So we've had some mornings dipping down below 5deg F recently here in WI.
The good: my stripped-down 1993 Mustang starts, can run, and drives just fine in this weather. This is the 2.3L N/A, with an after-market intake.
The bad: if you don't baby the throttle for the first few minutes, the motor will drop to idle, sputter, and just die.
We just changed the oil (5w30 full syntehtic). The motor has plenty of coolant. The temperature gauge never rises to normal-temp, but the block definitely holds heat just fine. We're investigating problems with the thermostat, temp gauge, and the radiator fan. The idle does drop as it should though after the car has warmed up, so this doesn't seem to be affecting the IAC at all.
My guess is possibly that the idle air control valve is not enjoying the cold weather. I'm not very familiar with this particular type of IAC. (I'm used to stepper motors.) -- Is this in effect just a set screw?
The other thing to note is during this time your foot must be on the brake, as the car currently has no handbrake [previous owner cut the cable; I assume it was seized or frozen]. Holding the brakes could impact the vacuum system, but this shouldn't kill the car even in these conditions.
How can you diagnose problems with the IAC aside from just replacing it? And is there anything else I should test first?
[For what it's worth: the car starts and idles fine once the temperature is above say 10-15 deg F; so it's definitely temperature dependent.]