If plugs are good, there are no vacuum leaks, fuel pressure is within spec, then we have to consider an injector (like you mentioned), a compression issue within that cylinder, and timing issues. I have to be honest, if it was timing, I suspect it would involve more than one cylinder.
To start, here is a link that explains how to test an injector:
Next, here is a link that explains how to test engine compression. Also, if you look through the first link below, it discusses symptoms of compression issues.
Here are the specific directions for your vehicle. The attached pic correlate with the directions.
2005 Buick Allure (CANADA) V6-3.8L
Engine Compression Test
Vehicle Powertrain Management Tune-up and Engine Performance Checks Compression Check Testing and Inspection Component Tests and General Diagnostics Engine Compression Test
ENGINE COMPRESSION TEST
Engine Compression Test
J38722 Compression Tester
A compression pressure test of the engine cylinders determines the condition of the rings, the valves, and the head gasket.
1. Run the engine until it reaches normal operating temperature. The battery must be at or near full charge.
2. Turn the engine OFF.
3. Disable the ignition.
Important: Remove the Powertrain Control Module (PCM) and the ignition fuses from the I/P fuse block.
4. Disable the fuel systems.
5. Remove the spark plugs from all the cylinders.
6. Remove the air duct from the throttle body.
7. Block the throttle plate in the open position.
8. Measure the engine compression, using the following procedure:
1. Firmly install J38722 to the spark plug hole.
2. Have an assistant crank the engine through at least four compression strokes in the testing cylinder.
3. Check and record the readings on J38722 at each stroke.
4. Disconnect J38722.
5. Repeat the compression test for each cylinder.
9. Record the compression readings from all of the cylinders.
The lowest reading should not be less than 70 percent of the highest reading.
No cylinder reading should be less than 689 kPa (100 psi).
10. The following are examples of the possible measurements:
When the compression measurement is normal, the compression builds up quickly and evenly to the specified compression on each cylinder.
When the compression is low on the first stroke and tends to build up on the following strokes, but does not reach the normal compression, or if the compression improves considerably with the addition of three squirts of oil, the piston rings may be the cause.
When the compression is low on the first stroke and does not build up in the following strokes, or the addition of oil does not affect the compression, the valves may be the cause.
When the compression is low on two adjacent cylinders, or coolant is present in the crankcase, the head gasket may be the cause.
11. Remove the block from the throttle plate.
12. Install the air duct to the throttle body.
13. Install the spark plugs.
14. Install the Powertrain Control Module (PCM) fuse.
15. Install the ignition fuse to the I/P fuse block.
Let me know if this helps.
Sunday, February 2nd, 2020 AT 7:07 PM