Hello all,

I have encountered a very strange overcooling problem and I am lost on ideas, so I thought perhaps someone has encountered this or has an idea what to try next. Sorry for the lenghty desicription but I have tried some stuff and sat around the problem with mechanics and better to give an overview at the start. Also I have searched forums for the solution and have not found one.

The boat is 1992 Four Winns Horizon 220, it has 5.8l (ford engine) inboard, OMC Cobra drive. Since I bought the boat, it always had riser overheating problems and too little water coming at low revs. I replaced manifolds, risers, impeller, hunted for blockages etc, and the risers always (especially the left side riser) overheated at low speeds or idle (engine temp was OK). It was impossible to hang around in marinas etc. Everything was OK at high speed.

So this winter I thought that the last thing to check was the water pipe in the lower section of the drive. When I took it apart, it had a gasket missing, and I replaced that (and the piping actually too).
So, it seemed to fix stuff, this year I had more water flow. Like last year, at idle the left riser overheated a bit, but not as much as last year so I was kind of happy. BUT, i noticed that the engine now overcools when revving. So, my symptoms are:
1) When idling, engine temp is about 76C, and risers are about 60-70C left, and 40C right.
2) When I rev to 2500rpm or more, the engine temp drops in about 1 minute to 45C. Risers stay about the same or get a bit cooler.
3) When I drop back to idle, the temp rises to 76C in a couple of minutes.
4) As I mentioned before, I have replaced manifolds, risers, impeller. I also replaced the thermostat, and gaskets (I have tried two thermostats (old and new), and two gaskets, also used gasket improvers so it is highly unlikely that the thermostat leaks.
5) The engine is rebuilt a couple of years ago so there should not be any in-block blockages.
6) I have checked that there are no blockages in the thermostat housing (thoroughly checked the bypass where incoming water goes directly to risers).
7) The temp sender unit is OK, I confirmed the temperature drop on the block with IR thermometer.
8) When the engine runs, the hoses are under quite a pressure. I did not notice it last year, For an example I cannot squeeze the large hose by hand. The hose temps are: Incoming cold, large one hot, riser hoses warm.

So - the only ideas I have so far, are that with the improved water flow it either:
a) forces the thermostat open because the bypass does not allow enough water by?
b) there is some kind of blockage further on?
c) any other ideas?

So, is it possible that the bypass in the thermostat housing has somehow rusted narrower in time? Or, is it possible that something else is blocking the exhaust water exit somehow... perhaps someone has had some kind of similar problem and has found a solution I could try.

Any ideas at all would be very welcome!