Boat mechanics input needed

Terryl

Terryl

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OK kids, I have a small problem with a Volvo Penta 5.0 liter engine in my boat.

Last week it overheated,(+210 degrees F) this due to a bad impeller on the raw water pump.

This has now been replaced, but the engine still overheated after the replacement impeller was installed.

So out of the water it came, after looking at the thermostat I found small bits of the old impeller inside the housing, so I cleaned out the housing, and back in the water it went, it still over heated, though not as bad,(under +200 F, normal is +170 F)) out it came again, this time I looked at the main circulating water pump, found nothing, and tested it, it worked fine.

So, I pulled all hoses and did a thorough cleaning, then a complete back flush of the engine block with the water pumps, and thermostat housing off the block, I made adapters to block off the one side of the water pump and one to push water through the thermostat entrance, this was a backwards flow, ran the water at street pressure for 10 min, I didn't see anything come out the discharge tube into the 5 gallon bucket I used to catch anything if there was anything blocking the water passages.

I tested the thermostat by boiling it in a pot of water, it opened right at 167 degrees F, and was fully open at 174 degrees F, so it's not the problem.

I tested the thermostat water sensor buy using an ohm meter and a pot of water on the stove, this along with a calibrated digital thermometer gave my the resistance readings for a wide range of water temp's, I know now that at 210 degrees I will have 64 ohms at the sender, and around 760 ohms at 70 degrees.

This way I can tell directly at the engine if the water temp gauge is right or not.

Now......Is there a filter somewhere that I am not seeing???? And if it still overheats after all this do I need to pull the heads to look for blockage?? (Not a job I'm looking forward too)

This is looking to be a big project, I don't want to take it in to the service center as that is a bit above my budget right now, any advice from you boat guys will help.
 
I am not a boat technician but car technician and having looked at the schematics for what parts are involved it looks very basic so I don't think you have missed anything.

Does then engine still start and perform perfectly ?

Can you carry out a combustion leak test to see if you have the starts of a head gasket failure ?

Although you replace a broken pump are you sure particles have not damaged the new pump ?

Do you have a laser thermometer to check if there is a specific point on the engine or cooling system that is getting hotter quicker than anywhere else ?

sonic1
 
Hey Sonic1 thanks for the quick reply.

Well all tests on the raw water pump show it is doing it's job, (stern-drive in a large tub) and the flow from the standard engine water pump show it's working fine.

I will have to dig out my compression gauge to have a look at the heads for a leak, maybe I can tell by the plugs?? If one or more look dirty or fouled???

I haven't worked on a Chevy engine block for some time, my knowledge of them was quite extensive, (in the past) but the old library is getting a bit dusty as I haven't used that part of the old brain for some time.

I don't have an IR or laser thermometer to check for any hot spots, for that I would have to take it in.

I'm just hoping it's not a blockage in one of the water jackets in the heads or block......But the reverse flush-out under pressure should have pushed then out correct???

As to the new pump, it is a raw water pump, it has a rubber impeller, and only sucks water from outside the boat, it then delivers cooling water to the exhaust manifolds (they are running cool to the touch, so it's working fine) and to the thermostat housing, this is where the cooling for the engine is done, the cool water is mixed with the water from the block, all this is controlled by the thermostat.
 
You can get an IR thermometer for £15 these days.

The fact that the manifolds are getting cool makes me think the pump is doings its job and the problem lies elsewhere.

An inspection of the plugs may help but I always prefer a a chemical combustion leak check., again you can get them cheap.

Its hard to help behind a laptop, I would rather stick my head in the engine bay. :p
 
Is is s straight 5 litre or are there some other factors or mod?
Sterndrive ?

Odd that someone on the other side of the pond uses litres, but there you go. The euro must be strong.
 
It is a standard Chevy block, in 5.0 liter, (that's the way they are measuring them now days) without all the smog junk on it, then it connects to a Penta outboard Stern drive.


Looks something like this one....Without the boat.

boat-five.webp

The water intake is the line of dots just to the right of the propeller, it is sucked in by the raw water pump, this is attached at the harmonic balancer, it then goes up from there to the water distribution/thermostat manafold, the cool water is split 3 ways, two go to the exhaust manifolds to cool them, the rest goes through the thermostat to cool down the engine block, this is done by a standard water pump like you would find on a car.
 
Is it possible to try with the thermostat removed? can't think why it should be the cause of the problem but seems the easiest next thing to try?
 
Failed core plug(s)? That's about the only thing I can think of with overheating when the rest of the system is "working" after having suffered an overheat...
 
2cvbloke said:
Failed core plug(s)? That's about the only thing I can think of with overheating when the rest of the system is "working" after having suffered an overheat...
Click to expand...
Would have thought that failed core plugs would result in noticeable water leaks!
 
pgh13 said:
Is it possible to try with the thermostat removed? can't think why it should be the cause of the problem but seems the easiest next thing to try?
Click to expand...
Not possible as the thermostat is pressed into the main water distribution block, you would have to buy a new water block without the thermostat in it, that's over $190 USD.

Thermostat was tested in the water block by putting in a pan of water, it opened at the correct temperature.

And no water leaks anywhere near the freeze plugs or anywhere else.
 
jeallen01 said:
Would have thought that failed core plugs would result in noticeable water leaks!
Click to expand...

Shows how much I know about water-cooled engines... :-rofl2
 
It does sound like you have warped one if not both heads in the first overheat, and combustion gas is getting past one side of the block into the cooling system.

You could try a pressure lock on each cylinder, see what gasses pass elsewhere than the rings. before taking the engine apart.
 
2cvbloke said:
Shows how much I know about water-cooled engines... :-rofl2
Click to expand...
Decades ago I bought a J2 Minibus with a 1500cc "B" series engine with blown core/freeze plugs - replaced that with a similar scrapyard unit that was OK. (OTOH, could have just replaced the core plugs on the original unit, but didn't trust that route because there could have been internal faults - notably, on that engine, cylinder head cracks/ head gasket leaks)
 
Going to do a cylinder pressure test this evening as it's close to 100 degrees F today, the shop has a swamp cooler in it, that will get it down to around 80 F.
 
Channel Hopper said:
It does sound like you have warped one if not both heads in the first overheat, and combustion gas is getting past one side of the block into the cooling system.

You could try a pressure lock on each cylinder, see what gasses pass elsewhere than the rings. before taking the engine apart.
Click to expand...

Chevy cast iron blocks and heads take quite a bit of heat to warp them, now if it was an aluminum block then I would be worried, will find out if I have to pull the heads tonight, or some time this week end, it's getting too close to that famous show around here called "Time to drain that Scotch bottle", it's already Beer30 time, so we will see what happens.
 
Fuel contamination?
 
In my Scotch.........Never.
 
Try a block tester combustion kit , sniffs for exhaust gases in coolant system cheap and easy way to test headgasket.
 
purplemarina said:
Try a block tester combustion kit , sniffs for exhaust gases in coolant system cheap and easy way to test headgasket.
Click to expand...

Do you have a link to one that has been used, or recommended ?
 
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