Mercury 305 CID, Service Manual, Page: 621 / 967

90-823225--1 1096 5E-80 - ELECTRONIC FUEL INJECTION (MULTI-PORT AND THROTTLE BODY)
Idle Air Control (IAC) Functional Test
(1 of 2)
IDLE AIR
CONTROL
(IAC) VALVE
441 BLU/WHT
442 BLU/BLK
443 GRN/WHT
444 GRN/BLK
CIRCUIT DESCRIPTION:
The ECM controls idle speed to a calculated, “de-
sired” RPM based on sensor inputs and actual en-
gine RPM, determined by the time between succes-
sive ignition reference pulses from the ignition
module. The ECM uses four circuits to move an Idle
Air Control (IAC) valve, which allows varying
amounts of air flow into the intake manifold, control-
ling idle speed.
IMPORTANT: Improper IAC readings or improper
idle speed can result from other faults ie: flood-
ing VST, fouled spark plugs, bad sensors. These
items should be in proper working order to en-
sure correct diagnosis.
TEST DESCRIPTION:
Number(s) below refer to circled number(s) on the
functional check chart.
1. This step determines if the IAC valve is function-
ing properly.
2. This step determines if the circuitry or the IAC
valve is faulty.
DIAGNOSTIC AIDS:
Check for vacuum leaks, unconnected or brittle vacu-
um hoses, cuts, etc. Examine manifold and throttle
body gaskets for proper seal. Check for cracked in-
take manifold/plenum. Check open, shorts, or poor
connections to IAC valve in CKTs 441, 442, 443 and
444.
An open, short, or poor connection in CKTs 441, 442,
443, or 444 will result in improper idle control and
may cause improper idle.
An IAC valve which is stopped and cannot respond
to the ECM, a throttle stop screw which has been
tampered with, or a damaged throttle body or linkage
could cause improper idle.