another case of guess and hope system. try to guess the problem and hope you fixed it
as always, the sequence is concern, cause, correction, confirm.

in short:
1-interview the driver, ask pertinent questions
2-make measurements and digitize results, e.g. engine vacuum in inches of mercury at idle speed, pressure of the suction side at idle speed and at what ambient temperature
3-make adustments, repairs, replace worn out or time change parts, service with the correct fluids
4-make measurements again comparing the before and after results