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Cooling the engine - understanding the system

September 11, 2019, 12:07:14 pm

I had a heat alarm and high engine temperatures when I shouldn't have, and that sent me on a project of renovating the cooling system. So far I have replaced the coolant 'filter', thoroughly cleaned the radiator exterior and air (moderator) that hangs in front of the radiator (both vacuumed out both sides and blown out with air), replaced the two thermostats, fixed the return stub (was crushed by too much clamp pressure) on the bottom of the expansion tank, and replaced the radiator cap on the surge tank. All that is remaining is to do the flushes and replacement of coolant and the job will be complete. It already runs at an operating temperature that is 25-30 degrees cooler than I have seen before in the past three years of ownership!
Wolfe 10's explanation about what I should see regarding changing levels of coolant in the expansion tank (hot versus cold levels) was an eye-opener. It is such an easy diagnostic tool to determine if an air bubble is in the system!
After my fiddling with the filter and the thermostats I had lost coolant and even though the surge tank was full and the expansion tank was full (I had deliberately overfilled it in the hope that any draw-down would make up for my renovation spillage), I took it for a short drive after a long idle. Interestingly the level in the coolant tank barely changed after the drive and only about 2" overnight, even though the surge tank, and thus the radiator, needed more coolant - so I did indeed have an air bubble in the system. I expect a few more as I take more trips down the road. The very small change in expansion tank level led me to drain it (thinking that perhaps the hose was plugged), ( it drained very very slowly and some air bubbles kept appearing in the runoff. Something was suspicious about this. So I took it off the frame, removed the hose and found the crimped stub. It is interesting how such a little thing, and an invisible crimp, can affect the whole system. Anyway all is good now and I have new knowledge that help with Mr. Fit-It confidence.
My question is about the pounds of pressure (options?) for the pressure cap. In reading the Forum it seemed that 10# and 15# were both acceptable. Since NAPA only had the 15#, I took it and used it to replace the one that had no pressure markings (and looked like it was the original 1993 OEM cap!!).
So...Mr. Wolfe, and others...what are the implications for the cooling system if my system needs 15# of pressure/vacuum to function, as opposed to 10# of pressure/vacuum (if I had found another cap)? Would the operating temperatures be different? If so, then in what way? Just curious enough to want to understand the components better.

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Re: Cooling the engine - understanding the system

Reply #1 – September 11, 2019, 12:46:08 pm

Every pound of pressure in the cooling system will raise the static boiling point three degrees so a 15 psi pressure cap will allow the coolant to rise 15 degrees more than the 10 psi cap before boiling (vaporizing).

The operating temperature of the engine will not change, just the boiling point.

Most cars will have a 15# cap.

Here is a site with a chart showing the boiling point of coolants and how different pressures effect the boiling point: https://durathermfluids.com/pdf/techpapers/pressure-boiling-point.pdf

A high boiling point has the advantage of the coolant not being able to "flash" boil in high temperature locations in the cylinder head, especially around the exhaust valve(s). If the coolant does vaporize, it allows the cylinder head metal to overheat and possibly cause damage to the valve seats and not allow the valve guides and seats to transfer as much heat from the valve seat face and stem to the cylinder head.

Pierce

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Re: Cooling the engine - understanding the system

Reply #2 – September 11, 2019, 01:48:38 pm

Mike_Sandra_Lloyd wrote:
"The very small change in expansion tank level led me to drain it (thinking that perhaps the hose was plugged), ( it drained very very slowly and some air bubbles kept appearing in the runoff. Something was suspicious about this. So I took it off the frame, removed the hose and found the crimped stub. It is interesting how such a little thing, and an invisible crimp, can affect the whole system. Anyway all is good now and I have new knowledge that help with Mr. Fit-It confidence."

Good for you and congratulations.  This is the sort of problem that is unique to your vehicle and prohibitively expensive to have repaired.


Mike_Sandra_Lloyd wrote:  "So...Mr. Wolfe, and others...what are the implications for the cooling system if my system needs 15# of pressure/vacuum to function, as opposed to 10# of pressure/vacuum (if I had found another cap)? Would the operating temperatures be different? If so, then in what way? Just curious enough to want to understand the components better."

Just to be blunt:  The thermostats control the operating temperature.  A combination of the coolant and the pressure on the system keeps the coolant from boiling.

You're doing great!

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Re: Cooling the engine - understanding the system

Reply #3 – September 11, 2019, 02:31:11 pm

Thanks guys! It feels pretty good to (a) have done the work myself, and (b) to continue learning about this amazing machine. Coincidental with the cooling system fixes, I renovated the engine compartment insulation and sheathing so that it is fireproof as well as pretty! Pictures to follow on another thread.

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Re: Cooling the engine - understanding the system

Reply #4 – September 11, 2019, 02:38:43 pm

My opionion would be if you can get by without overheating with the lower pressure cap then use that cap,less pressure less chance for leaks,internally or externally.

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