'88 Olds 88 Rough Idle, Dies Only After Warming Up

'88 Olds 88
3800 Engine Code C (more like an '89 than '87-88 but still quite similar)

Starts fine, runs fine when cold. Once warmed up enough (for emission control loop to kick in?, approx 5-10 minutes later) then the Check Engine Light comes on and at low RPM it idles very rough, occasionally dying stopped at stop lights but never moving (still rough at low RPM moving). Rough idle in park but a little worse in drive. When it dies, it always starts again but may die again.

The problem progressively got worse, at first it just idled rough but eventually bad enough to die. Soon thereafter the O2 sensor rusted through, fell off in my hand. At that point it was replaced, but this didn't fix the problem. Engine codes are 13 and 44 (Open O2 Sensor Circuit & O2 Lean Exhaust Indicated?).

Next spark plugs replaced (was due for it anyway), no change. Old plugs didn't look bad just old. MAF sensor was replaced a few years back, and it doesn't seem to be the same code.

I have not cleared the codes after I replaced the O2 sensor, yet. Should this matter? I thought it still should not die regardless of having cleared the codes or not, and I suspect the O2 sensor rusting through was just coincidental but I'll recheck the plug & contact for it, and clear the codes now.

Can it only store 2 codes? If so, that might explain why it only has the first two, or would it forget the oldest codes and store only the (n) # of most recent codes?

Edit: Cleared the codes, after about 8 minutes of driving it set code 44 again. I might now be more cricital of every little flaw but it almost seems like it idles a tiny bit better but still badly. It seems the RPM is just dipping too low for whatever reason. Do you agree it must be a sensor, or would it still revert to a failsafe mode to keep RPM up? Unfortunately I don't know the intended nor actual RPM at idle in gear.

Welcome to AF, a full scan may be required to look at all the sensor inputs also check the fuel pressure for starters.

OK, I didn't see anything wrong with the new O2 sensor plug or wiring. Adding to this, that the idle was rough before the O2 sensor broke off, I'm thinking it might've just been coincidence that the code still points to a part replaced.

Correct me if wrong but would it tend to suggest one of the following conditions, or could any be ruled out since there is no other code except 44?

MAF sensor - If while it's running rough, I unplug this, would a return to smooth idle indicate it's faulty, or just cause the ECM into revert to warm up (Is that "closed loop"?) mode and then masking the problem? Is it possible to just clean the MAF sensor and what's the best thing to use if so?

Vacuum leaks - I don't have a gauge to check this. Hoses look old, but intact. Can't tell about other areas. Is there a more basic test possible to confirm or rule this out?

Fuel Pressure - is this even a possible problem considering the car runs fine cold, accelerates good, runs ok at higher RPM? I was under the impression this was a (constant?) pressure system that just dumps excess fuel back into the (tank?), so if anything pressure at idle wouldn't be a factor.

Injectors - Similar thoughts to fuel pressure, if it runs cold and at higher speed, wouldn't they just stay open longer if exhaust measured too lean?

I'm happy to confess I overlooked something. At some point while working on the car, either I or the forces of nature, caused the vacuum line to the ventilation/AC to become unplugged and it was (at least after "some" point) a vacuum problem. I feel silly for not noticing this earlier but I'd have sworn I had gone over everything before I replaced the O2 sensor. Maybe during that, or the spark plugs, or a power steering pump swap (I didn't mention as it didn't seem relevant).

Anyway, it runs like the jolly old boat that it's supposed to be again! Thanks maxwedge for your input.