I am pritty certain this will be a vacume leak, get him to check all the vacume pipes on the intake manifold, some times you can hear a hissing if its leaking bad but sometimes not. Get him to check 1,2,3,4,7,8, i cant remember if 5,6 are water lines, just pull them off and check Get him to take off each one and put his finger over the solid tube from the intake and note if the revs rise or fall, do it for each one and with any luck you can find the leak