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Old 08-08-2022, 07:02 PM   #21
N147JK
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Default Re: One time overheat?

I'm guessing it has to do with limiting the number of on/off cycles the fan has to make. A lower fan-on temp would cause the fan to cycle on and off more often. I'll bet they wanted to limit that annoyance as much as possible, but still protect the engine, and figured 220 degrees was a safe compromise.
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Old 08-08-2022, 08:43 PM   #22
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Default Re: One time overheat?

I suspect the high temperature as compared to 70s cars is for fuel economy and emissions. I get significantly better MPG in summer vs winter. When it's hot the PCM runs the car lean.
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Old 08-08-2022, 09:38 PM   #23
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Default Re: One time overheat?

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Originally Posted by Waiex191 View Post
I suspect the high temperature as compared to 70s cars is for fuel economy and emissions. I get significantly better MPG in summer vs winter. When it's hot the PCM runs the car lean.
True, except I think for this car the ideal operating temp, which is maintained while underway, is around 185. So, one might expect the fan to try to hold somehting closer to that temp. But it doesn't - it allows the temp to reach 220. That can cause a boil over.. when the car isn't even moving! To me, it's a really strange design mentality. Of course, it wont overheat if every part of the cooling system is properly maintained, but how many owners actually do that? I can see why quite a few owners have installed manual override fan switches. But what an odd thing to have to do.
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Old 08-09-2022, 03:49 AM   #24
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Default Re: One time overheat?

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True, except I think for this car the ideal operating temp, which is maintained while underway, is around 185. So, one might expect the fan to try to hold somehting closer to that temp. But it doesn't - it allows the temp to reach 220. That can cause a boil over.. when the car isn't even moving! To me, it's a really strange design mentality. Of course, it wont overheat if every part of the cooling system is properly maintained, but how many owners actually do that? I can see why quite a few owners have installed manual override fan switches. But what an odd thing to have to do.
The boiling point is raised by the pressure within the system, and if everything is working correctly with water alone should be around 248 degrees F. In a proper working system the coolant/antifreeze is primarily to keep it from freezing and to act as corrosion protection, NOT to raise the boiling point. The boiling point being raised is simply a by product of the solution being used.


That being said, it leads me to think you might have a flow problem at some point, or that your ECTS is not reading correctly.

First test, is the small line on the back of the reservoir. Make sure there is a constant flow through this line, as it is important to cooling. Easiest way is to just remove it with the car cold, unscrew the reservoir cap and put the hose in the reservoir. Start the car, then lift the hose enough to check for constant flow. If it doesn't flow, you have clogged passages that need to be cleared.


If that line is fine, I'd check the ECTS connector, and if it looks good I'd pull the ECTS and check resistance vs temp. Though you can't get above boiling with water, you can see if it's close at boiling or near. I just popped the ECTS and a kitchen meat thermometer though a piece of cardboard on a coffee cup, filled with boiling water high enough to keep the sensor under the water, and probed it with a voltmeter as it cooled down.





You can also check the reservoir temp vs gauge reading using a meat thermometer, up until near boiling. This might give you an idea of where it should read vs temp.

But keep in mind, if you are using a reader, it is reading that temp through the sensor. So if the sensor is bad, the data the reader shows is bad as well.
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Old 08-09-2022, 08:26 AM   #25
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Default Re: One time overheat?

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The boiling point is raised by the pressure within the system, and if everything is working correctly with water alone should be around 248 degrees F.
Thank you for reminding me - I had forgotten. So even on straight water, it shouldn't be boiling over.

I replaced the ECTS with brand new. I use an OBD-II scanner to check the gauge against the coolant temp - it seems to be accurate. I disconnected the small line on the reservoir and blew through it - I get bubbles in the reservoir tank, which indicates it's open. Even changed the cap to brand new. I am at a loss here..
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Old 08-09-2022, 10:10 AM   #26
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Default Re: One time overheat?

Ok I changed out the thermostat (I was going to do that anyway) and fired it up. Filled system with plain water. Wait, what is that drip coming out of the recirculation hose where it connects to the reservoir tank? Shut it down, pulled the hose - sure enough there is a crack in the rubber, underneath the hose clamp. Water is dripping out. Would that explain the boil-over? If the system must be pressure-tight to function correctly, that might be the smoking gun right there.

Last edited by N147JK; 08-09-2022 at 10:23 AM.
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Old 08-09-2022, 12:20 PM   #27
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Default Re: One time overheat?

Seems promising!
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Old 08-10-2022, 12:30 AM   #28
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Default Re: One time overheat?

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Ok I changed out the thermostat (I was going to do that anyway) and fired it up. Filled system with plain water. Wait, what is that drip coming out of the recirculation hose where it connects to the reservoir tank? Shut it down, pulled the hose - sure enough there is a crack in the rubber, underneath the hose clamp. Water is dripping out. Would that explain the boil-over? If the system must be pressure-tight to function correctly, that might be the smoking gun right there.
Hopefully that is the cause. If it drips when cool, it's probably losing more coolant once it heats up, and dropping pressure as it does. At that point two things are going against you, the dropping level of the fluid and the fact that pressure helps with cooling.
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Old 08-12-2022, 04:21 PM   #29
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Default Re: One time overheat?

Well unfortunately my efforts have not fixed this. Frustrated enough to junk the car, take the $ loss and move on to somehting else.

The leaking hose was not the problem - it was just loose. I thought the cap was not seating properly, so I replaced both the tank and the cap with factory-new parts. Still boils over. BTW, this is using straight water, which is supposed to boil at 275 under 15 PSI. Maybe this isn't effective in a system like this?

What happens is, with the car idling, just as the temp reaches 212 the coolant tank suddenly fills from the bottom. It reaches the cap and overboard it goes.

It's as if the system is not fully pressurizing. However, when I open the cap after it cools some, I do get some pressure release from the tank. I see no leaks anywhere.

Should I try filling with fresh anti-freeze? I am running out of ideas.

Any suggestions appreciated.
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Old 08-12-2022, 06:20 PM   #30
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Default Re: One time overheat?

Problem solved!

My helper noticed that when I screwed the coolant cap on, it didn't look like it went on right. It was square, screwed down, and tight, but somehting didn't look right to him. He tried it and we found out the threads are such that it's possible to get the cap to look fully on, but it's not really sealing. After working with it and reinstalling it a few times, the system runs perfectly - fan come on right at 221, as expected, temps are perfect, no more boil over.

Thanks to everyone here who contributed and helped me out - I really appreciate all the help!
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Old 08-12-2022, 08:19 PM   #31
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Default Re: One time overheat?

Excellent! I really didn't know what to say to the prior post.
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Old 08-12-2022, 08:30 PM   #32
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Default Re: One time overheat?

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Excellent! I really didn't know what to say to the prior post.
Yeah there is something odd about the threads on the cap, or the threads on the tank, or maybe both. One can put the cap on tight, but it isn't actually sealing, and you wont know it until it boils over. A plastic cap? It should be simple and completely fool-proof.
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Old 08-16-2022, 10:28 PM   #33
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Default Re: One time overheat?

If you turn the air conditioner on the radiator fan should automatically turn on I just went through a bunch of stuff to figure out that my fan was intermittently working because the motor was going to bed
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