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Learning from this board that exhaust leak at the manifold can cause burned exhaust valves. I just didn't see the explanation. I have exhaust leak in my 87 P30 RV. Spark plug wires is way cheaper comparing to thousands of dollars for a new exhaust system. However, burning valves and damaging engine is not what I opt for. Give me your suggestion!
Michael, it works like this, when the exhaust valve opens at the end of the power stroke, there is still pressure present in the cylinder. The piston, now traveling upward, adds to this pressure and the exhaust gases travel rapidly out the cylinder and into the manifold as a somewhat compact mass, called a "slug". Behind this "slug" is an area of low pressure, lower than atmospheric pressure, this phenomenon can be taken advantage of by tapping into the exhaust manifold and using check valves to prevent back-flow when there is positive pressure in the manifold in order to introduce additional air, (Oxygen), into the exhaust stream to help control tailpipe emissions. This system is used in lieu of a "smog" pump, and doesn't take any power from the engine. All this said, when an older engine's working hard and the exhaust valves are nice and red, if there is an exhaust manifold leak near a port, the engine is apt to inhale a blast of cold air into the low pressure area around the valve, causing rapid oxidation of the valve seating area. Due to materials used in the exhaust valves of late model engines, this isn't much of a threat anymore. Can you believe the Model T Ford had cast iron valve heads? |
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