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Old 27-05-2021, 05:56   #1
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Volvo Penta 8.1 blowing cap off fresh water crossover

Hello,

I have a new to me 2006 Volvo penta 8.1 x2 in a doral grande boca. We’ve been having problems with the cap #19 blowing off on the port engine. When bringing the boat back home the high temp alarms went off, shut it down and coolant was everywhere. Either the engine overheated for some reason first, or the cap blew off for some unknown reason which cause the engine to overheat from lack of coolant. So either way somehow too much pressure is building in this crossover tube. The engine had been running for about an hr before that just fine. It seems to pump water fine as well.

We have the heat exchanger off now to be cleaned.

Any other thoughts?

Here is the link to the diagram.

https://www.volvopentashop.com/epc/en-US/Details/PentaPartsCatalog/7744240_057?path=MarineGasolineEngines%2F7744240%2 F7744240_26%2F7744240_057
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Old 28-05-2021, 11:00   #2
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Re: Volvo Penta 8.1 blowing cap off fresh water crossover

I wonder if a blown head gasket could be the cause? What did the temp gauge say before it happened?
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Old 28-05-2021, 11:26   #3
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Re: Volvo Penta 8.1 blowing cap off fresh water crossover

I wasn’t on the boat at the time. Cruising was about 3500 rpm I wanna say. It had been running about 1.5hr by that point. This is the second time now. I wanna guess the temp was normal, I know that day previous they held steady at 175. My other thought was a faulty thermostat.

Regardless the exchangers have been cleaned, they were dirty, but we will see tomo if this did the trick.

Still at a loss how this cap could rupture without the expansion tank cap blowing off first.
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Old 31-05-2021, 05:20   #4
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Re: Volvo Penta 8.1 blowing cap off fresh water crossover

Cap "blew off" ?

Caps are held in place by a bayonet mount. Short of creative use of minor explosives, they don't blow off. The spring loaded gasket underneath compresses and releases steam, coolant, and small fish all over the engine compartment. They do that at a particular over-pressure, defined by the spring. The underlying idea is to allow a higher operating temperature, by raising the boiling point through greater than atmospheric pressure in the coolant.

OK. To spray stuff there's gotta be that pressure. A leaking head gasket is one possibility, and you hope it's not that, because it means removing the head. The other is boiling even at the set pressure, which means you've not getting enough cooling. A stuck closed thermostat is one possibility. That's easy to test; heat the thermostat in water on the stove, and see at what temperature it opens. If it doesn't open by the time you reach boiling, shout EUREKA, toss it, and start over.

Then there's reduced heat flow through the cooling system. The heat exchanger being junked up is one part of it, but the rest of the system may need a thorough flushing as well. Drain it (lowest hose connection if you don't have a drain plug), flush it, and refill. Messy.

If you found a mess in the heat exchanger, you're on to something: flush the rest of the system and hope that you are home free. Then do the other engine. Good luck with it. You have a messy Saturday in your future.
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Old 31-05-2021, 06:12   #5
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Re: Volvo Penta 8.1 blowing cap off fresh water crossover

I think we are discussing different items. This “cap” is literally just a rubber cap that covers a port on the belt side of the port engine. It caps off the port on the circulating cross pipe that connects each head for fresh water coolant.

Long story short I took the heat exchanger to be cleaned, a little gunk in it but nothing major, took the thermostat out and took it for a full day of riding. No problems. However it does run a little hotter than the starboard engine. I feel like taking the thermostat out was a bandaid for a different issue. Now I’m thinking some sort of restriction somewhere. Sea strainer is clear, oil cooler I haven’t tested yet. My other thought is an impeller going out on the fresh water circulating pump. The raw water pump was replaced not long ago, risers less than a year ago.
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Old 01-06-2021, 05:10   #6
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Re: Volvo Penta 8.1 blowing cap off fresh water crossover

I'm sorry that I misunderstood. Does this cap prevent pressurization of the coolant? That seems odd.

It does sound like you are chasing down various sources of poor cooling. Is the discharge of heated raw water seem similar on the two engines? If not, it could suggest either a restriction due to gunk or a weak pump.
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Old 01-06-2021, 05:18   #7
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Re: Volvo Penta 8.1 blowing cap off fresh water crossover

From what I understand the capped off nipple/port, whatever we call it, is used as an alternative to fresh water coolant.

The interesting thing is that when both motors are running the raw water temps of the coolant hoses feel about the same. Obviously an infrared gun would be best. However, the fresh water side of the port engine is definitely a bit toastier. I think my next go to is replacing the circulating fresh water pump and seeing what happens.
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Old Yesterday, 05:32   #8
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Re: Volvo Penta 8.1 blowing cap off fresh water crossover

Hi
I had the same issue with my 8.1. Left many scratching their heads. Turned out to be that the pressure relief valve was clogged so all the coolant would vaporize and blow out the reservoir and the engine would overheat. Replaced the valve and all is good with the world. An interesting note, the previous owner must have had a leak and likely used a can of leak fix which ultimately clogged the valve.

Good luck, let us know how you make out.
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