Raw water cooling pressure test

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Mar 30, 2017
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I have a 1997 5.7 mercruiser. I know I have a manifold or riser that's plugged up with rust because one of the risers was getting hot when I ran the boat and I am getting just enough water in the oil to milk it up but not raise the oil level. I pressure tested the cooling system by plugging the hose from the outdrive and one of the manifold hoses and put my gauge and shrader valve on the other manifold hose. I pressurized to 15 psi and only lost 1/2psi in an hour and a half. My question is, how might I find this leak? It's too small of a leak to hear any air
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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You could be leaking at the gasket joint between the riser and manifold. However if the riser is that plugged with rust, assume the manifolds are too.


Have you pulled them for inspection?
 
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Mar 30, 2017
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The manifolds and risers were not included in my pressure test. I pulled the risers off and the water passages looked pretty scaled up but I didn't notice any rust in the exhaust passages. The riser that was getting hot had a bit of rust at the gasket but on the outside.
 

Brentathon

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Dec 29, 2009
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Risers ("elbows"), manifolds, circulating pump, and T-stat housings are all maintenance items. I replace elbows every 3 years, t-stat housing, circ pump and manifolds every 5 years. Never take shortcuts with cooling components...it's usually cooling that kills a raw-water engine.
Just my opinion.
 
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Mar 30, 2017
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Yes, I'm the same way, but this engine is new to me. I have already ordered new manifolds and risers knowing that the engine was used in salt water. I just need to figure out how to find where this half pound of air leaked from.
 

wahlejim

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Jul 23, 2015
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The riser that was getting hot had a bit of rust at the gasket but on the outside.

Possible gasket failure. This system needs to be water tight or water can flow back into your engine and milk up some oil, as mentioned in your first post.

If the boat was in salt water, check your exhaust shutters. I would suspect that you will find pieces of shutter in your exhaust system on the side you are having troubles with. When those things break, they block water flow and would cause your heating issue.
 

wahlejim

Master Chief Petty Officer
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Jul 23, 2015
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As far as the 1/2 psi loss, take soapy water in a spray bottle, pressurize the system, spray all hose connections and gaskets, including intake manifold. This should narrow down the simple fixes that would cause it. Another source of milky oil and loss of psi is a crack or failure in your intake manifold. I would inspect that as well especially with salt water exposure.
 
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Mar 30, 2017
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I just recently put this motor in the boat, I just have thru hull tips with internal flappers plus external flappers

And okay I'll do that, thank you
 
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Mar 30, 2017
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Left everything hooked up from yesterday, sprayed everything down with soapy water and couldn't find any bubbles but it's such a small leak I think the soapy water dries up before any bubbles will show.

It's lost 2psi in 24hrs
 
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Mar 30, 2017
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So would you all reccomend just running it after I replace my manifolds and risers? 2 psi in 24hrs just seems too small to mean anything.

I just don't want to hurt anything
 
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