Engine Overheating

I have a problem that just popped up today. I took the boat out (1999 SAN, GT-40) and when I got in it from putting up the truck, engine temp was climbing. Usually it sits around 140, but I watched it go past 200. I decided to run it to see if it cooled off. It cooled off when I was running it (water being forced in through the block), but as soon as I went back to idle, the temp began to climb again. Yesterday, a family member started it in the driveway for about 10-15 seconds while I was gone. Could starting it with no water running through it for that little amount of time messed something up? Does anyone have a clue as to what could be the problem? If this is the impeller, where is that located? Any help would be appreciated-

stranded boarder

It Sounds like you smoked your impeller !
(Dont ever run your boat without water)

it is the impeller. it has happened before.

Lucky for you they are fairly cheap! Easy to replace as
well. I learned the hard way. ooops!!!

50 bucks! make the family member pay for it!

Yup, definately the impeller. I have the same boat and parts are about $27 (skidim.com or southeast correct craft are good parts sources if you don't have a local dealer). Buy an extra when you get the replacement part. The impeller pump is the lowest belt-driven part on the very back of the engine, passenger side. A heat gun will make getting the hoses off a lot easier and I take the whole pump out to change the impeller. Seems to go smoother that way and I don't lose any bolts into the bildge. Not a hard item to change. The worst part is you have to take out the carpeted side boards next to the engine to get to it. While you are replacing the impeller, put a little bit of dish soap or binding lube on it when you put the new one in. It will lubricate it for the few seconds that it takes water to get to the pump. Lastly, check for leaks.

If you haven't figured it out yet from everyone else's response, it's the impeller.

Some of the vanes must be damaged. When you are running at idle, the engine isn't getting enough water to cool it down. As you increase speed, the impeller is spinning faster and pumping enough water to cool it even with the damaged vanes.

http://www.skidim.com

oh yeah---->impeller<---
never run your boat without water to the seawater intake scupper---NEVER

Something that everyone of you forgot. Clean out all lines around the impeller. The pieces of rubber are in the houses or possible in the thermostat housing. Here is a good tip for you Craig. Find your water strainer bowl (Should be clear) That is the first part where water comes in. Make sure that the strainer is circulating water through it. If there is no water bubbling around in it, you are not getting water anywhere else in your engine. Good Luck to you!