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Off-Idle Stumble

My 2007 Outback has had an off-idle stumble since I bought it a year ago. Otherwise runs well and uses less gas than the 2000 Forester it replaced. I’ve had a smoke test done, cleaned the throttle body, replaced front oxy sensor, and used a variety of gas treatments. I don’t trust the local Subaru dealer to work on my car and my local garage has run out of ideas, but suggested MAF sensor may help. Some days it runs better than others. Occasionally I’ll see a check engine light P0171 (lean) or P0420 (cat) but it will go weeks between setting those. Any ideas?

Sounds like a vacuum leak or maybe an EGR problem.

Can you elaborate on “off-idle stumble?” Do you mean it just stumbles momentarily just as you’re getting on the throttle and then clears up? So it’s kind of a “low throttle” on take off issue? Or do you mean it will stumble any old time when you are on the throttle? At idle is it perfectly smooth?

Has your idle air control valve been cleaned and checked for smooth operation? Has the shop checked the throttle position sensor?

The transition from idle to off-idle is partially controlled by the throttle position sensor in many cars, probably yours too. The engine diagnostic codes you mention suggest there may be a mixture problem too, the comments above address that well. But you might consider to have the throttle position sensor tested too. It may just need to be adjusted. Good idea to have the idle speed measured too, make sure it is spot on to Subaru’s specs. Here’s another thing to try. If you notice when you are going around a corner in gear with the throttle at idle position (because you want to go slow), then you press on the gas at the end of the turn to accelerate out of the turn and notice the car seems to jerk an unusual amount before returning to a steady speed, that would confirm the throttle position sensor needs to be checked.

The off idle stumble usually occurs from a dead stop. One or two seconds where the engine just shakes a bit before producing power. I have noticed some times too where moving with light throttle there seems to be no power and I have to push down hard on the pedal to get a response. In that case it just seems to have no power until it is called upon to produce maximum power then everything seems to work okay. Does not have either problem all the time. The car is a pure joy to drive sometimes.

This engine has throttle by wire. TPS is built into the throttle assembly and there is literally nothing to adjust. Normal idle there is no question it is not a Lexus but no more vibration than I would expect from a 4 cylinder. Looking through the service manual I get the impression that there is no discreet idle air control. They only address it as a ECM trouble code.

Smoke test is the way to look for a vacuum leak and the mechanic said everything was tight. No one else has suggested the EGR valve before and that does make sense. Worth a look see.

EGR valve was the problem. I’ve had it off 3 times getting more carbon out each time. I will probably end up replacing it but the car is running well. Thank you for the suggestion NYBo.

One last comment in case anyone has a similar issue. Beside the EGR valve carbonized, the MAF sensor was apparently out of calibration. Replacing both has the car running better than it has since I have owned it. Even handles better since the engine is no longer stumbling when driving through a turn. Have not seen a check engine light either.

Excellent news and thanks for reporting back. Out of curiosity, did you attempt to clean the MAF before replacing it? Sometimes they do just get gunk on them. On the off chance that issues crop up again, take a good look upstream from the MAF to make sure nothing is amiss that might contaminate it. Mostly I assume that you will just happily drive on.

Mass air flow is a pattern failure on this car. If you don’t feel like taking the plunge for a new mass air flow then perhaps try a used mass air flow sensor and see if the car acts differently.