Your ECM has what's called adaptive strategies which allows the ECM to learn known good values which can actually be "off" when there are other underlying issues. If an issue has been corrected, throttle body cleaned, tune up, etc you should reset the min IAC & TPS voltage. There are many ways in which you can do this.
GMC NUMBER: 88-T-55
GROUP: 6E - Engine Emissions
DATE: November, 1987
CORPORATENUMBER: 836302
SUBJECT: TRUCK ENGINE CONTROLLED IDLE SPEED AND MINIMUM IDLE AIR
RATE SPECIFICATIONS
MODELS: 1987 AND 1988 TRUCKS (ALL GASOLINE ENGINES) WITH THROTTLE BODY
INJECTION (TBI)
This bulletin defines the controlled idle speed and minimum idle air rate specifications for 1987 and 1988 model year trucks for all engine applications with TBI. Additionally, step by step procedures are provided for performing the controlled idle speed check and the minimum idle air rate check. It is important that these procedures and specifications be adhered to. An incorrect readjustment with a high minimum air rate will cause the Idle Air Control (IAC) valve pintle to constantly bottom on its seat and may result in early IAC valve failure. A minimum air rate that is too low may result in a no-start condition in cold weather, a stall after start or a stall during deceleration, because of poor air/fuel distribution through the throttle bore.
NOTICE: THESE SPECIFICATIONS AND PROCEDURES SUPERSEDE THOSE PROCEDURES DEFINED IN THE 1987 FUEL AND DRIVEABILITY MANUAL, X-8736.
Controlled Idle Speed Check
Before performing this check there should be no codes displayed, idle air control system has been checked and ignition timing correct. The procedures to check the idle air control system can be found in the Light Duty Truck Fuel and Emissions service manual at the following locations:
All Truck Engines Except 2.5L Page 4-11
2.5L Truck Engines Page 3-61
1. Set parking brake and block drive wheels.
2. Connect a "SCAN" tool to the Assembly Line Data Link (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 specifications chart at the end of this bulletin for controlled idle speed and IAC valve pintle position (counts).
6. If within specifications, the idle speed is being correctly controlled by the Electronic Control Module (ECM).
7. If not within specifications, refer to "Rough, Unstable or Incorrect Idle, Stalling" in Section "2" of the Fuel and Driveability Manual X-8736 and review information at the beginning of this check.
MINIMUM IDLE AIR RATE CHECK
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 temperatures (85-100 degrees Celsius). Turn engine "OFF".
4. Remove air cleaner, adapter and gaskets. On S/T series vehicles, leave THERMAC hose connected. Check that the throttle lever is not being bound by the throttle, Throttle Valve (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 (this allows IAC valve pintle to extend and seat in throttle body).
7. With ignition "ON", engine stopped, test terminal still grounded, disconnect IAC valve electrical connector. (This disables IAC valve in seated position). Remove ground from diagnostic terminal.
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.
NOTICE: ON 2.5L ENGINES A SERVICE ENGINE SOON LIGHT WILL COME ON AND A CODE 35 WILL BE SET.
10. Check rpm against specifications at the end of this bulletin. Disregard IAC counts on "Scan tool with the IAC disconnected. If the engine has less than 500 miles or is checked at altitudes above 1500 feet, the idle rpm with a seated IAC valve should be lower than values in chart.
11. If the minimum idle air rate is within specifications, no further check is required.
12. If the minimum idle air rate is not within specifications, perform the following procedures:
13. If present, remove stop screw plug by piercing it with an awl, then applying leverage. The screw is covered to discourage unauthorized adjustments.
14. With engine at normal operating temperature (85-100 degrees C), adjust stop screw to obtain nominal rpm per specifications with seated IAC valve.
15. Turn ignition "OFF" and reconnect IAC valve electrical connector.
16. Disconnect "Scan" tool or tachometer.
17. Use silicon sealant or equivalent to cover stop screw hole.
18. Install air cleaner, adapter and gasket.
19. On 2.5L engines, clear Code 35 that has been set.
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