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.]

  • Did it ever run well in that kind of cold before? I have similar issues with my Toyota MR2 when it gets down around 0F. Idle will get crazy low if I put any load on the engine (brake or power steering). When it gets down to -15F, it'll even do a thing where it starts to idle after start, then just shuts off and I have to restart it every time when the engine is cold (second start it'll stay running as long as I don't touch the brake until it gets some heat in it). I've always figured it was a fact of life with '90's fuel injected engines... – Brian Knoblauch Feb 1 '13 at 21:15
  • Just bought the car used, so I'm not sure. I'll look into getting it on a flat surface and see if it has the same issues without any load on the vacuum system. It has an aftermarket intake though. [Throttle body is stock; the airbox is gone and replaced with a cone filter bolted to the MAF.] – Robbie Feb 1 '13 at 22:22

Ahh, cold starts. In the DIY ECU world, this is always the most difficult part of tuning a vehicle. One reason is that the engine is horribly inefficient at low temperatures, and requires a very narrowly defined amount of fuel. Too much and it eventually bogs down and dies. Too little and it dies as well. Then if the engine's warmed up at all during this, you have to wait 6 hours for it to cool down completely. It's interesting to note that cars require 2, 2.5, 3 times as much fuel to idle when cold than when warmed up. And that idling is one of the hardest things for an engine to do, tuning wise. It requires just enough fuel to stay at a specific RPM, but not too much that you increase RPM, waste gas, or stall out. But I digress...

If it requires the gas pedal, this sort of gives me the impression that it is getting too much fuel. A faulty air-flow meter, perhaps. Or a leak in the intake between the AFM and the throttle body.

However, that's not set in stone. It could be that it's too lean and when you press the gas, you trigger acceleration enrichment, which dumps fuel, which allows it to run a bit until it needs another fix.

I recommend:

  • Like you mention, getting on a flat surface (with wheel chocks of somesort), and not using the brake, to avoid changing engine vacuum abruptly when it's in this delicate state.
  • If it has a simple adjustable IAC, like you seem to imply, attempt to adjust it to allow more air to flow. A higher RPM idle might be the result, but if it stays running it's progress in my book.
  • Check for a cold idle valve, and test it if it exists. IACs are typically for upping the idle when under load (power steering demands, A/C, etc.), where cold idle valves are for, well, idling when cold.

Side question: Is it a 5.0?

  • I would ask about the engine size as a comment to the question, especially if it’s a side question. – theUg Feb 10 '13 at 23:40
  • Not the 5.0, it's the 2.3L N/A. Going to add that to the question. – Robbie Feb 11 '13 at 16:01

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