulley, mounts to the block w/ 3 bolts. 2 of them work fine, and the 3rd needed a shim- I used a stack of washers. The KA24 ecu has an input for a power steering pump pressure switch, I think it's supposed to raise the idle a little bit when you're turning the wheel at low speeds. It does bog the engine down a little w/o that system in place, but not bad. Eventually I would like to find the right power steering pump from a D21 w/ that sensor and all.
I knew from all y'all that the oil pan on the new engine would not clear the front differential on the 720, and that the motor mount brackets would not line up right. Saw a several-year-old post from Patrick Smith where he was offering the service of modifying those parts for this type of engine swap. Contacted him and he agreed to do it, sent him the parts and he worked some magic on them. I thought they turned out super good.
Where the exhaust system bolts to the exhaust manifold is a different interface on the two engines. They look similar so I was kind of assuming they were the same, but nope. The exhaust on my 720 was in pretty bad shape though, and the 2wd D21 exhaust was a totally different routing under the truck. Ended up just using the first few inches of exhaust downstream of the manifold from the D21, and taking it to an muffler shop to bend a new exhaust from that point back.