I’ve got a 2008 Subaru Outback, manual transmission. When I come to a light, I may put the car in 1st gear with the clutch depressed while I wait to proceed. Then, all by itself, the accelerator revs up to about 2500 rpm for a brief moment. Then it comes down, rests for a second and then does the same thing all over again. The only thing that will make it stop is if I put the stick in neutral and release the clutch. Anybody know of any way to cure this?
Sounds like a dirty iac or throttle plate.
Idle surging is a fairly common complaint here, but the clutch/gear relationship to the symptom is something unusual, I’ll give you that one. hmm … well, I think @knfenimore 's idea is probably the place to start. It may be the release bearing is putting just enough load on the engine to confuse the iac. Another thing to check along these lines is the throttle position sensor function.
The IAC has a sticky spot in its plunger travel.
When the clutch is engaged there is just enough drag from spinning the shafts to cause the computer to move the IAC plunger from the bad position.
I bet turning on the AC or adding some electrical load by turning the blower on high and turning on the headlights would have the same effect.