Ok.. I have a 2001 supercrew, 4.6L, 150000 miles. I need to replace cycl.#2 injector. My question is what happens if I put eight new 30# injectors in place of the old ones? The only reson for this is I already have the 30's laying around from a different project. Would I have to get the PCM re-flashed? Any help would be great, thanks.

 

You would need a custom tune to run the 30# injectors.
Buy the time you pay for that. You could buy a new set of 19#ers.

 

add a turbo

 

Here is the background on it.
The PCM has fuel tables that are continiously updated as the motor runs.
These tables have about a 20% range change available to keep normal corrections and account for engine and parts wear.
The motor will run on 30s and the PCM will adjust for them being richer by shifting the fuel tables toward their lean limits.
This is were the issue comes in. You have no adjustment left in the tables before lean codes are set.
In addition, fuel injection control is by varying the pulsewidth or 'on' time of the injector.
With larger injectors the pulsewidth will be shorter to limit fuel to that amount the 19s would normally flow at the same fuel pressure. The PCM does not know there are 30 lb injectors in place. It only detects the excess richness and tries to react to it.
Another issue is the mass air meter is calibrated for 19s. With the larger injectors the airflow will not match the 'expected' injection amount called 'tracking' signals.
So the answer is yes and no.
You may get away with it and you may not.
The system is engineered to work the hardware and softwere in a 'centered' area such that there is room for normal shifting before it become a drivability issue.
One effect may be a rich lopeing idle and rich cold starts due to to much fuel and much poorer fuel mileage and possible drivability issues..
Good luck.