* Different Sensors and Components: The V6 and V8 engines use different sensors (crankshaft position sensor, camshaft position sensor, MAP sensor, etc.), injectors, and other components. The wiring harness for the V8 is designed to accommodate these, while the V6 harness is not. Trying to force a connection will lead to mismatched signals and prevent the engine from starting.
* Power Requirements: The V8 will likely draw more power than the V6. The existing wiring might not be adequately sized or fused for the V8's demands, leading to power issues.
* ECM Compatibility: The original PCM (Powertrain Control Module) or ECM (Engine Control Module) for the V6 is not programmed to control a V8 engine. This is a major hurdle. You almost certainly need a V8 ECM that's compatible with the year and model of the truck *and* the specific 350 V8 you installed.
* Grounding Issues: Improper grounding can cause a multitude of starting problems. The V8 might require different grounding points than the V6.
* Incorrect Wiring: Even if you have the correct harness, improper installation or connection could be the reason.
Troubleshooting Steps:
1. Verify the ECM: This is the most crucial step. You absolutely need the correct ECM for your 350 V8 engine and year of truck. Without it, the engine won't even attempt to start. A properly programmed ECM is essential for fuel delivery.
2. Fuel Pump Operation: Check if the fuel pump is getting power. If not, the problem could be in the wiring between the fuel pump relay and the pump itself. You can also check fuel pressure directly at the fuel rail with a gauge.
3. Inspect the Wiring Harness: Carefully examine the entire wiring harness for any damaged wires, loose connections, or incorrect routing. Pay close attention to the areas where sensors, injectors, and other components connect.
4. Check for Spark: Use an inline spark tester to verify if you're getting spark at each spark plug wire. If not, the problem could be in the ignition system or the ECM.
5. Relays and Fuses: Check all relevant relays and fuses related to the fuel system and ignition system.
6. Ground Connections: Ensure proper grounding of the engine and the wiring harness to the chassis.
7. Professional Help: If you're not comfortable working with automotive wiring, it's best to seek professional help. This type of engine swap is complex and requires a good understanding of automotive electrical systems. A miswired system can cause significant damage.
In short, the wiring harness is a very likely suspect, but the ECM is the critical element. Don't jump to conclusions about the harness until you've verified everything else.
