Re: 1990 f150 idle problems
....Ok, this tells me the IAC is able to work as designed but may still be sticking and need cleaning.
....As said in last reply, the Temperature sesnor could be out of tolerance. The resistance can be measured both cold and hot to see if it changes from cold to hot.
Look for a resistance range cold to hot of 37000 ohms cold to less than 1 ohm at about 200 °. Be sure the plugups are all on and seated.
....Here is how cold start sequence works;
....At ignition key on 'run' the computer looks at the values of the temperature sensor and the Intake temperature sensor to set the amount of fuel and ignition timing and opens the IAC wide to increase air bypass around the throttle body plate to raise idle rpm.
....As you can see any fault in the hardware involved will cause a start/idle issue.
I would clean the IAC, and the throttle plate first, then measure the resistance of both the coolant sensor and the intake air sensor for resistance range.
....Note both the coolant and the intake air sensor should have about the same resistance value over the temperature range because they are both the same element but in a different package for obvious reasons.
....The last part of the operation is the OX sensors. The should heat up to operating temperature in about 45 second and begin to take over fuel control if they are not to lazy from age. They need to get up to 600° to begin to operate.
....I'm sure you will find the issue somewhere in this sequence.
....Use can spray cleaner on the IAC and throtle body plate/bore until it flows out nearly clean on a cloth. Wait some time for it to dry up before starting the mo.
....After you find the issue and clear it, it would be a good idea to re boot the computer and allow it to re-learn the start sequence from scratch for good starts under all conditions.
....To do this, remove the battery cable for several minutes. Reconnect. Put headlites on and start the motor. This does a cold start under high load conditions so battery charging and the lights will cold load the motor such that the computer detects this and stores in memory, those conditions. Then allow engine to heat up and drop to normal idle of about 650-700 rpm and be stable by it'self.
....Drive about 5 miles and do several restarts under hot conditions so this is stored in memory. Note the IAC and coolant sensors are key to all this for memory storage between cold and hot conditions.
....After this procedure, all your starts should be perfect without intervention if there are no other faults.