233hp mercruiser high speed overheat.

[boxchevyman]

Hi all!
I have a 351w mercruiser that I am having some trouble with. Late last year it was starting to have a high speed over heat problem so I just put it away for the year and I was going to deal with it this year. So taking the advice on the forums I did the following:

New water pocket cover as the old one was leaking.
New impeller with base and housing torqued by a good torque wrench.
Pulled y pipe checking for burned or lost exhaust flappers ( everything was good)
Compression test checking for bad head gasket (all cylinders were a 120 psi.)
Timing check, (then while I was on the water I advanced and retarded with no change in behavior)
Remove thermostat (slowed overheat by 1-2 minutes if at all)
Clear hose test (no bubbles)
Checked one riser and it was fine did not pull the other one. (temp was consistently hot on both sides)
Checked raw water flow in a gallon bucket and while it was hard to be exact I feel like I got a little more than a gallon in 15 seconds at 1000 rpm.

Any ideas? It would probably be cheaper to fix this boat than it would be to buy another one.

Thanks a ton!
James

 

[Fun Times]

You may have a worn down water circulation pump impeller at the front of the engine or the impeller shaft might not be turning internally even though the pulley assembly is.

Depending on the pumps design, You might be able to remove the circulation pumps rear stainless steel cover and inspect the brass impeller for wear and tear and ensure it's turning properly..... Hope it's a simple problem for you, good luck.

 

[achris]

How did you check the elbow (riser)? if it was just visual, then it may still have restricted flow and you can't see it. Or the manifolds could have internal blockages. How old are they?

If you can hook up a pressure gauge on the last hose before it goes into the (exhaust) manifold that would be indicative. If you see the pressure rise, the problem is a blockage in manifold or elbow.

Chris.......

 

[boxchevyman]

Hey chris. Thanks for taking the time to respond. I flow checked by using a garden hose and checking to see if water came out. The iron got pretty cold on the garden hose.

I checked the riser visually and it looked ok but i let it soak for a while in straight muratic acid and rinsed. They are probably original but being that a new riser and manifold set would cost as much as the boat is worth i really want to prove that is whats wrong before i throw money at it.

I may try to rig up some sort of presure gauge on the lines to prove this theroy.

Thanks for the tip fun times, thats honestly where i was going next. I have an automotive circulation pump on the enginge right now, maybe it just craped out. I live in missouri so i dought this old boat ever saw salt. But im going to yank it off and inspect anyways.


Thanks guys.
James

 

[boxchevyman]

Hi all,

So i pulled the circulation pump and the impeller has all of its fins and seems to be in good shape. no play in pump shaft. so I guess I'm back to manifold and risers.

Is there a good way to test them? I'm assuming what Chris said is probably the only way, but just wanted to know if there was another way i could do it on the trailer.

Thanks,
James

 

[boxchevyman]

No Title

ok went looking in the manual and it seems i have my hose's routed wrong. I have my exhaust manifold on where my riser should be and vice versa.

The weird thing is i didn't switch the hoses. And it ran like this last year. so i guess this is my problem but its just weird that ran last year albeit a little on the warm side.