Strange Overheating Problem

[185GXSF-Guy]

I have a 2001 Glastron GX 185 SF (Volvo Penta 4.3 GL - I/O) and I am having a weird overheating issue and I was wondering if anyone has seen this before:

1) If I drop the boat in the water and just run at idle speed, the boat will overheat in about 10 minutes of run time.
2) If I drop the boat in the water and run faster than idle speed for a minute or two, I never overheat - boat comes up to temperature and stays at temperature just fine.  I can run at idle speed all day long after that.

At first, I thought the problem was the thermostat which I have never replaced (probably the original).  I replaced the thermostat successfully and still the same problem.  Now my thinking is that perhaps it is the raw water pump or something related.  Is is possible that at low idle speed, there isn't enough suction to get water into the engine?  Maybe a bad pump or other related issue?  I replace the impeller every couple of years when I winterize so I don't think that is the issue but you never know - if the impeller was bad, I would think it would overheat all the time.

If you've seen / heard about this before, I would love to hear about how you solved it!!

Thanks much!

[Plugcheck]

It would be my guess the raw water pump is failing to make enough pressure or volume at low speed.  Outboard performance guys run a water pressure gauge to insure they are getting enough water in the pickups.  Is it possible to somehow test the pressure at the output of the pump? The raw pump would be the place?   Other things to check, could be some blockage in the pickup, or the block pump, if there is one, has impeller issues as well.  Bringing the engine up off idle prevents the issue from occurring seems to me like not enough water being brought in.  Are there lines from the thermostat housing to the exhaust?  This is generally where they dump hot water, are these lines hot to touch?   

[Hyperacme]

My Buddy had a BBC Merc / Bravo with the same system ( water pump on motor) as the Volvo.
He had small rocks and stuff plugging up piping to pump.
As long as you have it apart replace impeller.

[185GXSF-Guy]

Thanks - good ideas and I will check - thanks again!

[carlsoncvx18]

If all else fails check your exhaust risers.

[Diamond Chad]

Impeller.   Just went through similar, overheat on idle but fine at speed.  The water inlet on the bottom is shaped so that at speed the water is scooped up with pressure so the impeller does not really do anything.   At idle it relies on the impeller to suck in fresh water.  I switched out this one and idle temp snapped back to 165 where it belongs, even after a hard run and drop to idle.

[185GXSF-Guy]

Winner winner chicken dinner - Diamond Chad nailed it - impeller.  The impeller was partially damaged so seems like it was able to keep the boat cool once pressure had been established but not able to establish enough pressure at idle with a few vanes missing.  Attached are some pictures.

Also, I mentioned that I replaced the thermostat - pictures also included (old and new).  The "trick" with the thermostats on these engines is that there is a rubber O ring that holds the thermostat in place that can be hard to see if the thermostat is all rusted, etc. 

Thanks much for the help!  Ran the boat for about 4 hours this evening non-stop with no issues.