If the BOV piston is slightly open at idle as a result of insufficient spring preload, it can cause some or all of the following symptoms:
- Engine stalls after venting, or when pulling to a stop
- When revs fall back to idle, they drop below normal idle rpm before stabilising
- Engine idles roughly or revs hunt up and down
- Significant black smoke or backfiring from the exhaust on deceleration
- Noticeable hesitation when changing gears
The solution is to increase the spring preload by turning the spring adjustment clockwise a turn or two at a time until none of the above symptoms are evident. The aim is to have the piston closed just before the engine returns to idle after being revved.