86' 300 Weekender - Merc 260's - Overheating at cruise speed

[joenofish]

We have an '86 300 Weekender with twin 260 mercruisers. The engines are freshwater cooled and have about 700 hours. She sits in salt water 24-7-365.
The raw water pumps have been changed to a "Volvo" style (these are actually made in Sweeden) and driven directly by the crank shaft, mounted on a bracket in front of the hamonic ballancer.

We have had this boat for a little less than a year (it was well maintained by the proir owner). I watch the instuments like a hawk and know both engines run steady 170F when running up on plane running @3300 rpm.

The last few trips (we run it weekly) I nervously watched the starboard engine running hotter, a little more each time out. The last trip we were approaching 210deg (the port was holding @170 as it always does). It only builds this temp when we are up on plane, if I pull back the throttles it cools right down.

I had our diver verify the thru hulls were clean (yesterday), I cleaned tehe strainers but there was almost nothing in them, a few tiny shells, I pulled the raw water hose off of the output side of the heat exchanger and started the engine, at 1000 rpm it had a robust flow out of the hose.

I have had many overheating issues as a younger man driving junky-worn out cars, swapping a thermostat NEVER corrected any of them.

Any Ideas? My manifolds and risers are two years old, are they due again?

Thanks in advance,

Joe in St. Pete

 

[KatzNDogs]

First thing I thought was risers? but then I read you changed them 2 years ago. Did you check the coolant level in your heat exchanger yet?

 

[SCORPIO]

Is the circulator drive belt tight? Are the impellars good? Could you have a hose that is collapsing internally blocking water flow?

 

[JVM225]

I would check the oil first to make sure there is no trace of water in it. If the oil is clear I would pull the plugs to see if there is any evidence of water getting in to the cylinder.
From the symptoms you describe it could be a bad head gasket or maybe even a cracked head.
Two years on risers and manifolds? In theory you should get much more, but I've seen the cheapo ones go bad in that amount of time.

 

[Fordiesel69]

Did you re-install new exhaust flappers? One or both could be restricting the exhaust.

 

[joenofish]

The fresh water coolant and over flow tank are at the correct levels, I ran engine with the cap off for a little while, when the thremostat opened (as the engine warmed) the antifreeze flowed through the heat exhanger at a good clip.
These raw water pumps are direct drive (from the crankshaft) no belts. Regarding a hose partially collapsing, I don't have a clue, I did not think of that. I'll check all of them and replace any "spongy" items (all of them if needed).
I did check the oil and no signs of water, I did not change anything in the exhaust, if I have flappers I don't know where they are??
I called the former owner and during our conversation it occured to me that on two occasions I started the engines with the sea cocks closed, he said he did that as well and did damage the raw water impellers (ran dry for over a min. on at least two occasions).
I'm going to re-assemble everything this weekend, stuff in a fresh set impellers (on the raw water pumps) and take it around the block and see what happens.

Thanks for the help, I'll post the results once I get this done.

Joe in St. Pete

 

[Lady Ashley SR560]

use dish soap in the impeller housing when you assm. It saves wear on the initial "dry" start and blows bubbles out the back to let you know with out a doubt there pumping water...

 

[johngus]

check the heat exchanger.I had a similar problem on mine.I removed the end plates and ran a coat hanger through every hole in the exchanger.I was amazed at how many were partially clogged.make sure the water pump is pumping enough water to.