Thermal Vacuum Valve: Description and Operation

4-151
All 4-151 equipped CJ vehicles use only manifold vacuum for distributor spark advance. When coolant temperature is below 120° F, a delay valve maintains the vacuum advance during sudden throttle openings. When coolant temperature is above 120° F, the delay valve is bypassed by the spark coolant temperature override (CTO) valve.

Fig. 23 Spark CTO valve:

SPARK COOLANT TEMPERATURE OVERRIDE (CTO) VALVE
The spark CTO valve, Fig. 23, is screwed into the thermostat housing so that the thermal sensor is in contact with the engine coolant. The CTO valve controls the distributor vacuum advance according to coolant temperature by permitting manifold vacuum with or without the delay function.
When coolant temperature is below 120° F, manifold vacuum at port 1 is applied to port D. Port D is connected to the distributor spark advance mechanism with a vacuum hose. When the valve is in this position, the delay valve is in the circuit.
When coolant temperature reaches 120° F, manifold vacuum at port 2 is also supplied to port D, but the delay valve is bypassed. This is the normal operating condition.

Fig. 24 Vacuum spark control delay valve:

VACUUM SPARK CONTROL DELAY VALVE
This valve is used in the vacuum advance circuit to provide improved driveability when the engine is cold, Fig. 24.
Ports 1 and 2 and ports 3 and 4 of this valve are connected internally. When vacuum is greater at port 4 then at port 1 (such as during sudden acceleration), air must flow through the orifice to equalize pressure. This creates a momentary delay that prevents a sudden decrease in spark advance. When vacuum is greater at port 1 than at port 4, air flows freely through the unseated check valve and the pressure is instantly equalized.




COOLANT TEMPERATURE OVER RIDE (CTO) VALVE-STANDARD COOLING SYSTEM
On engines equipped with a spark (vacuum advance) CTO valve, the distributor vacuum spark advance is controlled by the carburetor after the engine coolant heats to a specified temperature. Driveability during warm-up is improved by controlling the vacuum spark advance by manifold vacuum when the engine is cold. This is accomplished by the spark control system.
The CTO valve, Fig. 23, is screwed into the intake manifold coolant passage on six cylinder engines. On all V8 engines, the CTO valve is screwed into the thermostat housing or intake manifold coolant passage. A thermal sensor on the valve is in contact with the engine coolant. The CTO valve allows either manifold vacuum or carburetor ported vacuum to control the vacuum spark advance, according to coolant temperature.
When coolant reaches the specified temperature, the valve moves upward and blocks manifold vacuum at port 1. Carburetor ported vacuum is now exposed at port 1 and applied to port D. Ported vacuum now controls the vacuum advance mechanism diaphragm. This is the normal operating condition.



COOLANT TEMPERATURE OVERRIDE (CTO) VALVE-HEAVY DUTY COOLING SYSTEM
This is a single function valve used in conjunction with a heavy duty cooling system to prevent engine overheating. This valve is installed in the same location as that used with a standard cooling system.
When coolant temperature is below 220° F, ported vacuum is exposed at port 1 and applies to port D to control the distributor vacuum advance. When coolant temperature exceeds 220° F, port 1 closes and port 2 is connected to port D to allow manifold vacuum to control the distributor vacuum advance. With manifold vacuum applied to the vacuum advance mechanism, engine idle speed increases and engine cooling efficiency is improved. Some engines use both standard and heavy duty cooling system type CTO valves.

NON-LINEAR VACUUM REGULATOR (NLVR) VALVE
On engines equipped with a NLVR valve, distributor vacuum advance is controlled primarily by regulated vacuum. Regulation is determined by engine load.
The NLVR valve has two input ports (one from manifold vacuum and one from carburetor ported vacuum sources) and one outlet port to CTO valve. Under low-load or no-load operating conditions, the valve provides regulated vacuum. Under these conditions, manifold vacuum is high and there is little or no ported vacuum. The NLVR valve provides a vacuum level that is somewhere between the two vacuum levels, determined by the calibration of the valve. As engine load increases and ported vacuum increases above the regulated value, the valve switches to ported vacuum.

FORWARD DELAY VALVE
Some engines incorporate a one-way forward delay valve in the vacuum advance circuit to reduce hydrocarbon emission and improve driveability. The valve delays the effects of sudden increases in vacuum during rapid throttle closings. This action prevents sudden spark advance during deceleration.

REVERSE DELAY VALVE
This valve, used in conjunction with the forward delay valve, functions to improve cold engine driveability and reduce hydrocarbon emission.
When the engine is started, manifold vacuum applied to the distributor advance mechanism advances ignition timing. During acceleration, manifold vacuum decreases and ignition timing is retarded. The one-way reverse delay valve delays the effects of the decrease in manifold vacuum, thereby preventing the sudden retarding of ignition timing during acceleration.