I have a 2003 Subaru Outback that has recently started to lug down / vibrate when I bring it to a complete stop and hold the car in place by applying the brake. It does not do this when in park or when in neutral. When I bring it to a stop it is ok for several seconds, but then lugs down and vibrates. If I drop it into park or neutral it stops doing it, but starts again as soon as I put it in Drive.
Have you had the motor mounts checked? While I have never heard of bad motor mounts on a Subaru–especially one that is only 6 years old, it is still possible that you have a bad mount–or two. Have there been any oil leaks that could have damaged the mounts?
Motor mounts check out ok.
Is everything up to date–in terms of both odometer mileage and elapsed time–with maintenance, as per the manufacturer’s maintenance schedule?
Possibly an Idle Air Control valve problem. A scan for codes should be done and it may show something; or not.
It’s possible to have an IAC problem with no code present. The idle RPMs should remain the same no matter if the car is in PARK or DRIVE.
It’s also possible there could be another problem causing a rich running condition at idle and doing what is called “loading up” the engine, possibly an air leak in the brake booster, etc. but the IAC should be investigated first.
(The brake booster could be verified with the use of a vacuum gauge.)
I’m thinking “something with the transmission”. This is an automatic, so something may be failing to disengage when stopped while still in gear, and dragging down the engine to the point where it can barely keep going at idle. Be sure to check the ATF level, and has the tranny been serviced on schedule (fluid changes, etc.)?
Since you have an automatic the torque converter normally locks up at higher speeds to increase mileage. It sounds like yours is not unlocking as it should.
I am not much on automatics, but this is a common problem. Some one who knows them should be able to tell you what is required for the fix.
If transman shows up and leaves you a message, just totally ignore any reply that conflicts with anything he says, including my message.
If anything is “not disengaging” it would have to be the converter.