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post #1 of 28 Old 06-10-2015 Thread Starter
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Coolant puzzle

This is the school O'day 35 with an Universal engine.

Their is no overflow tube or expansion tank.

Actually their is a tube but it is capped off.

Even if the engine has been off for a couple days if you crack the radiator cap, anti-freeze starts bubbling out.

I've never seen that happen before. What does it mean?

The anti-freeze that bubbles out looks OK, greenish.

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post #2 of 28 Old 06-10-2015
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Re: Coolant puzzle

It indicates you have a nice tight leak-free coolant system. And that it is plenty full.

I'm not so sure it is a good idea to cap off the overflow tube as that would disable the pressure relief feature of the cap, yes?

Excessive coolant pressure, and many bubbles while running can indicate a head gasket problem. Or, very rarely, a crack in the block or head.

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Re: Coolant puzzle

Which cap are you cracking off? The one on the exhaust manifold, or the one on the header tank? It's normal for coolant to flow out of the cap on the exhaust manifold if you crack it, as the header tank is located well above that. Indeed, that is part of the bleeding procedure.

At least, that's how my Universal is set up.
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Re: Coolant puzzle

This is the cap the coolant comes out of.

This is a different boat because as you can see their is an overflow tube but other wise the setup is about the same.
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Re: Coolant puzzle

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Which cap are you cracking off? The one on the exhaust manifold, or the one on the header tank? It's normal for coolant to flow out of the cap on the exhaust manifold if you crack it, as the header tank is located well above that. Indeed, that is part of the bleeding procedure.

At least, that's how my Universal is set up.
Not sure what you mean by header tank. If you are talking about one of those plastic expansion tanks we don't have one.

The overflow tube is capped off.

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Re: Coolant puzzle

OK, that is the cap on top of the exhaust manifold. Somewhere else, there will be a header tank. On my boat it is a grey metal tank, located high up in the top of the starboard lazarette. Look for it.

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Re: Coolant puzzle

My old universal m18 had no coolant recovery tank. Just make sure the coolant in the exhaust manifold is topped off to within an inch. The hose should not be capped off. It is a pressure release if the pressure gets too high. It should not leak though unless your cap is broken.
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Re: Coolant puzzle

You're talking about a coolant recovery tank, which catches coolant that passes through the pressure cap if it releases. What I'm talking about is a header / expansion tank, which is in the cooling system and is pressurised. It has a second pressure cap on it. It should be the highest point in the system.

If you have a header tank, the pressure cap on it should release at a lower pressure than the one on the exhaust manifold, which is thus not functioning as a pressure cap. It's just a plug.

You can of course have a coolant recovery tank, AND a header tank, which is what I have. The coolant recovery tank is connected to the cap on the header tank, NOT the exhaust manifold one.

You need a coolant recovery and an expansion tank to minimise the air in the system, if you are going to use modern long-life coolants. They hate being exposed to hot oxygen and then begin to crystallise. If you don't have either, better stick to the old 2-year green coolant.

However, the OP asked why coolant gushes out of the cap on the exhaust manifold. Answer : because there is a header tank he hasn't found yet.

"The overflow tube is capped off" : Because it's doing nothing, because the overflow tube that actually works is connected to the header tank.
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Last edited by MarkSF; 06-11-2015 at 02:49 AM.
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Re: Coolant puzzle

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why coolant gushes out of the cap on the exhaust manifold. Answer : because there is a header tank he hasn't found yet.
I had exactly the same problem AND a header tank (that I didn't know was a header tank, and was baffled by) right up in the lazarette. This sure will make my engine checklist a lot less messy! Better check the level in the header tank...

Wow, that really should have been obvious. Now I'm having a "doh!" moment that just won't go away.

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Re: Coolant puzzle

Back in the day, the converted car engine's cooling system included a big plastic pop bottle with the bottom cut off.This was tied ,bottom upwith twine to the aft wheel house bulkhead At a glance you knew the situation re expansion,temp,and fluid level.What's more ,you had a real understanding of the systems that allows your equipment to be happy
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