Have 1989 mercruiser 3.0 with digital ignition. Was out on water and cruising along at about 3500 rpm. Was running good for about 15 min then started losing power. Noticed temp was in red. Not sure what temp as guage doesn't have degrees just lines. Sat anchored for about an hour to let cool down. Pulled water inlet hose off and noticed no water pumping in. Got a tow back to marina and tried to start just to get on to trailer. Was very hard to start and keep running just to get it out of water. Of course, the impeller had gone out. Replaced impeller, and housing. Back flushed hoses to be sure no remnants of bad impeller in system. Also while having lower unit off , i replaced the oil seal where drive shaft enters upper unit. Figure while in process , I cleaned spark arrested and installed new spark plugs. Got everything back together. Now it has a hard time starting. Have to give it a few pumps and hold at half down to get engine running. Once return to idle position, it shakes violently then dies. After about 5 min of playinh with throttle and get it a little warm, it will finally idle but it's at 800-900 rpm with a lot of shaking. Thought maybe one of the new plugs wasn't quite gapped right so I rechecked all. Choke looks to be opening and closing as it should. Before impeller failing and overheating ,engine purred smooth at the 600-700 rpm idle. It'll run smooth above 1000 rpm. Took it out and ran around lake for about 1-1/2 hrs. Runs great but when put into neutral and idle it'll start shaking and die in a few seconds. Have to give it about 3/4 throttle to get started again. Need help. As stated, engine ran great with no issues before impeller fail and overheating ..