Trying to solve what should be a simple fix.

Upon starting my '99 f150 4.6 and revving accelerator up to about 1500, engine briefly acts like it wants to stall, but doesn't. This dipping of the RPMS seems to resolve once motor is fully warmed up. Seems like RPMS dip at 1500 when AC is on as well. The sound is almost as if there is water in the gas.

What Ive done on this renovation project:

New plugs, coils, fuel filter, AC, mass air sensor, gas tank drained of old gas.

Question: I dont know the name of the part where the 2 wire harness for each plug connects just under fuel rail. Could one of these fail when cold? What is the name of that part?

What else should I be checking?

Many thanks!

 

Learn first how the cold start works, then relate to the issue.
Cold start:
Key to Run only, runs the fuel pump about 3 seconds then shuts off.
At the same time the PCM looks at the values of the Intake Air Temp sensor and the Cylinder Head sensor to tell what the starting conditions are. (this is the only way the system has to know what to set parameter at.)
This sets the IAC full open for air and high idle, along with richer fuel injection and advanced Ignition timing.
At this point the OX sensors are not active as they are not hot but electric supplied to them begins to heat them to shorten the time they get up to temperature once the engine starts and exhaust begins to heat them.
At cranking, the fuel pump is restarted.
Depending on the beginning ambient temperature, the Idle should increase and remain until reduced by the program as the engine heats, coolant heats and intake air heats and Ox sensors come up to temperature at about 600 degrees to go closed loop.
This whole operation should allow the vehicle to be driven off cold without hesitation unless there is a fault in the system.
You need to look at the IAT, IAC and TPS for correct operation.
A Scanner looking at each item should tell which one is not up to proper operation.
It's not simple to get hold of unless you just happen to be lucky changing parts, otherwise.
I would look at cleaning the IAC and the throttle body first.
.
Hot restart is exactly the same except the values the PCM see from the Cylinder Head Sensor and the Intake Air Sensor dictateing how fuel, timing and IAC are set for hot operation..
The only thing your throttle does is open the throttle body plate and operate the TPS sensor to tell the PCM what angle the plate is at as well as add more air. Nothing 'directly' to do with adding fuel 'like a Carburetor' does.
Good luck.