twojtyniak

Well, the results are in and the new front O2 sensor (Fuel Trim Sensor) fixed the problem COMPLETELY. I thank you all so much for the help.

Okay, apparently I tested the rear O2 sensor first. After testing the front O2 sensor I find that...it appears to be a (the) culprit. With it disconnected the car did not exhibit the problem, but right after plugging it in the problem showed up. I'll replace it and report back in. My girlfriend, the owner of the car, was having a rough day, so it was nice to have something positive to report to her. Thanks, all. I'll report in after the sensor is replaced.

I finally time to do some testing. I unplugged the O2 sensor on the front cat and found it didn't change the behavior. I'm going to go thump the MAF now...

Thank you again for the ideas. I'll do some investigating today and let you know what I find.

Thank you for the ideas. Other than remove and replace does anyone have ideas how to test/rule out any of these particular problems? The mechanics checked the TPS according to procedure and ruled that out. It sounds like the problem ronemus had matches pretty well, as the car will idle just fine. Any ideas how to test the MAF? And the codes are (drum roll, please): PO301 PO302 PO303 PO304 PO172 I'll replace the fuel filter just to be sure, but it sure seems electrical in the way it turns off and on at will. Shaking the car around doesn't seem to affect the problem. It did seem to run better for quite a while after being on a bumpy road, but it also has had long periods of running well after a bad spell without any bumps involved.

Thank you for the preliminary thoughts and the guidance in providing further data. I will get the codes ASAP, but note that for a few days of driving the CEL was not on although the car was exhibiting the behavior. It's a puzzlement.

The Facts: 2002 Legacy Outback wagon 2.5 l 5-speed New, correct NGK plugs New wires New coil pack The Problem: Intermittently (of course) when driving, either accelerating or constant speed, the car will stumble and lose power as if the power was pulled back to idle. It does not stall. Sometimes it recovers on its own, other times I clutch it and rev it and it recovers. Sometimes it backfires if the throttle is held steady. Sometimes it doesn't recover, but if the revs are high enough (2200+?) I can floor it and the engine will run well enough to maintain speed or keep accelerating, but it does not run correctly. The behavior stops and starts instantly, like a switch being thrown. The check engine light was not on for quite a long time while the behavior was occurring, but is on now. Earlier when exhibiting this behavior the codes had been misfire and running rich, but I haven't had the code pulled again yet. The Hunch: Some sensor is misbehaving, but not going out of range, so the computer can't identify the cause. On a Ford EEC-IV at full throttle the car would be in open loop, running strictly off of default values from a table rather than sensor input. At part throttle it is in closed loop mode, dependent upon sensors. I just don't know which sensor might cause this behavior. Things that do not appear to be factors: Outside temp, engine temp, engine speed, vehicle speed. The coil was swapped but did not change the behavior. The new wires did not change the behavior. The plugs were changed quite some time before the behavior started. Any ideas? TIA, Tim