5.7 mercuriser overheating

[lloane]

1987 Sea Ray 5.7 alpha one gen one. When I put my boat away last winter, it was running perfectly fine, I bring it out of storage, and do all my usual checks, and run it. It's a 350 Mercruiser and it slowly overheats at idle, faster with increased rpm.
I have replaced the impeller, the water pump on the engine, both manifolds, and removed the thermostat with no change in the condition. Engine and manifolds are always drained of water every winter before boat goes into storage.
The reason for changing the manifolds, is when I bought the boat 13 years ago, the person didn't winterize it, and the block was cracked, and both exhaust manifolds. I replaced the engine brand new, and had the manifolds welded, and never had a problem for the last 13 years. Always in fresh water, and lake water cooled engine.
I'm at a loss for what could happen during storage for this to happen. Risers are spotless, no build-up what so ever. Just did a compression test, forgot to warm engine first, but got 130-135 on all 8 cylinders.
Thanks for any help.
posted this in wrong forum to start with, so moved it here.

 

[greno]

This sounds just like what happened to me two seasons ago. Did all the service prior to start of season. New impeller kit. Launched boat and watched the temp slowly keep going up. Couldn't figure it out. Finally went and got another impeller kit and installed it and problem went away. Still don't know what was wrong. Maybe I installed it incorrectly.

 

[lloane]

The impeller I took out was perfectly fine, took it to the mechanic I bought the new one from, said old one was fine, new one installed same way old one came out. Took lower unit off twice to make sure.

 

[BudL1te]

I wonder if the rod that drives the impeller is turning? Prop working?

 

[greno]

The only other thing that comes to mind is leaking head gasket. But with raw cooling that shouldn't be able to overheat it.
Most likely impeller or blockage in water system. I've had old impeller pieces that I've had to hunt down and remove from my system.

 

[lloane]

Its getting good water flow, it's not gushing out when I take the inlet hose off the power steering cooler, but there as much as coming out of the garden hose. prop is turning, did a compression test, all 8 cylinders are between 130-135 psi cold...forgot fo warm it before I took all the plugs out.

 

[greno]

A real long shot would be water pump has backwards impeller.

 

[lloane]

Made sure it was turning clockwise when I installed it, even took lower unit off a second time to confirm.

 

[greno]

I meant the engine water pump. If the impeller is a backwards impeller it will move very little water.

 

[lloane]

Replaced brand new with same one that came off.

 

[greno]

Do you notice one exhaust manifold getting hotter than the other?

 

[lloane]

Yes, the starboard was 30* hotter then the port??

 

[greno]

Ok then, either frozen flapper or you have blockage in water passages on that side.

 

[lloane]

How far down are these "flappers" I put my hand down the exhaust tube and didn't feel anything??

 

[greno]

If you have the older 3-piece Y-pipe they are all the way at the bottom of the tubes.
If you have the newer oval type they are at the top of the tubes.

 

[greno]

And if you have the newer type and you don't see any flapper at the top then it could have rusted off and fell down the tube.

 

[lloane]

I believe they are the 3-piece Y pipe, there are 2 big rubber boots, one off a riser to a 45* pipe, then another rubber boot. on both sides. But if those weren't opening, why am I getting water flow out the exhaust on both sides of the outdrive?

 

[lloane]

I believe they are the 3-piece Y pipe, there are 2 big rubber boots, one off a riser to a 45* pipe, then another rubber boot. on both sides. But if those weren't opening, why am I getting water flow out the exhaust on both sides of the outdrive?

 

[greno]

Cause the flapper has little openings in them that allow water through. These aren't air tight or anything. But if they get stuck in there they restrict the flow of exhaust enough to overheat the engine.

 

[lloane]

I wish I could post the video of it running, with the water & exhaust coming out, but what I did notice, is I have very little water coming out the center of the prop, but there is water and exhaust coming out both holes on each side of the outdrive.
Do you know, is there a limit on the size picture/video you can upload to the forum?