2001 forester shudder while idling

I have the same problem in my Impreza. Has been this way for a while. Although the problem is intermittent, it only shows up when the engine is cold. Invariably, it disappears when engine temp reaches its steady point. Also, I noticed that the problem (hesitation and in worst cases shudder) occurs more frequently in wet weather.

A friend told me it's likely to be a bad/corroded contact to an oxygen sensor. I am waiting for the thing to become more reproducible before bringing the car to mechanic.

DK

If I had to bet, i would suspect torque-converter "stalling"..I.E. Failure....

The "wet weather" correlation suggests ignition. O2 is possible, I suppose.

In the dark, with a spray bottle, you can mist over teh engine as it runs. If you see arcing, replace plug wires and maybe the coil pack.

-- Todd H.

Correct. If insulation is beginning to fail, that'll cause you to see arcing inthe presence of aerosol moisture.

Not necessarily. These aren't binary devices. They can be leaky and still mostly work before they entirely poop out.

I ran into a subaru tech in a social context and he said a lot of his day is mired in plug wires and coil packs to correct driveability issues.

-- Todd H.