Question for the resident experts. Would a motor from a 1986 240 work properly in a 1990 240, or would the electronics (ECU, fuel injection system, etc) need to be changed over as well. Thinking it would be a fairly straight across swap.......or at least hoping.

Tanks for any insight.

 

Yes, Both have the same b230 motor. Simplest to keep the electronics in the receiving car. Swap the intake, whatever sensors are different, distributor and the engine wiring harness onto the doner engine.
Before starting the process I would make sure there are drilled and tapped holes on the block to bolt the flywheel speed sensor to and you will need the flywheel from the 1990 engine. (and install it correctly)

 

That's a very big question. Unless someone has actually done it, a lot of research into the schematics for both would be required. Generally, the later the model the more complex the electronics became. As you may have noticed Volvo was always on a continuous "improvement" trip.

 

I've done it a few times - and always used the wiring/fuel injection/ignition from the original car

 

Hoonk has some good advice there. But the LH2.4 system on the 1990 will need a crankshaft position sensor ( Like the one you are currently trying to remove in another post) placed in a dilled large hole and a smaller tapped hole in the 86 block. Use the original '90 block as a guide for the location, and save the flywheel . The '86 won't work on this system, as it uses no CPS (AKA flywheel speed sensor). I would use all of the recipient car's wiring; as the '86 was smack in the "biodegradable wiring harness insulation" years. You may have to modify some of the wiring harness connections...I had to in order to put an '88 engine into an '86. At least, with the engine out of your '90, you should be able to tap out that CPS from the inside