I pulled the heads off my 1991 Mercruiser 454 (a.k.a. 7.4L) yesterday. The pistons/cylinders look pretty good, Head gaskets were good, There was some rust on the exhaust ports of the heads on cylinders #4 & #5. Also some rust in the exhaust manifolds of ports #3,#5 & #4,#6. Pics below.
Engine history=540 hours total over the course of 20 years. Moored seasonally in salt water.
I believe that Cylinders #4 & #5 share an exhaust cross-over port in the intake manifold to help with cold starts (?) (funky square port in the middle of the row of intake ports on the head). Anyway, this area of the intake manifold had serious rust on it when I took the intake manifold off. I am thinking that this was the source of water intrusion into the area of the exhaust ports on the middle cylinders.
What do you all think? I hope the pictures are helpful.
I am wondering if this is a typical problem with the 454's of that vintage (Mark IV, 1991). The way the intake manifold meets the cylinder head in that area does not seem to insure a good seal, and would allow the gasket to de-laminate over time.
BTW - I leak tested both exhaust manifolds, and they are fine. So I am thinking the water is from the crossover port.
Next steps are to take the valves out of the heads and get a look at the seats. I also have to get the engine mounted on an engine stand so I can have a peek at the lower end.