Got a wrecked 99 with a 3.0/5speed that I want to put in my "nice" 91. Started tearing down the 99 and I now have a lot of questions...

I had planned to use the motor wiring and ignition wiring from the 99, and just cut everything else out and run the 91 body harness and 99 engine harness side by side. Is this insane or will it work? Will the 91 ecu work on the 99, so I can run without cats etc? What other systems will need to be messed with if the 91 ecu would work? What am I gonna need to cut/modify on the 99 harness? Is everything gonna bolt up or no? Is it a good idea to mod the 91 to use the one piece driveshaft from the 99?

 

Don't start cutting harnesses. Use your harness from your 91
Use the block and heads from the 99, those are the same. Change everything else.
Keep the upper intake manifold from your 91, it will bolt up on the 99 heads and block.

The 99 will have a cam shaft position sensor, if your 91 has a distributor, that is where it will go, then the engine will have to be timed as normal.
If your 91 has a cam shaft position sensor and it has a different number of wires, then use that on the 99 block.

And yes, the ecu from the 91 will work, but you will have to keep all the things from the 91, don't think you can use any of the 99's wiring at all.
Use your injectors from your 91.
If your 91 has cats, then leave them there, the computer and oxygen sensors expect them there.
The block will be the same, you current drive will bolt up to the block, I don't know about changing the drive shaft.

 

+1 ^^^

1991 can be run without Cats, its OBD1
1999 Can not be run without Cats, OBD2, well can be but CEL will be on all the time, so you would never know when there was a real problem

1991 is a 60-wire computer
1999 is a 104-wire computer.................40 more wires to deal with

1991 has distributor
1999 does not

1991 uses 30psi return fuel system
1999 uses 60psi Returnless fuel system

As said, swap over just the 1999 block and heads, and bolt the 1991 stuff to it

 

This info has been very helpful, I'm tearing down the motor right now and noticed the cps is present on the 99 and not the 91. Will cps delete be harmful to the 91 motor as long as I'm using the 91 stuff on it?

 

1991 3.0l used a distributor in place of Cam sensor, so no problem swapping them out, 1991 computer needs a distributor signal