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I have had a few people doing conversions from 4.3 V6 to 5.0 or 5.7 V8 in 1996 to 1999 vehicles.  I will try to focus on this a lot to get all the info out there you'll need.  First of all, you can USE your v6 engine harness to run the v8.  It should fit just fine.  You do need to add two additional injector wires to the PCM, and two key hot power wire to the injector connector.  Of course you need to change out the large injector connector.  The V6 PCM can also be reprogrammed for the V8 engine.  Use the SAME PCM.

The thing to keep in mind here is that the injector # assignment is not the same from v6 to v8.  Also if you read the special note above, you'll know that injector pin assignment changed from 1997 to 1998 for cylinder 1 & 8 on a V8, and 1 & 6 on a V6.

So you are putting in a V8 in place of V6 engine.

Your V6 engine uses the following wire colors for these cylinders.  It does not matter the year 96 to 99, colors remain same, only pin location from 97 to 98 changed. Also V6 to V8 pin locations changed.

1: BLACK

2: LT GREEN/BLACK

3: PINK/BLACK

4: LT BLUE/BLACK

5: BLACK/WHITE

6: YELLOW/BLACK

V8 engine uses this color coding.  Injectors 1-6 are the same color at the injector connector. 7 & 8 are new colors.

1: BLACK

2: LT GREEN/BLACK

3: PINK/BLACK

4: LT BLUE/BLACK

5: BLACK/WHITE

6: YELLOW/BLACK

7: RED/BLACK

8: DK BLUE/WHITE

Now, here is where you need to pay attention.  PIN LOCATION AT THE PCM is different between V6 and V8  and 97-98 for Inj 1,6 on V6, and 1,8 on V8.

V6 1996-1997      to   V8 1996-1997.                       Location of #1 and #6 V6 or #8 V8 changed from 97-98

Inj 1: RED pin 3         Inj 1: RED pin 3                    1998+ =====>    Inj 1: V6 RED pin 8    Inj 1: V8 RED pin 8

Inj 2: RED pin 15       Inj 2: RED pin 11

Inj 3: RED pin 16       Inj 3: RED pin 32

Inj 4: RED pin 32       Inj 4: RED pin 31

Inj 5: RED pin 31       Inj 5: RED pin 15

Inj 6: BLUE pin 5       Inj 6: RED pin 16                    1998+ ======> Inj 6: V6 RED pin 9

                               Inj 7: RED pin 12

                               Inj 8: BLUE pin 5                    1998+ =========================> Inj 8: V8 RED pin 9

As you can see, going from v6 to v8, only Injector #1 is in the correct location in the PCM connector.  You have TWO options to correct this. 

1. You can MOVE all the pins around AT the PCM so the color is correct per factory wiring pinouts/diagrams, then you can wire in the v8 connector COLOR to COLOR, exact match up.  This takes a little more work, but will ensure no confusion down the road if you ever had to diagnose a problem (or someone else for that matter)

2. You can LEAVE all the pins in the current PCM connectors, and then match the V6 color/location to the V8 color/location.  This will make it easy and quick, but might cause confusion if you have to diagnose.  But if you want to do this.  Follow this info.  AGAIN, this LEAVES THE V6 PINS in the STOCK location AT THE PCM, and you are wiring in a V8 injector connector.

V6 harness BLACK to V8 connector BLACK

V6 harness LT BLU/BLK to V8 connector PNK/BLK

V6 harness LT GRN/BLK to V8 connector BLK/WHT

V6 harness BLK/WHT to V8 connector LT BLU/BLK

V6 harness PNK/BLK to V8 connector YEL/BLK

V6 harness YEL/BLK to V8 connector DK BLU/WHT

you will then need two additional PINS to the PCM.

Pin 12 of PCM RED connector needs wire run to V8 connector RED/BLK

Pin 11 of PCM RED connector needs wire run to V8 connector LT GRN/BLK