This problem may or may not be related to another problem I am having: Link Annoyingly, this problem is intermittent. Sometimes the car will start to increase idle speed, up to 3K or more. I can go days without it occurring, other times it happens regularly on the ride to/from work. Often I can fix it by turning the ignition off and back on. This workaround seems more successful if I do this with high vacuum, e.g. engine braking conditions. What I mean by that is if I can tell it's doing it's high idle nonsense by its behavior during gear changes, I'll put it in the lowest appropriate gear for my speed, let out the clutch and turn the ignition off for a second or two and back on. I have looked through this thread and found no help. I have checked all potential vacuum leaks, replaced the soft parts of the PCV plumbing, etc. The ISV moves freely, and I have a spare ISV that my brother put a plexiglas window on to check the function. Problem occurs with either ISV. The one thing I suspect at this point is the brake booster. I have noticed that the problem occurs most frequently if I am very light on the brakes coming to a stop. Sometimes fiddling with the brake pedal makes the idle increase more rapidly. I can't get the car to do this in my driveway and, living in the mountains I am not anxious to try disconnecting the brake booster for long enough to be sure. I can test by putting a 12V solenoid valve in the brake booster vac line and closing the valve to see if that fixes it, but I thought I would see what the hive mind says before I go to that expense and effort.