1988 Mariner 150 - help requested

[ksimpson]

My 1988 Mariner 150 V6 has a good tell tale at tick over but overheats and steams after approx 1 mile @ 4000 RPM. Thermostats, poppet and pump and delivery pipe check out ok. I suspect exhaust gas is forcing into the cooling water and have replaced head gaskets, exhaust divider and manifold gaskets and powerhead and intermediate plate gaskets, also checking all waterways were clean when the heads and plates were off.

One bore has slight scoring but performance is unaffected.

Does anyone please have any previous experience of this and know what else to look for?

[Frosty]

It would appear that you have checked most stuff out already. If the engine was mine I would first of all run the engine with the muffs on ( in the drive with the garden hose on the motor). Impellors sometimes look ok but have in actuall fact become un-bonded from the inner bush,-- like a spun prop. This would give you your problem.

It also might help if you could throw any light onto the history of the engine before this problem occured, such as been standing etc. Do you fresh flush?