Check/ remove the end caps on the main heat exchanger to check the rubber cap seals and than the oil cooler for debris, on the port side under exhaust manifold, for clog.

Change the rubber seals on each end of the heat exchanger,when they get old they can get stretched and can block the flow of water through the heat exchanger

Forgot to mention that I already changed the gaskets on the sides of the heat exchanger..... ordered a new T drain with the ball check valve .... I will put it next week and see what happens...... talk to the parts guy and they no longer sell the parts to rebuild it .... had to buy the whole part for $52 ... in update as I go

You can pull both hoses from the manifold with the engine running to see how much flow and if the ball valve is working.Dont run for to long with no water going through the manifold

Flush it with 80% water 20% muriatic maybe you have some calcium/salt buildup.