When you ran it without the intake on, did you get a Check Engine Light ? You removed the MAFS ( Mass Air Flow Sensor ) which tells the truck how much air is flowing into the throttle body ( for Air / Fuel ratio with pulse width on the injectors ). This is in the body of the air filter element
You can clean this with MAFS safe spray cleaner ( not all throttle body cleaners are MAFS safe, it will say on the can if it is. If it does not say MAFS safe, it is not.
The other is the IAT ( Incoming Air Temperature ) sensor. This indicates the air temperature, so coupled with the Air Flow, this gives the density of the air, to make a correct A/F ratio adjustment.
This is the one after the air filter and the MAFS, before the throttle body.
You can clean this as well, it is a bulb sensor, be careful with cleaning it.
If the RPM are jumping around at idle, this would be a function of the IAC ( Idle Air Control ) on the back of the throttle body. When the butterfly is closed, this is what lets air in.
It is under the plastic throttle body cover, held on to the back of the throttle body with 2 8mm ( 9mm ? ) bolts.
This could be gummed up, and slow to open, so the truck opens it too much, causing the rev up, and then closes it too much to make up for it.
It is a motor operating a piston to let air into the back of the throttle body.
Removing this and cleaning it with throttle body cleaner might be a good idea. Also check the vac line to the back of the throttle body for cracks, the rubber 90* elbow is prone to cracking over time.
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