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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 15634
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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Well maybe you can. I have a customers Fountain with the 525EFI

Customer Question

Well maybe you can. I have a customers Fountain with the 525EFI Mercruiser 502. It overheats after about five minutes and won't cool down at idle. Checked and replaced thermostat,Heat exchanger not plugged, good water flow, good water psi, exhaust cooling good, antifreeze boils over when this happens, still reserve antifreeze in reservoir, but stays hot like 220. I think it is a blown head gasket keeping air in crossover assembly, bad circulation pump.
Don't know right now.
Submitted: 2 years ago.
Category: Boat
Expert:  Jason replied 2 years ago.
Hello my name is ***** ***** am I speaking with?
Have you done a compression test, or checked the heat exchanger for signs of hydrocarbons with a dye tester?
When did this problem start, and when did it last cool properly?
Does the boat have a bravo sterndrive?
Customer: replied 2 years ago.
My name is***** have been a marine technician for about 15 years.
Yes, compression test. all at 160 except 7 and 8 a little low at 128.
Had water in #8 that took out the starter. replaced starter. Don't know for sure how water got there. I think customer left hose on after flushing motor and it back filled header.
Found water in oil, valve covers had milky residue, pcv valve system same, blew out oil cooler, pressure checked cooler, blew out all oil lines, filter assembly, replaced oil thermostat in oil block, flushed motor, new oil.
Customer states he has never had to fill coolant reservoir.
Pressure checked CMI Headers found water leaking into #8 exhaust port. Found other leaks and had welded and repaired. Pressure check good now both sides of headers.
Heat exchanger looks great both sides flows good. Have not checked closed cooling system with dye tester yet.
Oil pressure used to be low but now at rated spec.
Oil temp used to be high but now at rated spec.
Water pressure is good at running at any speed.
Have heard air pockets in crossover can cause problems, some suggest drilling holes around thermostat to relieve air pockets.
Overheats rapidly after about three minutes running almost WOT. Wont cool down even at idle with good water flowing.
Took inlet hose off of x changer and tested flow, water pumping excellent at idle.
Bravo XR outdrive. I have seen the problems with inlet fitting on older bravos. Corrosion crimps of the tube. This should not have that problem, newer model. 2005 I can check.
Problem started after the above fixes.
I think possible head gasket leaking air bubbles into closed cooling system causing overheating of closed cooling system. Thus creating air pocket around thermostat preventing it from opening causing the overheating regardless of raw water flow.So what do you think?
Customer: replied 2 years ago.
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Expert:  Jason replied 2 years ago.
I think possible head gasket leaking air bubbles into closed cooling system causing overheating of closed cooling system. Thus creating air pocket around thermostat preventing it from opening causing the overheating regardless of raw water flow.
So what do you think?
That's definitely a possibility, and that's why they make the dye testers, to confirm that theory.
Let me ask you another question. Do you have an IR temp gun, and if yes. If you go and duplicate the problem do the exhaust risers measure hotter or colder than engine temperature?
Customer: replied 2 years ago.
During the overheating episode the exhaust headers ran nice and cool and continued to do so while under way back to the ramp under guardian mode with temps spiking to about 240. Also some coolant expelled right at the start of the overheat but there was reserve in the tank that would not go down while back to the ramp.
Yes I have a gun.
If you know about this series of engines it has a cast aluminum crossover that retains the heat exchanger above it. The engine temp sender is in this crossover assembly. So if there is any air trapped in this crossover the temps will spike high I predict and the thermostat won't open in an air pocket. I have heard of drilling holes in the thermostat housing to relieve air.
What do you suggest.
Expert:  Jason replied 2 years ago.
If the headers ran cool then it's not a lack of water issue, if it was they would have read hot as well. So now you are down to either a circulation problem, or a head gasket issue. Do you have a radiator pressure tester that reads pressure if you stick it on the heat exchanger?
Customer: replied 2 years ago.
That's kinda what I thought.
The water circulation pump.......the shaft is still turning so I would assume that it is fine. I don't think I ever recall coming across one that wouldn't turn the blades without the shaft breaking off or something.
I don't have that tester. I can only stick it on the coolant reservoir tank, that's where the radiator cap is. If I pressurize the coolant system and it doesn't hold, head gasket bad, but it might not leak when motor is cool......possible right?
I can go buy a hydrocarbon test kit and a radiator pressure kit today.
Expert:  Jason replied 2 years ago.
The reason I asked on the radiator pressure tester I certain ones will measure the pressure inside of the cooling system. I have to double check but I think these run a 15 pound cap, which means the pressure should normally run 10 to 12 psi. (on the antifreeze side). If you hook up the tester and the pressure runs up as the engine gets hot, especially if it shoots up high and quickly, that extra pressure is more than likely coming from a head gasket leak. Does that make sense?
Customer: replied 2 years ago.
Yes it has a 15 lb. cap. I will buy a tester and perform that test.
Now why do you think the motor won't cool back down at all, even with the coolant cap off and reserved coolant present?
Expert:  Jason replied 2 years ago.
If there is a big air bubble in the block it's not going to cool until that bubble burps.... Or collapses back down on itself.
Customer: replied 2 years ago.
So I have read that many suggest to drill the body of the thermostat with three of four 1/8 inch holes to relieve air that gets trapped in the system on these motors. At least it would probably let the motor cool back down after overheat you think?
Expert:  Jason replied 2 years ago.
I wouldn't bother. Thermostats are not air tight. Air will get past it.

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