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Old 04-27-2013, 06:22 AM
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Lets Talk Cavitation and Coolant

I tried doing some searching and came up empty, so if this has already been addressed at length, please point me to it!

I have been doing a lot of reading about cavitation, thermostats, O-rings, Radiators, Front Covers, etc. Seems to me like Ford keeps throwing more and more parts at it without solving the underlying problem.

Does anyone really know what the root cause of all this is?

I know this all started with a simple "T" fix at the degas bottle.

IMO, More O-rings shouldn;t be needed to hold less then 15psi of pressure. What is causing the leaks to begin with? Excessive pressure? Deforming of the pastic? Bad O-rings?

For the last 6 months I have had a leaky HG(finally fixed) and a pressure gage hooked up to my degas bottle. Occasionally when I got on it my pressure would shoot to almost 20 psi, but yet my "single O-ring" hoses never leaked.

So what is causing the cavitation? is it air in the cooling system at the water pump? If so where is it coming from?

Or is it just the coolant itself forming miniature air bubbles from what? Heat? Pressure from the Water pump?

What do you guys think? Let's get to the bottom of this!!
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Old 04-27-2013, 06:24 AM
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I'll even be the first to reply to my own thread.... Here is a picture of one of the thermostats I replaced. What the hell caused this? The Thermostat had just been replaced about 20,000 miles ago. The other one looked new-


Last edited by Fordnut74; 04-27-2013 at 06:28 AM.
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Old 04-27-2013, 07:07 PM
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Cavitation is caused by the suction pressure being too low which can also be associated with coolant that's too hot...
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Old 04-28-2013, 05:56 PM
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Quote:
Originally Posted by RUFFSTUFF View Post
Cavitation is caused by the suction pressure being too low which can also be associated with coolant that's too hot...
You know, it's interesting you mention that. Led me down and avenue that makes sense of what I have been seeing.

For the last 6 months or so I have had a pressure gauge hoked up to my degas bottle to monitor my leaking head gasket. However I have noticed that pressure always builds up to about 5 psi as the truck is warming up, then drops to almost 0 when the thermostat opens. Which one, I don't know.

The degas bottle drain is connected to the water pump inlet hose- which is about 3 ft long. Is it possible that with such a huge water pump(compared to the 7.3 and 6.0) and so much pump head that when the dual thermostat's open the pump has far more vacuum on the inlet then it does resistance at the outlet creating prime conditions for cavitation? And possibly "sucking" air past the o-rings designed for a pressure system?
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Old 04-28-2013, 06:39 PM
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Well, the higher flow raises the required NPSH and lowers the available NPSH. In other words... Anything is possible.
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Old 04-29-2013, 06:53 AM
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I dont know the answer to the question but i am interested to see what everyone says.
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Old 04-29-2013, 06:20 PM
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Originally Posted by RUFFSTUFF View Post
Well, the higher flow raises the required NPSH and lowers the available NPSH. In other words... Anything is possible.
I am not even going to pretend I know what NPSH is beyond what I googled. Sounds to me like creating a low pressure delta near point of boiling will induce boiling/cavitation exactly at the pump, until the fluid reaches the positive pressure side where the fluid is again above boiling point.

But if the water is coming from the radiator(cooled) it shouldn't be anywhere close to point of boiling. I guess unless there is a lot of vacuum significantly lowering boiling point, or we are pulling air past the O-rings. I remember this problem showing up on the old 7.3 fuel system.

At this point I am wondering if this cavitation is what ate the seal from around the Thermostat I pictured earlier. If so- why just the one and not the other?

Does anyone know at what temperatures they open? Same or different?

The so called "Venturi Tee", according to Ford, "On vehicles equipped with a 6.4L diesel engine, a venturi tee is used to prevent the reverse flow of air from the degas bottle to the radiator.". Say what?? So Ford knows air is getting in the system and thinks it's from the Degas bottle?

I am getting ready to make a 300 mile trip tomorrow, maybe I will run the small hose from the top of the radiator, through the cab with a clear hose, then to the Degas to see if I have air coming from the radiator or the degas.
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Old 04-29-2013, 06:50 PM
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Jason - I have nothing to contribute to this thread other than a bump. Hopefully the collective mind here is able to figure this out.
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Old 05-03-2013, 06:00 AM
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Originally Posted by Snake View Post
Jason - I have nothing to contribute to this thread other than a bump. Hopefully the collective mind here is able to figure this out.
Thanks Mike. You know, you should make a Road trip North for the Summer-

Alright guys help me out here. I know some others have put some work into figuring this out??


Back to topic at hand. So I hooked up some clear hose to the 3/8 coolant return running from the upper right corner of the radiator back to the Degas running a loop up past the bottom of the windshield so I could see exactly what was happening the results were intriguing.

My coolant system has been flushed for several hundred miles so there shouldn't be any air in the system.

Normal cruising doing about 75 at just under 2000rpm is exactly as expected- Constant steady flow of coolant with no air. This did not appear affected by load or boost. I was able to reach about 28psi before she would shift down. And that is when things became interesting. Tangent- I bloody hate these damn Transmissions. Impossible to control gears unless I want 3rd. After flowing solid coolant for miles, I would hit the gas making it shift down and about 5-10 seconds later I started getting tons of very small air bubbles flowing past my view. This continued as long as I was able to hold the RPM at 2500rpm or above. Not sure if this is being created from cavitation, or from air being pulled into the system somewhere. These results were very consistent. Anytime I revved the motor, I got air. This does not appear to be load related, but I wasn't towing anything so can't be certain

At idle, there is no flow at all through the hose. In fact I did see some reverse flow where Air was going from the Degas to the radiator. Everytime I would take off from a stop there was an enormous amount of air being purged back to the Degas bottle. These results are a bit ambiguous right now as my time to experiment is limited.

I did have a pressure gauge connected as well, but that never budged much. But I am curious to know, what is causing the flow that is obviously RPM related. Is there a low pressure on the inlet hose pulling fluid, or a high pressure at the outlet pushing fluid back to the degas bottle. Does it matter?

In some of my reading about cavitation(maybe it was the NPSH) I remember something about not enough pressure at the outlet of a pump, or too much restriction at the inlet. FWIW, my coolant temps are stable at 197. Could a low pressure at the inlet be artificially reducing the boiling point enough to induce flash boiling(cavitation?) causing the miniature air bubbles I was seeing?

Is anyone even paying attention?
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Old 05-03-2013, 06:05 AM
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Paying attention yes, understanding no.

Its a ford fix it as it breaks and don't ask questions?
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