My week has been plagued by car problems. I drive a terrible old V6 Firebird, build long after after the classic Firebirds were gone, when it was nothing more than a knockoff Camaro with a different name. It's had no problems in the last few months of course. My wife drives a '09 Corolla. She's been driving it a few years, but we only got around to buying it from my father-in-law this summer. So now it developed problems. Last week a bearing went bad on the alternator, that was pretty easy to diagnose, and obvious when the alternator was off -- turn the pulley and it made a nasty crunch. But occasionally from a cold start, it idles slowly, sounds like it's going to stall out our something, but then after a couple seconds, it idles normally and shows no more symptoms. And also occasionally at a cold start it has a fairly loud rattle, again for only about a second, and then no more symptoms. My father law was pretty sure this was a starter going bad -- odd idle sounds from the starter gear not retracting right way, and rattle from the gear not retracting completely and remaining in partial contact for another second. So we replaced the starter, and the sounds went away for 24 hours, and then came back with no change whatsoever. So now I have no clue about the weird idle sounds, and I'm starting to think the rattle might be this:
A bad Bendix on a starter motor presents itself in a greatly more dramatic fashion than that. How did the starter look when you pulled it? I'll bet it was fine. Loping idle while cold is generally related to the vacuum logic not adding up. This means a cracked vacuum line or a plugged vacuum valve or something like that. Basically the ECM is getting the wrong manifold vacuum values so it's got the wrong advance but it can't self-compensate by dropping the RPM so it throws it into that regime and waits for something to change. Usually the ECM ignores that value once temperature has risen to operating values so the problem goes away once it's warmed up. It's become increasingly common for manufacturers to lower weights by going with smaller batteries and smaller starter motors, which requires some sort of intake/exhaust bypass to lower compression pressure. This bypass is often centrifugal or solenoidal and closes once the engine hits idle. If it doesn't hit that idle quickly it may hammer, which often sounds a lot like your problem. The hose that's compromised is small, non-obvious and deep in the engine. Should you find it you will never be sure if you fixed it, and you may fuck up two or three others in the process of diagnosis. This is a great thing to farm out.
Yeah, the old starter seemed OK. I'll look into some more troubleshooting for vacuum issues. I do think it's odd that both sounds only occasionally happen, and never for more than 3-4 seconds after ignition.
The 3-4 seconds after ignition is because the ECM is in "START" mode as opposed to "RUN" mode or "WARM UP" mode. Fuel curve is different, ignition timing is different, combustion mix is different, exhaust gas has different makeup. It's all a big mess of look-up tables and "START" is a whole different table than "WARM UP" or "RUN." There are varied indicators that push the engine onto a different table. One of them can be vacuum.