The 1987 Subaru GL had failed to pass its state-madated annual emissions test. All of the important sensors appeared to check out, and a recent tune-up failed to solve the problem. Was a lack of control over the heating and ventilation system somehow related to the uneven idle, or was the lower than normal compression in one cylinder somehow to blame? Reconnecting an accidentally dislodged vacuum hose brought the car back into compliance with state standards, but a pronounced unevenness during prolonged idling persisted until the base idle speed was raised to the high end of the specs.