B757 RB211 Low duct pressure below switchover
I'm Stumped.
I have a low duct pressure in #1 eng at idle and LP while #2 is normal, split of about 5-15psi. This difference in duct pressures disappears above switchover when when engine bleed is from HP2, duct pressures are then equal throughout the HP range.
We have worked through the FIM and replaced PRSOV, pilot valves and checked all lines for leaks, IP check valve was also checked to see if HP 6 was recycling back through HP2.
This only occurs about 15mins after initial start before then it seems to operate as it should.
The characteristics of this fault indicate that below switchover #1 eng is operating in a low flow mode eg it is above 31,000 ft. Altitude switches were swapped over but fault remained in #1.
If anyone has experienced this or has any ideas on where to look next that would be great.
Cheers