Manifold corrosion is clearly visible once the elbow is unbolted from the manifold.
When the exhaust condition is acceptable, it is possible to grab / hold the manifold and elbow while the engine is operating. When corroded, the exterior exhaust temperature is much higher, making touching / holding the operating exhaust difficult.
If the exhaust has deteriorated w/ the flushing procedure described, then a procedure change is required. The expensive parts need to be protected.
When I flush my engines after salt water operation, I shut off the raw water, run the engines w/ fresh water, run the engines w/ soapy water, run the engines w/ Salt Away, then shut down. I then manually drain the manifolds bone dry.
My boat was designed to keep raw water in the bottom of the exhaust 24/7/365 w/ no way to flush. I redesigned the exhaust to permit the described flush / drain during the last replacement cycle because of the obscene parts cost and short OEM lifetime, about 3-4 years.
If the manifolds are degraded, then don't neglect the expensive service because engine failure is the expected result. There are lower cost aftermarket parts available. I have always used Mercruiser OEM parts.
