C 3500 Truck 2WD V8-305 5.0L (1988)
Idle Speed: Adjustments
Idle Speed
CONTROLLED IDLE SPEED CHECK
***UPDATED BY TSB # 88-T-55 DATED NOVEMBER 1987
Before performing this check, there should be no codes displayed, idle air control system has been checked and ignition timing correct.
1.
Set parking brake and block drive wheels.
2.
Connect a "SCAN" tool to the ALDL connector with tool in Open Mode.
3.
Start engine and bring it to normal operating temperature.
4.
Check for correct state of Park/Neutral switch on "Scan" tool.
5.
Check specification chart under SPECIFICATIONS for controlled idle speed and IAC valve pintle positions counts. See: Specifications
6.
If within specifications, the idle speed is being controlled correctly by the ECM.
7.
If not within specifications, refer to Rough Unstable Idle or Stalls at Idle, under DIAGNOSIS BY SYMPTOM.
MINIMUM IDLE AIR RATE CHECK
***UPDATED BY TSB # 88-T-55 DATED NOVEMBER 1987
1.
Check controlled idle speed and perform idle air control system check first.
2.
Set parking brake and block drive wheels.
3.
Start engine and bring it to normal operating conditions. Turn engine "OFF".
4.
Remove air cleaner, adapter and gaskets. On ST series models leave THERMAC hose connected. Check that the throttle lever is not being bound
by the throttle, TV or cruise control cables.
5.
With IAC valve connected, ground the diagnostic terminal (ALDL) connector.
6.
Turn "ON" ignition, do not start engine. Wait at least 10 seconds.
7.
With ignition "ON", engine stopped, test terminal still grounded, disconnect IAC valve electrical connector. Remove ground from the ALDL
connector.
8.
Connect a SCAN tool to the ALDL connector and place in open mode. If a tool is not available connect a tachometer to the engine.
9.
Start engine. With transmission in neutral, allow engine rpm to stabilize.
10.
Check rpm against specification chart under SPECIFICATIONS. See: Specifications Disregard IAC counts on "SCAN" tool with IAC valve
disconnected. If the engine has less than 500 miles on it or is checked at altitudes above 1500 feet, the idle rpm should be lower than the
specifications chart list, with IAC valve fully seated.
11.
If the minimum idle air rate is within the specifications, no further check is required.
12.
If the minimum idle air rate is not within specifications, perform the following procedures:
Stop Screw Plug Removal
13.
If present, remove stop screw plug by piercing it with an awl, then apply leverage.
14.
With engine at normal operating conditions, adjust stop screw to obtain nominal rpm specifications with seated IAC valve.
15.
Turn ignition "OFF" and reconnect the IAC valve electrical connector.
16.
Disconnect the "SCAN" tool or tachometer.
