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.
Coolant Temperature Override (CTO): Dual-Function
This valve supplies manifold vacuum to vacuum advance at both cold and hot coolant temperatures. At normal coolant temperatures, carburetor ported vacuum is supplied. When coolant temperature is below 149°F, manifold vacuum enters through port "3" and is applied through port "2". This allows full manifold vacuum to distributor.
From 149-220°F, port "3" is blocked and ported vacuum enters through port "1" and connects to port "2". This allows full ported vacuum to distributor. Above 220°F, port "1" is again blocked and full manifold is applied to distributor through port "2". See Fig 2.
Use the Manual With the Right Hardware
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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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