Air in cooling system, any ideas?

[staintx]

I am dealing with an interesting air in the cooling problem.
garden hose directly connected to water intake hose with a clear vinyl bubble and i can see no air bubbles are going into the engine. I have 2 pieces of clear tubing on the exhaust houses as well. At idle no air bubbles are present, at 1500 no bubbles but over 3000 rpm bubbles show up on exhaust cooling hoses.

Looks like a cracked head or gasket but how come bubbles are not present around 1500 rpm?
if its a blown gasket I would assume that it would still leak air into the system at 1500.
what are your thoughts on this issue?
97 5.7 Lx with double barrel carb, rebuilt is only 2 hours old.

 

[Bt Doctur]

bad head gasket allowing combustion gas into the cooling system

 

[kmarine]

If you are running on a hose you will also suck air in with your impeller. The hose does not supply enough volume to keep air bubbles out. It only keeps the engine and impeller cool enough to run at low rpm without damage. some large outboards will over heat at idle on a hose and are not recommended to be hose ran at all. On my old 5.7 volvo I had clear hoses and would see air bubbles Had the boat for 20 years without any engine issues. If you are looking to see about head gasket leak read the spark plugs. The water will wash off the carbon. A compression test will also show a head gasket leak, Bubbles in the exhaust water hose mean Nothing.

 

[staintx]

I connected the garden hose directly to thermostat housing via clear tubing. impeller is completely out of the test scope at this point. What puzzles me is that I get the bubbles after 3000 rpm, Im guessing failure is at the beginning stage that's why I don't see bubbles while idling or at low rpm. I will check the spark plugs tomorrow and pickup a compression test to gather a little more data.

Here is a video of the bubbles.

 

[kmarine]

What I am seeing on the video is completely normal. My volvo 5.7 looked exactly the same. I added the clear hose after an impeller priming issue to insure water flow. The boat did that for 20 years and is still running strong today. What other performance issues do you have.

 

[tpenfield]

Looks like a cracked head or gasket but how come bubbles are not present around 1500 rpm?
if its a blown gasket I would assume that it would still leak air into the system at 1500.
what are your thoughts on this issue?
97 5.7 Lx with double barrel carb, rebuilt is only 2 hours old.

Typically, you don't get air at lower RPM with an early stage head gasket issue. It could be a crack, rather than a gasket. A cooling system pressure test may help isolate the issue. Sounds like a bad rebuild.

 

[staintx]

After initial start last summer, I left the engine idling and didnt realize that water well switch shut off and caused engine to over heat. It wasnt too bad ir temp gauge did show the heads were close to 230s.
Weather changed and i stopped working on it for 7 months. I took the boat out last weekend and noticed engine temp was fluctuating so brought it back in and took it home. Replaced the thermostat and put a little more tension on the belt and its not fluctuating or overheating anymore. But now i see the bubbles and it upsets me that i may have to pull this engine out one more time. Im going to do a leak down test tomorrow and see how bad of a damage Im dealing with.