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.
1990 Lexus LS 400FACTORY OEM SERVICE PROCEDURE
Poor Fuel Economy
- Check for fuel leaks.
- Check fuel quality.
- Ensure brakes do not drag.
- Check accelerator pedal linkage.
- Ensure cooling system is functioning properly.
- Check water temperature sensor circuit. See CODE 22 in TESTS W/CODES article.
- Check intake air temperature sensor circuit. See CODE 24 in TESTS W/CODES article.
- Check throttle position sensor circuit. See CODE 41 and CODE 47 in TESTS W/CODES article.
- Check spark plugs.
- Check ignition coil. See SYSTEM/COMP TESTS article.
- Check distributor.
- Check engine compression.
- Check fuel system circuit. See FUEL SYSTEM CIRCUIT chart in TESTS W/CODES article.
- Check injector circuit. See INJECTOR CIRCUIT chart in TESTS W/CODES article.
- Check fuel pressure control vacuum switching valve circuit. See FUEL PRESSURE CONTROL VACUUM SWITCHING VALVE (VSV) CIRCUIT chart in TESTS W/CODES article.
- Check cold start injector circuit. See COLD START INJECTOR CIRCUIT chart in TESTS W/CODES article.
- Check idle speed control valve circuit. See IDLE SPEED CONTROL (ISC) VALVE CIRCUIT chart in TESTS W/CODES article.
- Check neutral start switch circuit. See CODE 51 in TESTS W/CODES article.
- Check main oxygen sensor circuit. See CODE 25 and CODE 26 in TESTS W/CODES article.
- Check sub-oxygen sensor circuit. See CODE 27 and CODE 29 in TESTS W/CODES article.
- Ensure transmission is functioning properly.
- Check engine for mechanical failure.
- Check ECU. See SYSTEM/COMP TESTS article.
Factory Service Tools
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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.