92 Legacy runs rough when warmed up.

Hi, I need some advice about a 92 Legacy that recently started running rough. This is my son's car and I'm trying to diagnose the problem without having driven the car much recently. According to him the problem just started as opposed to gradually getting bad. He initially told me about it, because we put a transmission in the car about a month ago and he thought the problem was the tranny. After driving it, I think it's not the tranny. The engine is stumbly, hesitant and low power. These traits are there even when idling in park. The problem seems to start after it warms up. I drove the car this morning and until it warmed up it seemed ok. I'm not sure, but I would geuss that it runs in a closed loop until warmed up. So this lead me to believe that one of the sensors that the computer gets info from after it's warmed up might be the culprit. So far I have changed the MAF, TPS, and fuel filter. I have a parts car and those were easy to change just to see if it helped. It did not. I also ran the on board diagnostics using the black and green plugs under the dash. It is producing no codes. This is the part that really confused me. This car is running pretty rough, there's no mistaking that there is some kind of problem. I can't imagine that this much disfunction is not producing any trouble codes.
When first taking off, the stumbly effect is very noticable, it almost seems like it's going to die, but does not. Then while driving the problem is erratic sometimes running almost normal for short periods, sometimes surging or hesitating. Just basically running like crap. Anybody had a similar problem, or suggestions on what to check or change next.
Thanks
Diggler1971

You are close as far as open/closed loop go. The computer runs in open loop, base programming without much sensor input, until it reaches a certain temp at which it switches to closed loop and starts looking at several sensors. Make sure there aren't any vacuum leaks, sometimes those things won't pop up until the engine is warmed up. A can of carb cleaner will usually steer you in the right direction for those, even the small ones. Second, How are those o2 sensors? The computer won't usually pay attention to those until it goes into closed loop. Checked the catalytic converter? Any unusual noises, smoke, leaks, etc?

From Last night:

The plugs and wires have less than 5000 miles on them and I will start checking the vacuum hoses. I disconnected the o2 sensor and drove the car just now and initially thought that was the problem. It was cold when I drove it and ran ok till warmed up as always, however it kept running normal for a good while after warmed up (maybe ten minutes). I thought, aha, that's it. But before I got it back home it started sputtering again. It seemed not as bad as previous drives today though. I will prolly go ahead and swap the o2 sensor off my parts car, just to rule it out. Thanks and any opinions will be appreciated.
Diggler1971

From this morning:

I checked the air filter yesterday. I wasn't clear in my orig. post, but I checked for trouble codes with black and green plugs, but not with both plugged together at the same time. Black with car not running but ign on, green with ign off at first, then depress gas pedal (fully down, then to half throttle for two seconds), then start the car and drive for at least a mile at more than 7 mph. The green plug test did not produce any codes, and in fact did not even flash the cel unit I actually took off. It just blinked steady for the entire drive. I took this to mean "no codes present". When I drove it last night with the o2 sensor disconnected the cel came on, once it warmed up. That was helpful, because it confirms that the cel is working to come on when it gets bad or no info, at least from the o2 sensor.

I'm gonna drive again this morning with the o2 sensor still disconnected, to confirm that the previous "better" performance last night was not just a coincidence. If that the case, then as the previous poster said, the problem should more likely lie with the air and/or fuel. That being the case, it would seem that since I already replaced the fuel filter and maf sensor with known good parts, the most likely place to start is vacuum leaks. I'll spray around with carb or brake cleaner to see if the engine is affected.

Also, is the IAC only a factor at idle speed, or could it affect performance whilst driving at higher rpm as well? I'll prolly go ahead and pull it, and clean it or replace from the parts car to rule it out.

Sorry about the lengthy posts, but typing it out helps me put things together in my head better. Again any advice or opinions appreciated.
Diggler1971

Oh, and I have an appointment with a local import repair shop to try to get them to diagnose the problem (hopefully with a diagnostic computer), but they can't get to me till Tuesday. If I haven't figured it out by then, I'll take it down there. But, it would be great to not have to take it to them, because they are a pricey operation. But they do good work.

Well, I think it's fixed, and I feel a little embarassed to say what the problem was. Yesterday, I drove it again with o2 unplugged (perofrmed better than with o2 plugged in), then changed the o2 out of my parts car (performed a little better yet). This reaffirmed to me that the problem lie in the fuel or air stuff. After this, I decided to (at the suggestion of a poster) start pulling plug wires to see if one did not produce much change in idle performance, assuming that if one did not change engine performance, then prolly that is were the problem is. I started off pulling pass. side front wire. As soon as I pulled it, the engine died. I found this odd, because I have run one of these motors missing one plug, in the past. Missing a plug the motor ran rough and had low power but it did run, so I found it odd that this motor died straight away when losing one cylinder. When I moved around to pull the dr. side rear plug wire, I noticed that it didn't appear to be pushed all the way down. The rubber boot that flushes up against the valve cover was maybe 3/8" away from valve cover. I pulled this wire and saw no, or very little change in engine performance. Aha! I pushed this wire back on (all the way) and drove the car about 15 miles with no problem. Then I drove it to work last night (about 70 mile r/t) again with no problem.
I had not checked this basic thing before, because the plugs and plug wires are only about 6000 miles old. The person I bought it from had just done some work on the car before I got it (including plugs and plug wires). So I think either he didn't get this wire on all the way and it finally backed out enough to not arc from wire to plug, or maybe we snagged it and pulled it back a little when we changed the tranny a month ago. Although, I think that this plug has maybe been like this (to a lesser degree) for the whole year we've had it. It might be just because it has been running rough for a few days, but it seems to be running better than it has since we've had the car. It has always stumbled a bit when first taking off from dead stop, and it is not doing that now.
So, in conclusion, I should have done what I always tell people I give troubleshooting advice to. That's to always check the easiest most common things first, even if, like in my case, they seem like they should not be the problem. Thanks to all that posted comments and advice.

Diggler1971