Exhaust Manifold

 

As the burned gases leave the engine cylinders, they pass into the exhaust manifold. The manifold is usually made of cast iron or stainless steel and is attached to the cylinder head with a series of fasteners. It is designed to route the exhaust gases with a minimum of sharp bends. Once affixed to the engine, periodic service (other than lubrication of the exhaust manifold heat control valve) is usually not required.

To remove the manifold, disconnect the exhaust pipe flange and any braces or tubing that may be connected to the manifold. Then, remove the manifold fasteners. If the fasteners are stuck, apply penetrating oil and allow it to soak for 15 to 20 minutes before attempting to loosen the fasteners.

When installing a manifold, all mounting surfaces must be clean. Use a file to remove burrs and bits of hardened gasket material. Install new gaskets where needed. Torque fasteners in proper sequence. If the new manifold-to-head gaskets are made of a composition material (not steel), the , fasteners should be retorqued after the engine has been operated. This will bring fastener torque back up to compensate for the torque lost due to the gasket flattening out after heating.

Use fastener locks when required to prevent the fasteners (especially end ones) from loosening. Connect the exhaust pipe (use new gasket) and hook up the parts originally attached to the manifold.

Figure 23-1 illustrates a typical exhaust manifold. Note that in this particular case, the intake manifold is connected to the exhaust manifold by a metal tube. The tube provides exhaust gases for EGR (exhaust gas recirculation) valve operation. Exhaust manifolds for V-type engines use similar construction.

Heat Control Valve

To provide heat to help vaporize the fuel charge during engine warm-up, a heat control valve, or heat riser, may be installed in the exhaust manifold. It may also be installed between the exhaust manifold and the exhaust pipe. When the valve is used on an older engine, it is actuated by a bimetallic spring. In newer vehicles, the heat riser is operated by a vacuum motor (diaphragm unit). The vacuum- operated heat control valve is often called an early fuel evaporation device (EFE device).

To provide heat to help vaporize the fuel charge during engine warm-up, a heat control valve, or heat riser, may be installed in the exhaust manifold. It may also be installed between the exhaust manifold and the exhaust pipe. When the valve is used on an older engine, it is actuated by a bimetallic spring. In newer vehicles, the heat riser is operated by a vacuum motor (diaphragm unit). The vacuum- operated heat control valve is often called an early fuel evaporation device (EFE device).

In Figure 23-2C, the heat control valve is shown between the exhaust manifold and the exhaust pipe. This is a popular arrangement on V-type engines. On a typical V-type engine, only one exhaust manifold. is fitted with a heat control valve. When it closes, the hot gases are forced to travel up through the heat riser and to the other exhaust manifold. They are then discharged into the exhaust system, Figure 23-3. As the gases pass through the intake manifold, the area beneath the carburetor or throttle body is heated, thus aiding fuel vaporization during cold engine operation.

Vacuum-operated heat valves are pictured in Figures 23-4 and 23-5. In each of these setups, a thermal vacuum switch is used to apply enough engine vacuum to match engine temperature. When the engine is cold, full vacuum is applied and the valve is pulled closed. As the engine warms, the vacuum signal weakens. When the engine reaches operating temperature, no vacuum is applied and the valve is held open by spring pressure.

The heat control valve will often stick due to an accumulation of carbon. When stuck in the open position, slow warmup, carburetor icing and stalling, flat spots during acceleration, and crankcase dilution may occur. If the valve sticks closed, overheating, detonation, burned valves, and a warped manifold may result. It is important to have the heat control valve free in its bushings. The thermostatic spring should not be distorted.

To check an older spring-operated heat control valve, accelerate the engine quickly while watching the heat valve. The counterweight should move, indicating that the shaft is free. To double check, allow the manifold to cool off thoroughly and then try to move the weight by hand to make sure the valve has full travel.

Vacuum-operated heat control valves can be checked by removing the vacuum line when the engine is cold. If the spring does not return the valve to the open position when vacuum is removed, the valve is stuck, the linkage is binding or disconnected, or the diaphragm is leaking.

If the heat control valve is stuck, allow the manifold to cool off and apply several drops of penetrating oil to both ends of the shaft where it passes through the manifold. Work the valve back and forth until it is free. Add more penetrating oil as needed. When the valve is stuck so tight that it cannot be moved by hand, tap the ends of the shaft after applying the penetrating oil. After the valve is free, lubricate the shaft with a special heat-resistant graphite mixture or leave it dry and clean. Never use engine oil, as it will burn and form more carbon.

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