My 1999 Legacy GT 2.5 (Automatic--115,000 mi) runs great with one exception: when the engine is fully warm in traffic and the throttle is pressed very gently(just enough to edge the vehicle forward in traffic), the engine "stumbles" or surges. If you step on the accelerator normally, like accelerating away from a stop sign, the engine responds properly. It's not an engine misfire--just a momentary stumble at about 1100 RPM when the accelerator is pressed very gently from a standstill. I've removed the throttle body, cleaned it out, and replaced the T/B gasket. I replaced spark plugs and plug wires as normal maintenance just a few months ago, but that was not an effort to correct the stumble condition. The "check engine" light is NOT coming on--I borrowed a friend's Snap-On scanner and there are no codes stored. Anyone have any ideas?

The finicky knock sensor on Subarus will sometimes pick up engine resonance at a specific RPM resulting in sudden "corrective" timing retard.

 

Do a USMB search or a net search on the subject, there's quite a bit posted about different methods of either reducing the signal passively or mounting the sensor on a rubber bushing to dampen it's physical coupling to the crankase.

 

I think most of the documented cases applied to an earlier year Legacy than yours, but I'm convinced that the problem still exists in certain individual cars, just to a lesser degree than it did previously.

The finicky knock sensor on Subarus will sometimes pick up engine resonance at a specific RPM resulting in sudden "corrective" timing retard.

 

Do a USMB search or a net search on the subject, there's quite a bit posted about different methods of either reducing the signal passively or mounting the sensor on a rubber bushing to dampen it's physical coupling to the crankase.

 

I think most of the documented cases applied to an earlier year Legacy than yours, but I'm convinced that the problem still exists in certain individual cars, just to a lesser degree than it did previously.

Thank you for the great advice--after I read more about knock sensor issues posted by other owners, I think you've probably pointed me in the right direction. The possibility of the knock sensor causing timing-retard when it shouldn't makes sense because if I deliberately hold the throttle at about 1100 RPM in "D", I can replicate a continuous "up and down" surge, although it's rather subtle. I'll let you know what I come up with....thanks again.

had similar issues with my 2.5 rs, no check light I replace my A/F sensor and haven't had any probelms since

had similar issues with my 2.5 rs, no check light I replace my A/F sensor and haven't had any probelms since

How did you diagnose mass air flow sensor with no code stored?

How did you diagnose mass air flow sensor with no code stored?

air/fuel sensor, I was a service advisor for Honda for a very long time and had seen the same symptoms on 92-95 civics took a while to diagnose but each time we replace the primary o2 sensor and it fixed the problem.

air/fuel sensor, I was a service advisor for Honda for a very long time and had seen the same symptoms on 92-95 civics took a while to diagnose but each time we replace the primary o2 sensor and it fixed the problem.

Thanks... now I understand the abbreviation. I was a service advisor at a Chevrolet dealer for years (that's why I own a Subaru) so I was used to seeing the letters "AF" applied to mass air flow sensor.

I wrote for Honda for 9 years and I now work for a Toyota bodyshop and I drive a Subaru and have for most of my life...

Another strong posibility. Carbon build up in the engine.

 

This happened twice with my 97 OB. I got "exactly" the symptoms you describe. A stumble as you feathered into the throttle from a stop. It gradually got worse and affected higher rpms (up to 1500). Over the counter stuff won't (likely) do if this is your problem. A MotorVac fixed me up.

 

The second time it happened a few years later, I ended up changing the O2 sensor. (This was about 1.5 yr ago.) It was not throwing any codes, but the mechanic said it was 'slow'. My problem gradually went away after several weeks. I tried to help it along with some carbon cleaner and premium gas.

 

As I understand it, the knock sensor issue affects things around the 2000 / 2500 rpm range. One shouldn't be getting any knock if you are just feathering into the gas from idle.

 

These cars are known for running rich. The fuel maps are that way I guess.

 

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