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Old 09-01-2012, 10:22   #1
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Weird Engine Overheating Problem ...

I have been fighting this one for awhile. No, it's not the raw water system (and therefore not the impeller) -- spew raw water like a champ. No, it's not the thermostat (replaced).

Here's what happens. I make sure the coolant is topped off (including "back filling" to the circulator pump to make sure there's no air lock). It appears to run fine indefinitely, but once it's shut down and restarted, the coolant stops circulating and it overheats fairly quickly. I know the circulation is the problem, because I can stick my finger in the coolant in the heat exchanger once it overheats and it's cool.

So, apparently a vapor lock forms after the engine has been run hot and shut down and then that prevents the coolant from circulating on re-start and causes an overheating situation.

Although I have been unable to locate any coolant leaks during a cursory inspection, my theory goes that there's a leak that releases just enough coolant when the thing is running to cause an air-lock on restart.

Does this sound at all plausible?
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Old 09-01-2012, 10:29   #2
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Re: weird engine overheating problem ...

Have you considered and checked the possibility of the water heater being the cause of an airlock? You can check by isolating the hoses to the heater and bypassing them.
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Old 09-01-2012, 10:36   #3
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Re: weird engine overheating problem ...

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Have you considered and checked the possibility of the water heater being the cause of an airlock? You can check by isolating the hoses to the heater and bypassing them.
thanks for the tip, but there's no water heater in the system.
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Old 09-01-2012, 10:38   #4
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Re: weird engine overheating problem ...

Have you pulled the fresh water pump to see if the impeller is working. Sometimes they've been know to spin on the shaft. A hot motor maybe just enough to heat up the impeller to expand.
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Old 09-01-2012, 10:51   #5
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Re: weird engine overheating problem ...

Another note!
Is the reservoir cap in good shape (7 psi). Cooling systems expand and force out a little coolant into an overflow tank and when the motor cools it's supposed to suck the coolant back in.

If your overflow hose has a leak, or you don't have one, then it will suck air back in past the reservoir cap and look as if your loosing coolant. But when it gets hot again it will expand back to a full system again.
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Old 09-01-2012, 10:51   #6
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Re: weird engine overheating problem ...

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Originally Posted by delmarrey View Post
Have you pulled the fresh water pump to see if the impeller is working. Sometimes they've been know to spin on the shaft. A hot motor maybe just enough to heat up the impeller to expand.
No. I haven't. I was thinking it could be something like that, but was unsure why it would only be the case intermittently. Your theory makes sense. Luckily, I have a new pump -- I bought one a few months ago thinking that might be the problem, but subsequently convinced myself it might not be. I think I might just swap it anyway. Can't hurt, right?
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Old 09-01-2012, 10:54   #7
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Re: weird engine overheating problem ...

Are you 100% sure you've not got a small blow in the head gasket? It might be the leak and also add to the heating problem.
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Old 09-01-2012, 10:58   #8
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Re: weird engine overheating problem ...

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Are you 100% sure you've not got a small blow in the head gasket? It might be the leak and also add to the heating problem.
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No. I am not sure about that -- hadn't even thought of it. Would that likely be easily visible when the engine is running?
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Old 09-01-2012, 10:58   #9
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Re: weird engine overheating problem ...

If you've changed the pump already, I'd go with swagman's theory.

Have you pressurized the system to see if there is actually a leak. One can rent radiator pumps.
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Old 09-01-2012, 11:04   #10
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Re: weird engine overheating problem ...

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Originally Posted by delmarrey View Post
If you've changed the pump already, I'd go with swagman's theory.

Have you pressurized the system to see if there is actually a leak. One can rent radiator pumps.
Haven't changed the pump yet, but I think that's next on my list.

If it is a leak in the head gasket, we're talking big $$$, I assume.
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Old 09-01-2012, 11:17   #11
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Re: weird engine overheating problem ...

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Originally Posted by delmarrey View Post
Another note!
Is the reservoir cap in good shape (7 psi). Cooling systems expand and force out a little coolant into an overflow tank and when the motor cools it's supposed to suck the coolant back in.

If your overflow hose has a leak, or you don't have one, then it will suck air back in past the reservoir cap and look as if your loosing coolant. But when it gets hot again it will expand back to a full system again.
That does remind me of a problem I had with a V12 Jaguar. The line to the coolant catch tank was kinked, so it worked as a one way valve. Coolant went to the catch tank on expansion, but didn't get sucked back on cool-down. The result is a cooling system that constantly has air in it.

As the line gets suction on it on during cool-down, it needs to be a re-inforced hose - but mostly you find some cheap silicone tubing installed.
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Old 09-01-2012, 11:18   #12
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Re: weird engine overheating problem ...

Depending on it's age you may not need to, but get a gasket and inspect it, at least, to eliminate that. Use the automotive RTV (blue) sealer on both sides of the gasket. There is a weep hole on the underside of the pump's shaft. If it's rusty, replace the pump.

Then pressurize the system. If the gauge drops look for a leak. Cold water will show up, unlike hot water. If no leak I'd suspect the head gasket. The cost is in relation to how much work you can do yourself. But a mechanic can give you an estimate after he sees the motor.
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Old 09-01-2012, 11:22   #13
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Re: weird engine overheating problem ...

Quote:
Originally Posted by delmarrey View Post
Depending on it's age you may not need to, but get a gasket and inspect it, at least, to eliminate that. Use the automotive RTV (blue) sealer on both sides of the gasket. There is a weep hole on the underside of the pump's shaft. If it's rusty, replace the pump.

Then pressurize the system. If the gauge drops look for a leak. Cold water will show up, unlike hot water. If no leak I'd suspect the head gasket. The cost is in relation to how much work you can do yourself. But a mechanic can give you an estimate after he sees the motor.
I've taken a diesel 101 class and am otherwise pretty handy. That said, engines are not my forte. Is it something a reasonably bright, adroit human could accomplish?

Meantime, I will take your advice before looking into a more invasive fix. Thanks very much.
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Old 09-01-2012, 11:31   #14
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Re: weird engine overheating problem ...

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Originally Posted by MarkSF View Post
That does remind me of a problem I had with a V12 Jaguar. The line to the coolant catch tank was kinked, so it worked as a one way valve. Coolant went to the catch tank on expansion, but didn't get sucked back on cool-down. The result is a cooling system that constantly has air in it.

As the line gets suction on it on during cool-down, it needs to be a re-inforced hose - but mostly you find some cheap silicone tubing installed.
Thanks. I have kind of a funky set up for the heat exchanger, and it's definitely worth my double checking that the in/out isn't pinched.
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Old 09-01-2012, 11:35   #15
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Re: weird engine overheating problem ...

Assuming you have eliminated the possibility that the water pump is the problem, you might want to make sure your thermostat is actually working; I realize you have a relatively new one in there, but it doesn't hurt to double check. Run the engine for a bit without it, then shut down and wait a spell, and finally try running it again. If you have proper circulation, then I would install a new thermostat and give it a try again. You may also want to change out the coolant fluid and give the freshwater system a good flushing out.

If you have a eliminated the other possibilities, take a look at you head gasket. However, you would usually notice a mysterious loss of coolant over a short time, and I would think you could smell the glycol aroma in the exhaust smoke. You might not be able to detect a pinhole leak in the gasket from the outside.
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