See Figure 1
To further aid cold start driveability during engine warm-up, many V6 and V8 engines use a VOHV located between the exhaust manifold and the exhaust inlet (header) pipe.
When the engine is first started, the valve is closed, blocking exhaust gases from exiting one bank of cylinders. These gases are then diverted back through the intake manifold crossover passage under the carburetor or fuel injection throttle body. The result is quick heat to the carburetor and choke assembly or to the CFI throttle body.
The VOHV is controlled by a ported vacuum switch which uses manifold vacuum to keep the vacuum motor on the valve closed until the coolant reaches a predetermined warm-up value. When the engine is warmed-up, the ported switch shuts off vacuum to the VOHV, and a strong return spring opens the VOHV butterfly. The valve will also open before warm-up if engine speed or load condition causes a drop in intake manifold vacuum below a specified value.
- Using a hand vacuum pump, apply 10-15 in. Hg (34-51 kpa) of vacuum to the vacuum motor for 60 seconds.
- The valve should close and not leak more than 2 in. Hg (6.75 kpa) during this time.
- Release the vacuum and the valve should open.
- If these conditions are not met, the valve is defective.