Coolant Resevoir Keeps Filling Up.

bigpursuit

New Member
Mar 30, 2009
52
Michigan
Boat Info
34 pursuit
Engines
7.4 crusaders
My engines are 7.4 Crusader. I recently noticed the port engine ran a little hotter. When I looked down below the coolant resevoir was filled to the top and had overflowed. I opened the radiator cap on the engine and the engine needed fluid. I drained some of the fluid from the resevoir and filled the engine to the top.

I ran the boat at idle for a half hour or so and everything seemed fine. The next day I went out about 11 miles and the resevoir had filled again.

I have had a mechanic check out the boat and check the pressure etc. Says everything is fine.

Any ideas?
 
Did you drain the fluid recently? If so, could there be air trapped in the coolant loop? Air expands when heated, fluids don't....
 
My theory is that there was air in the system. Would that cause the resevoir to fill? I filled the system once and it happened again. Is it possible still more air?
 
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If it continues, it could be a sign of a blown head gasket. Combustion gasses might be forced into the cooling passages via the missing gasket, then force coolant up and into the reservoir? There is a test strip that can be dipped into the coolant that will indicate if combustion air is present.

I'd be doing a quick compression test at the very least.

FYI I'm not an expert - just thinkin out loud!
 
Everyone's on the right track.

Could be a stuck thermostat (overheating causes fluid expansion), a worn impeller (overheating causes fluid expansion), a bad coolant water pump (overheating causes fluid expansion), a blown head gasket (pressurizes the cooling system - but if the mechanic did a pressure check, you could eliminate this one), or did you recently change antifreeze (could be air in the system).

Eliminate them one at a time...
 
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Did you drain the fluid recently? If so, could there be air trapped in the coolant loop? Air expands when heated, fluids don't....

i am pretty sure fluids can expand when heated....in most (if not all) pressurized cooling systems for internal combustion engines there is an 'expansion tank' that is part of the system and connected to the radiator...when the coolant heats up it expands and some of the heated coolant flows into the expansion tank because the radiator, hoses and engine coolant passages cannot handle the increased volume of coolant...as the engine cools when it is shut down the coolant starts to contract and a vaccum is created and the coolant that was in the expansion tank is now sucked back into the radiator....a non-pressurized cooling system will have a simple 'over flow' tank in lieu of an 'expansion tank'....in this type system the over flow tank will catch any 'super heated' coolant that escapes from the radiator but the coolant will not flow back to the radiator when the engine cools...

not sure if any of this helps...
 
I had the same problem with the port engine our 330DA (mercruiser 7.4L) and it turned out to be the prime culprit was a heat exchanger leaking between fresh and raw water chambers.

We guess that initially the expansion tank would take the extra fluid as intended but the leak was a weird type that was closed off when under pressure or hot but allowed air to be drawn into the closed side of the system when cooling down. it took weeks to track down since the initial pressure tests were all ok.
 
Found the Problem.

After checking every possible variable, it turned out to be the thermostat. I know we should have just replaced it to begin with but the symptoms were strange. The boat would run forever at idle with no problems. Also the temperature shown at the gauge was never very high.
 
Re: Found the Problem.

After checking every possible variable, it turned out to be the thermostat. I know we should have just replaced it to begin with but the symptoms were strange. The boat would run forever at idle with no problems. Also the temperature shown at the gauge was never very high.

typical symptom of a partially failing t'stat...when the engine is at idle there is not as much cooling needed...it sounds like the t'stat was allowing 'some' coolant to flow which was sufficient to cool things down at idle...then when the rpm's of the engine increased and the cooling demand increased the flow of coolant was not sufficient, thus causing overheating...

glad it was something simple and you got it fixed...
 
Re: Found the Problem.

Problem still exists!
Took the boat out this weekend and the problem with the resevoir filling over occured again. This time I notices bubbles in the resevoir as I was running. Head Gasket?
 
Re: Found the Problem.

This time I notices bubbles in the resevoir as I was running. Head Gasket?

yep....that would be my guess...extremely hot combustion gases escaping from the combustion chamber and mixing with the coolant due to a bad head gasket....this causes the coolant to be super heated and the coolant expands and flows into to resevoir...
 
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Re: Found the Problem.

Just had a compression test over the weekend. Results: 1-170 2-160 3-145 4-150 5-142 6-155 7-157 8-165. So does this sound like a head gasket?
 
Re: Found the Problem.

Just had a compression test over the weekend. Results: 1-170 2-160 3-145 4-150 5-142 6-155 7-157 8-165. So does this sound like a head gasket?


i am not familiar with the 7.4 engines so i'll not tryto guide you on how the numbers look....someone with knowledge of the 7.4's will likely offer their opinion...

personally i would be suspisious of any number in the 140's and maybe even in the 150's....if you had a mechanic perform the compression test, what were his comments?
 
Re: Found the Problem.

i am not familiar with the 7.4 engines so i'll not tryto guide you on how the numbers look....someone with knowledge of the 7.4's will likely offer their opinion...

personally i would be suspisious of any number in the 140's and maybe even in the 150's....if you had a mechanic perform the compression test, what were his comments?

The mechanic believed the 3- 145 and 5- 142 showed a head gasket problem. He also atached a pressure gauge to the cooling system and pump up the pressure. He started the engine and the needle on the gauge jumped as the engine cycled. He believes the gaskets need to be replaced. I was just looking for more insight before I commit.
 
You have a blown head gasket!!!!!!!! seen this before, combustion gases getting into the coolant.
 
You have a blown head gasket!!!!!!!! seen this before, combustion gases getting into the coolant.
Thanks for the response. I really appreciate your response and the certainty of what is actually wrong. The biggest trouble with this problem is getting to the actual conclusion. All the mechanics I have worked with had trouble with thinking it is the head gasket. I understand the thought process since there is no other major symptom with the boat. However it appears not all head gasket problems first appear as dramatic as others. The compression test was simple and should have been done first in my opinion.
 
Thanks for the response. I really appreciate your response and the certainty of what is actually wrong. The biggest trouble with this problem is getting to the actual conclusion. All the mechanics I have worked with had trouble with thinking it is the head gasket. I understand the thought process since there is no other major symptom with the boat. However it appears not all head gasket problems first appear as dramatic as others. The compression test was simple and should have been done first in my opinion.

You have a problem your mechanics have only done a compression test that shows something is wrong there should never be more 15% difference between any cylinder, if you want to keep testing do a leak down test. At this point you already know something is wrong and need to tear it down and find the problem it could be just about anything from a blown head gasket to a cracked cylinder. Your just guessing till you pull it apart.
 

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