After changing intake gaskets number 5 cylinder floods with fuel and headlights flash while turning over

Which engine do you have?

Injectors are controlled by PCM and if the headlights are flashing, could it be due to the security system?

I would suggest checking for a bad body ground, as this is the only thing common between the two. If not, it could be separate problems and you might have a bad PCM.

Diagram indicates the location of the various grounds in engine compartment and I would suggest starting with those.

Sorry for the delay in replying, was away for a few days. Hope you have not lighted the fire yet.

INJECTOR CIRCUIT DIAGNOSIS

NOTE:
This test only applies to vehicles equipped with a VCM.

4.3L, 5.0L, 5.7L & 7.4L

1. Perform On-Board Diagnostic (OBD) system check. After performing OBD system check, go to next step.

2. Disconnect injector harness connector at intake manifold. Turn ignition on with engine off. Using a test light connected to ground, probe injector ignition feed circuits. If test light illuminates at all injector feed circuits, go to step 6. If test light does not illuminate at all injector feed circuits, go to next step.

3. If test light illuminates for only some of the injector circuits, go to step 5. If test light does not illuminate for any injector circuit, go to next step.

4. Check ECM I fuse (15-amp on 4.3L; 20-amp on all others), located in underhood fuse block. If fuse is open, go to step 10. If fuse is okay, go to step 11.

5. Locate open in faulty injector feed circuit. After locating faulty circuit, go to step 12.

6. Install injector test light to each injector circuit, one at a time. Crank engine. If injector test light flashes on each injector, check fuel injector(s). If injector test light does not flash for each injector circuit, go to next step.

7. If injector test light was on steady on any injector circuit, go to next step. If injector test light did not illuminate on any injector circuit, go to step 9.

8. Install injector test light to faulty injector circuit. Turn ignition off. Disconnect Red VCM harness connector. Turn ignition on with engine off. If injector test light is off, go to step 16. If injector test light remains illuminated, go to step 13.

9. Check for an open in injector control circuit. If circuit is open, go to step 12. If circuit is okay, go to step 14.

10. Locate short to ground in injector control circuit. After locating short, go to step 12.

11. Repair open or short to ground for all injector feed circuits. After repairs, go to step 17.

12. Repair circuit as necessary. After repairs, go to step 17.

13. Repair short to ground in injector control circuit. After repairs, go to step 17.

14. Check terminal contact at VCM harness connector. If terminal contact is faulty, go to next step. If terminal contact is okay, go to step 16.

15. Repair terminal contact as necessary. After repairs, go to step 17.

16. Replace VCM. Perform VCM reprogramming procedures. After repairs, go to next step.

17. Using scan tool, select DTC, CLEAR INFO function. Attempt to start engine. If engine starts and continues to run, go to next step. If engine does not start, or starts and stalls, go to step 2.

18. Allow engine to reach normal operating temperature. Using scan tool, select DTC, FAILED THIS IGN function. If any other DTCs are present, diagnose DTCs. If no other DTCs are present, go to next step.

19. Using scan tool, select CAPTURE INFO, REVIEW INFO function. If additional undiagnosed DTCs are present, diagnose DTCs. If additional DTCs are not present, system is okay.