Cooling issues

[Joestrong]

I have a 1993 bayliner with a 3L mercruiser and alpha one sterndrive. The engine runs and idles well but I can’t figure out this cooling problem. Recently replaced the water pump and impeller with no change.
When in the water and in “neutral” if I run the engine up to 2500 rpm it maintains engine temp aroun 150 f. But when I’m drive and moving forward at 2500 rpm the engine rises to 180 f within a few minutes and would keep climbing if I didn’t stop. Then if I put it in neutral “no prop” and run the engine to 2500rpm the temp drops rapidly.. so it appears when not in motion, the cooling system moves water through and everything works but when in motion (even if only going 15 mph@ 2000rpm or less) basically no water is being moved through. Any ideas for me? Thanks in advance

 

[Dieter]

when not moving the engine is not under load and not generating as much heat. When in the water take the supply from outdrive hose off thermostat housing run the engine briefly you should get 3 or so qts of water in 15 seconds. There is a merc test in the manual 14 for this attached. If you have enough flow look at thermostat for proper operation and any debris or old impeller pieces. May have a clogged exhaust manifold or riser that the spent water needs to pass through, Especially common in salt water boats.