2009 Toyota Corolla 175K mi, automatic transmission. 

This saga started when my alternator failed and it was replaced. I was ok with the repair attempts listwd below because they were mostly deferred maintenance that needed to happen anyway, but now I still have an idle which hunts and I'm not confident the shop knows how to diagnose the root cause. Hoping to have some next steps for ideas to possibly track down. Or the next things to check.

My goal is to hold on to this car for at least a few more years until the used market gets sane again. But currently with the occasional stall on start its hard to rely on this car in current condition.

Also Im North Denver Metro Area, CO if anyone has a mechanic recommendation.

Current sumptoms.

• No current error codes
• The car always feels like there is significantly less power. A gentle acceleration off a light requires pressing the acceletor more than I remember. And the take off is still sluggish.
• car starts fine
• Idle works ok in park, minimal to no hunting.
• when in reverse or drive and the brake is depressed the idle hunts. Sometimes very low around 400rpm and the engine shudders.
• Idle hunts when in R or D after engine is up to temp. Hunting is less extreme before the engine is up to temp.
• the idle hunts over a SMALLER range when the AC is ON with blower. less frequent, hunts appx every 20s instead of constantly without AC and blower on.
• But if I put the AC blower on the highest setting the idle struggles again and will kill the engine eventually.
• I noticed that the idle appears to be sensitive to ambient temperature. We were have days in the high 80s or 90s and the idle would hunt as described above. At night when the temp dropped to mid 70s the car would have no issues with the idle--even after driving awhile to get the engine to temp.
• If I change from park to R or D too soon. After starting the car the engine will stall. I wait for the rpm to settle and then switch into gear.
• If the car stalls from switching into gear the next time I start it the engine turns over, but I need to pump the accelerator to keep it from stalling.
• There is a high pitched sputtering noise at around 2k rpm only when the engine is under load. I do not remember this being present before this saga. I can reproduce this by using the EBrake, putting it into D and slowly accelerating to 1.5Krpm. The sound just starts there, but gets louder closer to 2k and then dropps off getting up into 3k--but that dropoff might just be road/wind boise drowning it out.

Work that was done in order.

1. Car does start, alternator not providing charge. Battery was removed for an evening during my self-diagnosis.
2. Mechanic confirms and replaces alternator.
3. Misfire on cyl 1 error code
4. Mechanic replaces ignition coil.
5. Having issues with idle hunting, no codes
6. Maintenance work: All Spark plugs replaced, injectors cleaned, transmission oil pan removed and fluid replaced, mass air flow sensor cleaned.
7. Still had idle hunting issues, idle now stalls the engine when at a light too long. Misfire codes on 3 cylinders.
8. Mechanic replaces remaining engine coils
9. Still having idle hunting issues, but no codes and engine no longer stalls at a light.
10. Mechanic cleans throttle body, resets fuel injection and puts it into learning mode.
11. Didn't notice any idle issues for a couple trips, but then idle hunting returned.
12. Mechanic suggest replacing timing chain and then resets fuel injection again and puts it into learning mode again. Suggests to drive 200mi on city streets for computer to learn.
13. After 400 mi the idle hunting is still present with no AC or max AC. See current symptoms.

I'm hoping someone can make sense of this and give me some steps to make progress. Before this saga the car had been running fine with no issues for the 6 years we've owned it. Really not wanting to buy a new vehicle in the current market.

Thank you!