Intermittent stopping of water flow through engine causes overheating

[virtual]

Looking for some help or suggestions. It's a 2007 Stingray with a mercruiser 3.0 and alpha gen 2 out drive

Beginning of a new season, during the first outing before even getting out of the marina my engine overheated while just idling. I took the boat back out of the water, and back at home with the muffs on everything starts off fine with water clearly going through the engine and coming out the out drive as normal, but after the engine was up to operating temperature the flow of water through the engine came to a stop and the engine over heated (again). The engine was in idle throughout. Basically what I observe is that the amount of water coming out of the out drive reduced and then came to a stop, followed by the predictable engine over heating that followed.

I then let things cool down a little, restarted the engine and the same thing happened. First things were fine with water coming out of the out drive and the engine temperature was fine, but once again after about 5-10 minutes the water flow came to a stop and it over heated again. Again the engine was at idle throughout most of this.

I then wanted to replicate the problem, but see what would happen to the water flow if I increased the engine revs when the water flow stopped, but sure enough.. Just to drive me insane it's been running fine ever since. I've now run it 3-4 more times and the water flow has been good and remained good, the engine temp is fine, and I've run it for more than 20 minutes each time without having the problem re-occur?

So I'm hesitant to think that the problem has magically fixed itself, but it's hard to know what to check now that I'm not seeing the problem anymore. I've also never experienced an intermittent water flow problem like this before. Usually there's either water flow through the engine or there isn't. Not flow for a while, and then stopping. I'm also hesitant to just start pulling things apart and replacing things, so I'm hoping that someone might help me come up with some plausible explanation of what might be happening and help with plan of attack to try and figure out what the problem is.

Thanks for your help.

 

[stork55]

When was the last time you changed your water pump impeller?Could be a piece of the impeller blocking the water outlet.Was the outdrive tilted the same each time,possibly pinching the rubber hose or the hose seperated.Maybe try removing the hose on the engine a try back flushing to see if water comes out the drive.

 

[virtual]

The Impeller is about 3 years old. Everything I've done has had the out drive at the same angle, and I checked carefully no hoses were pinched and the muffs have been in the same spot throughout, with good flow going to them. Basically I haven't moved anything since I set it up to start troubleshooting the problem. The impeller is (at least now) pumping water just fine, and since before the whole system was pumping sometimes, and not pumping other times, I was not sure how an impeller would work sometimes but not others, and then start working again when you start the engine again later. But the same holds for the rest of it. Hence my puzzlement.