'89 4.3 mercruiser overheat at idle issue (standard cooling)

[Augoose]

All,
I've been doing some research about an overheating issue I'm having. At cruise, the temp stays right at 150 +/- 5 degrees. When I stop and idle, three out of five times if I sit for longer than 2-3 minutes, the temp will slowly start to creep up towards 160-170 until I put it back into gear and begin moving again. I've already inspected the impeller which was new this season and I'm positive I've not overheated the engine (never went higher than 180) and I've not run the boat with the lower unit out of the water. I don't hear any sounds (like ball bearings bouncing around) from the water pump/circulator and its not leaking, so I'm assuming its likely ok. I've also flushed the cooling system and used a coat hanger to poke around in all drain holes to clean out blockages - the water seems to flow unobstructed.

I then read about the check balls which are located in the thermostat and read that they need to be tight? I also read the service manual but can't find anywhere where it discusses how to inspect the balls, only ensure that they seat well.

Can someone explain how to inspect these check balls and what to look for?

Thanks!

 

[Augoose]

Re: '89 4.3 mercruiser overheat at idle issue (standard cooling)

Alright, I think I figured it out.

I pulled the upper hoses off either side of the thermostat housing and inside was a metal rod, about 6" in length, with 1" nylon balls at either end held on with tension by one spring each. The springs were so weak that they were no longer forcing the balls against the thermostat housing to block water flow at the appropriate time. I replaced the springs and cleaned off the rust so that each nylon ball had a nice mating surface against the thermostat housing.

As I removed the rod I also found what appeared to be a piece of impeller- clearly that wasn't helping and could have also attributed to the problem.

Ran the boat today and it held a perfect 150 no matter what I did - idle, cruise, etc.

I hope this can help someone else out