Spark Control Systems Diagnosis & Testing

Fig. 45 Orifice spark advance control (OSAC) valve:

Ambient temperature in area of vehicle must be above 68° F. for this test.
Inspect hose connections. Replace any hoses that may allow air leakage. Check operation of valve, Fig. 45. With engine running at 2000 RPM in neutral, disconnect hose at the OSAC valve leading to distributor and attach a vacuum gauge to this port. If very gradual increase in vacuum is observed, approximately 15 seconds to a stabilized level, the valve is operating properly (some variations between engines). If vacuum immediately goes up to manifold vacuum or there is no vacuum, the OSAC valve is not operating properly and must be replaced.