As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER T/O AND DURING CLIMB THE LEFT DUCT PRESSURE WAS LOW. PASSING 10,000' PRESSURES WERE 41 AND 13. WHEN WE LEVELED OFF THE DUCT PRESSURES EVENED OUT. WHEN WE INCREASED THE POWER THE SPLIT WOULD COME BACK. GROUNDED - YES. AIR TURNBACK - DIVERSION TIME 24 MINUTES. REMOVED AND REPLACED NR 1 ENGINE PRE-COOLER CONTROL VALVE REF AMM 36-12-02-000-801 AND AMM 36-12-02-400-801. OPS AND LEAK CHECKS GOOD.