Cold idle

[elerner]

I'm curious, when I start my car when it's cold (the car), the idle is very low (about 500), almost ready to stall it'll slowly build up to around 900rpm. Is this how it should be behaving? Stock, the idle when first started was high and then it came down. I'm wondering what's going to happen when the temps dip into the 30's or lower.

Once the car is even a little warm, it idles fine.

[Banshee]

My car does this too. But it only dips down to 500 rpm once per cold start, then bounce right up to 1200 rpm. Doesn't matter if its cold outside or not.

[elerner]

My car does this too. But it only dips down to 500 rpm once per cold start, then bounce right up to 1200 rpm. Doesn't matter if its cold outside or not.

What the hell does the IAC do then? I figured it would raise the idle when the engine is cold (when conditions vary) and then once the engine is warm(when conditions don't really vary) the hard idle screw manages were it idles.

[Tom @ Fast Forward]

Now that you have some cold starts, have you tried un plugging the IAC connector and seeing if the idle shifts. The ECU has long and short term memory and it takes awhile for it to learn the conditions. If you are talking about the first 90 seconds or so, the O2 sensor does a lot to tell the ECU where to set the fuel and, during the first 90 seconds or so is warming up so the ECU goes to a set of look up tables for fuel and timing. Those are based on the original throttle body and manifold and engine loads and are not changeable.

Is it just the first 90 seconds or so or does it last longer? Not much you can do about it on any boosted engine.

Once it is all warmed up and and the oil is all warmed up, does the idle change if you unplug the IAC connector? Adjust the idle stop screw for no change in idle.

When the engine is warm, the conditions vary quite a bit. AC on or off, headlights on or off, cold incoming air (dense with lots of O2) or hot incoming air (significantly less O2), Power steering in motion to park the car, etc. The ECU tries to maintain good idle via IAC and timing. To do that it needs feedback from the MAF, IAT sensor, TPS and O2. Until the O2 is warm and stable, it only has the lookup tables.

If it has bad idle in cold weather and the engine is warm and you have properly adjusted the idle screw, you probably have a vacuum leak somewhere. One thing about the coldside above other boosted systems is it's ability to idle correctly.

[elerner]

It's just the first I'd say 20 seconds or so that the idle is low (around 500). It slowly picks up after that. I've played around with the idle a little (set it a little high, then turned it back). I'll have to do the "pull the IAC connector) routine tomorrow and check to see if I've got it right.

I still don't understand what the setting controled by the allen screw does then. Does it establish an absolute minimum throttle position setting? When does the idle control logic rely on that?

[Tom @ Fast Forward]

The throttle stop screw sets a target for the IAC to shoot for. But we need the ECU to learn for a couple days where it wants that target. It varies with O2 sensor, TPS, MAF, timing the new load on the engine trying to spin that big blower at idle (takes about the same as the alternator with the lights on). Lots of stuff and it averages it all out over a coule days and finds the 'sweet spot. Then we need to pre-set the TB to drop close to that sweet spot so all the IAC has to do is correct for lights on or off, AC on or off, warm day, cold day, etc. Both of my cars started and idled perfectly on cold start at 35F and 95F after setting that per the procedure. I will admit, the 99 is better during that first 90 seconds during O2 warmup than the 05 but I think that is back to the cold oil and VVT. However, after the O2 warms up, it idles perfectly under all load variations and, after the oil warms up, the get go hesitation from the cold VVT is gone as well. Probably takes another 2-3 minutes for that.