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When the balance spool bore in Mazda G4FA-EL units wears, the valve won't stroke to regulate EPC pressure. If the middle spool bore wears, line pressure will be allowed to exhaust. Sonnax oversized pressure modifier valve 74846-04 restores hydraulic integrity to the circuit by sealing these locations in the bore.
It is highly recommended that the pressure regulator bore also be checked for wear. A worn pressure regulator bore can result in low/high line pressure, harsh shifts, lack of cooler flow, and/or blued converters.
To wet air test the bore for leakage, place a small amount of oil in the balance EPC circuit. While holding the valve in the unstroked position, follow with low air pressure (Figure 1). There should be minimal or no leakage through the neighboring line port. If excessive leakage occurs, the bore should be reamed and the OE valve replaced.