OMC overheating, no apparent cause

The port engine is not getting any water, and is overheating. We have checked and changed the thermostats, hoses, ampeller, all water passages are open, gear drive is turning ampeller, there is no apparent reason why there is no water flow We have had a mechanic look at it also.
We have a Bayliner Avanti 1989 with OMC Cobras 5.0 , gms , 230 hp.
Any ideas? We just bought the boat this year, it ran fine when we went for the test drive, and the first time we took it out. After that the problems started.

 

Did you get any suggestions on this? The fix must be something very simple hopefully and no damage has been done do to the overheat.

 

Troubleshooting can be sooooo much fun. Just a few suggestions, based on hard-earned experience, maybe you checked some of these already:
1. Check the pump shaft seal, and look to see if the bilge pump runs when the engine runs. A bad shaft seal will put water into the bilge instead of through the engine.
2. Disconnect the hose from the pump to the engine block, run water from a garden hose and see if it circulates through the engine.
3. Remove exhaust manifolds, check to be sure there are no blockages, inspect the gaskets to be sure an incorrect gasket isn't blocking a water channel.
4. Check for clogged intake grill/screen
5. Check impellor, some are directional; same for pump drive, the pump could be rotating backwards.
6. Check the valve or seacock. Be sure it is open, all the way, and there is nothing clogged or blocking flow

If there is no water at all flowing through the engine, focus in order on each item starting at the intake screen/grill. Eliminate a stage only after you see water flowing through it under pressure, or, in the case of a passage, valve, hose, etc, only after you see daylight or have poked a wire or rod all the way through it. There will come a point at which there was water flowing, and at the next point there is none. That point will contain the problem...and the solution.

These steps are tedious, sometimes difficult, and some may seem too simple and basic. But when there is no obvious cause, you have to get dirty and basic.

Let us know what you find, please.

Good hunting,

Charlie