Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
SYS.OK|MANUAL.DB● LIVE
HOMESERVICE MANUALSGMC1991SUBURBAN R1500, 7.4 NREPAIR AND DIAGNOSISENGINE PERFORMANCEENGINE CONTROLS - ADJUSTMENTSIDLE SPEED & MIXTUREGASOLINEMINIMUM IDLE SPEED ADJUSTMENT (7.4L ONLY)
1991 GMC Suburban R1500, 7.4 N
Minimum Idle Speed Adjustment (7.4L Only)
1991 GMC Suburban R1500, 7.4 NSECTION Minimum Idle Speed Adjustment (7.4L Only)
- Check controlled idle speed and perform IAC system check first. Set parking brake and block drive wheels. Start engine and operate to normal operating temperature, about 185-212°F (85-100°C). Turn engine off.
- Remove air cleaner, adapter and gaskets. Ensure that throttle lever is not binding TV or cruise control cables. Using a jumper, ground ALDL diagnostic connector. Turn ignition on (engine off). Wait for at least 10 seconds (this allows IAC pintle to extend and seat in throttle body).
- Disconnect IAC harness connector. Remove jumper from ALDL diagnostic connector. Connect scan tester to ALDL diagnostic connector. Place scan tester in "Open Mode". Place transmission in Neutral. Start engine and allow engine RPM to stabilize. Check engine RPM. See MINIMUM IDLE SPEED table. Disregard IAC count on scan tester with IAC disconnected.
- 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.
- Reset IAC valve. To reset IAC valve, turn ignition on (engine off) for 5 seconds. Turn ignition off for 10 seconds. Start engine and check for proper idle operation. Repeat procedure if proper idle operation is not obtained.
MINIMUM IDLE SPEED (1)
| Application | RPM | |||||||
|---|---|---|---|---|---|---|---|---|
| 7.4L | ||||||||
| Auto. Trans. (2) | 600-650 | |||||||
| Man. Trans. (3) | 675-725 | |||||||
|
||||||||
RENDER: 1.0x
NO RELATED
Recommended Tools & Savings
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.