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.
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HOMESERVICE MANUALSLEXUS1998SC 300REPAIR AND DIAGNOSISENGINE PERFORMANCEENGINE CONTROLS - ADJUSTMENTSIGNITION TIMINGGS400, LS400 & SC400
1998 Lexus SC 300
GS400, LS400 & SC400
1998 Lexus SC 300SECTION GS400, LS400 & SC400
- Warm engine to normal operating temperature. Turn ignition off. ON LS400 & SC400, remove battery cover. Remove air cleaner and air intake duct. On all models, remove engine cover. Connect timing light to Yellow lead wire at ignition coil connector for cylinder No. 1.
- Connect scan tool to Data Link Connector (DLC) No. 3. See Figure. Scan tool is used to read engine RPM. Apply parking brake. Ensure all accessories are off. Place transmission Neutral. Start engine and maintain engine speed at 2500 RPM for 90 seconds and then allow engine to idle. On GS400, ensure cooling fan is off. On all models, ensure idle speed is 700-800 RPM.
- Install Jumper Wire (SST 09843-18020) between terminals TC and E1 of DLC No. 1. See Figure. Connector is located near air intake chamber. Check base timing. See IGNITION TIMING table.
- If base timing is correct, procedure is complete. If base timing is not correct, check for one or more of the following conditions. Repair as necessary.
- Improper operation of air intake system.
- Throttle valve is being prevented from fully closing.
- Throttle position sensor is misadjusted. See THROTTLE POSITION (TP) SENSOR .
- Valve timing is incorrect (timing belt is not correctly installed).
IGNITION TIMING (Degrees BTDC @ RPM)
| Application | (1) Base Timing | (2) Controlled Timing | ||||||
|---|---|---|---|---|---|---|---|---|
| ES300 | 8-12 @ 650-750 | 7-24 @ 650-750 | ||||||
| GS300 & SC300 | 8-12 @ 650-750 | 6-16 @ 650-750 | ||||||
| GS400, LS400 & SC400 | 8-12 @ 650-750 | (3)* | ||||||
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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.