F150 2001 5.4 4X4 CrewCab 138,000 miles idle speed oscillates between 700 and 1100 when extremely cold while the engine warms up and then runs normally when warm. Runs for a couple of seconds at 700 rpm and then speeds up to 900-1100 rpm for a couple of seconds and then back to 700 rpm for a couple of seconds.
Purchase OBD2 device
Found defective thermostat, changed thermostat
Engine P-0171 and P-0174, fuel pressure 30-40, changed fuel filter resolved codes.
Cleaned mass air flow sensor.

 

The codes point towards a vacuum leak. Did changing the T-stat cure the cold idle issue? or does that still persist? Changing the fuel filter and cleaning the MAF sensor is just good maintenance. Fuel pressure looks good, was that after the filter change?

 

Engine heats to proper temperature with Tstat replacement. Engine ramping at extreme cold temperature during warm-up persists.

 

Check for vacuum leaks, the elbow in the PCV hose behind the throttle body rots out on these engines. Wouldn't hurt to clean the IAC, use WD-40 and work the plunger with a small screwdriver. Cleaning the throttle body itself wouldn't be a bad idea either.

 

Does this happen when the compressor clutch engages and disengages? I was wondering because I have a similar problem on a friend's car.

 

COLD START operation:
Key to Run samples the CHT and IAT sensor values. The system has no other way to know the difference between cold and hot starts.
This sets the IAC open for high idle 'depending' on the cold temperature detected by the above sensors.
The Ignition timing is advanced.
The fuel is richened up.
.
High idle is normal and drops on a time/engine temperature control basis.
In very low temps the idle can go as high as 1200 rpm and take 3 to 5 minutes to slowly come down to normal idle of 750 +/- 50 rpm.
Within about 1 minute the Ox sensors heat to 600 degrees and begin to control fuel injection levels and takes control from a previous fixed fuel table.
.
This is all normal operation as long and there is no faults with the sensors and IAC.
Additionally, the transmission will not shift into OD until it's fluid temperature get to about 98 degrees on a cold drive-away.
The radiator fluid cooler helps to heat the fluid a bit faster before drive-away if the time element fits.
If the above operations were not built into the operating system, the vehicle cold drive-away could not be done without stalling and hesitation.
Good luck.