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Engine Controls - Adjustments - 7.4l GMC Suburban I

Adjustments ~620 words

ENGINE MECHANICAL

Note. The Assembly Line Data Link (ALDL) connector may also be referred to as the Data Link Connector (DLC). The SERVICE ENGINE SOON (SES) light may also be referred to as the Malfunction Indicator Light (MIL).

Before performing any on-vehicle adjustments to fuel or ignition systems, ensure engine mechanical condition is okay.

VALVE CLEARANCE

Note. All models use hydraulic lifters. No adjustments are required.

IGNITION TIMING

  1. Set parking brake. Block drive wheels. Place transmission in Neutral or Park. Warm engine to operating temperature. Turn A/C off (if equipped). Ensure Malfunction Indicator Light (MIL) is not on.
  2. Disconnect Tan/Black Electronic Spark Timing (EST) by-pass connector wire located near distributor, or in cab under glove box. DO NOT disconnect 4-wire connector at distributor.
  3. Connect timing light to No. 1 spark plug wire. Check timing and adjust if necessary. See «IGNITION TIMING»(/gmc/suburban/i-1992-1999/remont/adjustments/#engine-controls-adjustments-74l) . Reconnect EST by-pass connector. Clear DTC.
ApplicationMan. Trans.Auto. Trans.
7.4L (VIN N)4 @ Idle4 @ Idle

IGNITION TIMING (DEGREES BTDC @ RPM)

CONTROLLED IDLE SPEED CHECK

Note. DO NOT attempt to adjust idle mixture and idle speed. Both are controlled by Electronic Control Module (ECM). Incorrect idle speeds are normally caused by dirty throttle plate or vacuum leaks. Ensure all vacuum components are functioning properly.

Note. Controlled idle speed and IAC count can be checked using scan tester. See CONTROLLED IDLE SPEED & IAC COUNT . Minimum idle speed adjustment can only be performed on 7.4L engines. See MINIMUM IDLE SPEED ADJUSTMENT .

  1. Ensure no trouble code(s) are present, IAC system is okay, and ignition timing is correct. Block drive wheels. Apply parking brake. Connect Tech 1 scan tester to Data Link Connector (DLC). Put scan tester into OPEN mode.
  2. Start engine and bring to normal operating temperature. Check for correct state of Park/Neutral switch on scan tester. Check if idle speed and IAC valve pintle position (counts) are as specified. See «CONTROLLED IDLE SPEED & IAC COUNT»(/gmc/suburban/i-1992-1999/remont/adjustments/#engine-controls-adjustments-74l) .
  3. If idle speed is not within specification, refer to «TESTS W/O CODES - GASOLINE»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline) article in this section. Perform «MINIMUM IDLE SPEED ADJUSTMENT»(/gmc/suburban/i-1992-1999/remont/adjustments/#engine-controls-adjustments-74l__minimum-idle-speed-adjustment) .
ApplicationIdle RPM(1) IAC Counts
Auto. Trans. (2)6755-30
Man. Trans. (3)8005-30
(1) Add 2 counts for engines with less than 500 miles. On V8 engines, add 2 counts for every 1000 ft. above sea level. (2) Automatic transmission in Drive. (3) Manual transmission in Neutral. Tech 1 scan tester will display "RDL" with transmission in Neutral.
(1)Add 2 counts for engines with less than 500 miles. On V8 engines, add 2 counts for every 1000 ft. above sea level.
(2)Automatic transmission in Drive.
(3)Manual transmission in Neutral. Tech 1 scan tester will display "RDL" with transmission in Neutral.

CONTROLLED IDLE SPEED & IAC COUNT

MINIMUM IDLE SPEED ADJUSTMENT

  1. Check controlled idle speed and perform IAC system check first. Refer to «CONTROLLED IDLE SPEED CHECK»(/gmc/suburban/i-1992-1999/remont/adjustments/#engine-controls-adjustments-74l__controlled-idle-speed-check) . Set parking brake and block drive wheels. Start engine and operate to normal operating temperature of about 185-212°F (85-100°C). Turn engine off.
  2. Remove air cleaner, adapter and gaskets. Ensure throttle lever is not binding TV cable or cruise control cable. At Diagnostic Link Connector (DLC), ground the White/Black wire terminal (upper row, second terminal from right end). Turn ignition on (engine off). Wait for at least 10 seconds (this allows IAC pintle to extend and seat in throttle body).
  3. Disconnect IAC valve harness connector. Remove jumper from DLC. Connect scan tester to DLC. Put scan tester into OPEN mode. Place transmission in Neutral. Start engine. Allow engine RPM to stabilize. Check engine RPM. See «MINIMUM IDLE SPEED»(/gmc/suburban/i-1992-1999/remont/adjustments/#engine-controls-adjustments-74l) . Disregard IAC count on scan tester while IAC valve is disconnected.
  4. If idle speed is not as specified, remove stop screw plug by using an awl. With engine at normal operating temperature, adjust stop screw to obtain nominal RPM with seated IAC valve. Turn ignition off. Reconnect IAC harness connector. Disconnect scan tester. Reseal stop screw hole and reinstall air cleaner assembly.
  5. Reset IAC valve as follows: Turn ignition on (engine off) for 5 seconds. Turn ignition off for 10 seconds. Start engine. If engine does not idle correctly, repeat procedure.
ApplicationRPM
Auto. Trans. (In Drive)600-650
Manual Trans. (In Neutral)675-725
(1) If engine has less than 500 miles or if checking idle speed at altitudes above 1500 feet, idle speed with seated IAC is lower than specified.
(1)If engine has less than 500 miles or if checking idle speed at altitudes above 1500 feet, idle speed with seated IAC is lower than specified.

MINIMUM IDLE SPEED (1)

THROTTLE POSITION (TP) SENSOR

TP sensor output voltage should be .85 volt or less at idle (closed throttle).

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