IO-390-D Series Engine Maintenance Manual
12-30
© 2020 Avco Corporation. All Rights Reserved
Page 86
March 2020
IO-390-D Series Engine Maintenance Manual
Table 1 (Cont.)
Fault Isolation Guide
Problem
Cause
Corrective Action
Ref.
Rough Idle (Cont.)
Low fuel pressure
1.
Examine the fuel filter for blockage.
Clean a blocked fuel filter as per the
aircraft manufacturer’s instructions.
2.
Replace the fuel pump as per the
“Fuel Pump Replacement”
procedure.*
73-10
Valve sticking in fuel
manifold
1.
Complete the “Fuel Manifold
Removal” procedure.
2.
Send to either Lycoming Engines or
an authorized overhaul facility for
evaluation.
3.
Complete the “Fuel Manifold
Installation” procedure.
73-10
Engine will not idle
unless the boost
pump is on
Lean idle mixture
1.
Enrich the idle mixture as per the
section "Idle Speed and Mixture
Adjustment.”
2.
Readjust the idle speed.
72-00
Fuel pressure too low at
idle speed (engine could
also lose fuel pressure as
the aircraft climbs)
1.
Look for loose fuel line fitting.
Torque any loose fuel line fitting as
per the latest revision in the
Service
Table of Limits - SSP-1776
.
2.
Make sure the fuel pump is operating
properly.
73-10
Idle mixture is extremely
rich (evident by excess
black exhaust)
1.
Lean the idle mixture per "Idle Speed
and Mixture Adjustment” procedure.
2.
Readjust the idle speed.
72-00
Fuel vaporizing in lines
1.
Operate with cowl flaps in the FULL
OPEN position and keep ground
operation to a minimum.
2.
Operate with boost pump ON as
necessary.
* Review any applicable warranty information relative to replacement of a fuel component as per the
latest revision of Service Letters L221 and L253.
