2001 forester shudder while idling

[jfriedman]

I have a 2001 Forester automatic trans. It has started shuddering
intermittently when it idles, almost like a rocking forward and back.
It also has a rough time getting moving coming out of stop lights, and
this is also intermittent. My check engine light hasn't come on. Does
anybody have an idea what might be the cause of this?

 

[DK]

I have a 2001 Forester automatic trans. It has started shuddering
intermittently when it idles, almost like a rocking forward and back.
It also has a rough time getting moving coming out of stop lights, and
this is also intermittent. My check engine light hasn't come on. Does
anybody have an idea what might be the cause of this?

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

 

[Roger Buttsnort]

I have a 2001 Forester automatic trans. It has started shuddering
intermittently when it idles, almost like a rocking forward and back.
It also has a rough time getting moving coming out of stop lights, and
this is also intermittent. My check engine light hasn't come on. Does
anybody have an idea what might be the cause of this?

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

 

[Todd H.]

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.

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.

 

[DK]

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.

Could you please explain this one for me? What exactly does the
"mist test" tell? Lack of proper electrical isolation?

Also, WRT coil pack (which I understand is an expensive replacement):
If it were bad, wouldn't you expect the car to have trouble starting?

Thanks,

DK

 

[Todd H.]

Could you please explain this one for me? What exactly does the
"mist test" tell? Lack of proper electrical isolation?

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

Also, WRT coil pack (which I understand is an expensive replacement):
If it were bad, wouldn't you expect the car to have trouble
starting?

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.