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.
High Altitude Compensation (HAC) System: Description
As altitude increases, air pressure decreases and the air/fuel mixture becomes richer. The HAC system supplies additional air to the primary low and high speed circuits on all models, and secondary high speed circuits on Celica, Corolla, Corona, and Pickup models. This helps to reduce emissions and improve driveability. Ignition timing is also advanced for better driveability.
The systems include a high altitude compensation valve, a vacuum advance sub-diaphragm, and a check valve. Starlet models use a thermostatic vacuum switching valve (TVSV) which serves to eliminate compensation while the engine is cold.
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
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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.
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