The Cliff's Notes (pun intended):

I had a coolant leak. It turned out to be the radiator. I found out when it catastrophically failed. Replaced the radiator, then after a few weeks, had another leak. Turned out to be the heater core. Again I found out when it catastrophically failed. Replaced the heater core. Then had a leak at the thermostat housing. Turned out to be the thermostat cap not closing properly. Fixed that.

Then, a few weeks after that, I had white smoke billowing out of the passenger side engine vent. When I opened the hood I saw coolant spraying everywhere, coming from somewhere to the left of the battery. Since the weather was pretty bad, I was unable to find out where the leak was. I was able to make short trips with the car (as in 5-10 minutes) before it overheated. I knew the leak was somewhere behind the thermostat because the engine would gradually heat up until it reached 190*F after which temperature would shot up to the point the needle was far over the red. Since I also had an an electrical problem causing my battery to go bad, I didn't drive my car for a while since I had to find and order a new battery first. But by now the weather has gotten so bad I was unable to do any work on my car at all.

So 2 days ago, the weather finally cleared up and I was able to install the new battery. I filled up the coolant and started the car. I waited until the engine reached about 190*F and expected to see the spray of coolant again. Nothing. Not a leak to be found. I let it run for a while and temperature raised to about 230*F and still no leak in sight. I found out however that there's a bad radiator fan wire somewhere because the radiator did not come on. Only after I fiddled with the radiator cables a bit, did the radiator come on. But that's an easy fix.

But my question is: where did the leak go? The thermostat was most definitely open because I could feel the pressure (and hot coolant) at the hose after the thermostat, yet there was no leak to be found...

Also at about which temperature does the system reach 16 PSI? Since I also notice that the coolant in the overflow bottle always stays at ambient temperature.

quote Originally posted by Cliff Pennock: But my question is: where did the leak go? The thermostat was most definitely open because I could feel the pressure (and hot coolant) at the hose after the thermostat, yet there was no leak to be found... Also at about which temperature does the system reach 16 PSI? Since I also notice that the coolant in the overflow bottle always stays at ambient temperature.

I am not going to venture a guess as to where the leak went. That is something to be determined by watching it in person as the engine is running. Without being there and able to see it in person, anyone replying would just be guessing.

Now as to the second question, system pressure is not temperature related. The system is pressurized by the water pump on the engine. Total system pressure will change based upon engine RPM.

quote Originally posted by lateFormula: Now as to the second question, system pressure is not temperature related. The system is pressurized by the water pump on the engine. Total system pressure will change based upon engine RPM.

I'm pretty sure system pressure is directly related to temperature. The water pump affects flow, not pressure - at least not much in a properly working cooling system. The water pump only affect pressure in a faulty system, where flow is restricted or when it's pushing out coolant faster than the system can handle (at prolonged high RPMs). The only way the pressure in the entire system can increase, is by temperature due to the expansion of the coolant.

quote Originally posted by Cliff Pennock: I'm pretty sure system pressure is directly related to temperature. The water pump affects flow, not pressure - at least not much in a properly working cooling system. The water pump only affect pressure in a faulty system, where flow is restricted or when it's pushing out coolant faster than the system can handle (at prolonged high RPMs). The only way the pressure in the entire system can increase, is by temperature due to the expansion of the coolant.

I agree with this completely.

As for where the leak went? That's a mystery. But I'm pretty sure it will be back. Probably at the most inconvenient time possible.
Good luck.

quote Originally posted by Cliff Pennock: But my question is: where did the leak go? The thermostat was most definitely open because I could feel the pressure (and hot coolant) at the hose after the thermostat, yet there was no leak to be found...

My concern, though probably unfounded, is that the leak is still there, but it's spitting air, or being used for expansion. Did you properly burp the cooling system? I assume you probably did... but just throwing that out there. I'd be concerned that there's a lot of air in the system, but if you know that you've resolved all of that, then you should be good to go. It's also possible that the leak was so minute, and was only visible when the car was overheating... and with it not overheating, it doesn't leak.

If the car is running and driving fine, and not blowing smoke, then I wouldn't be too concerned at this point. I would just keep an eye on it.

quote Originally posted by Cliff Pennock: Also at about which temperature does the system reach 16 PSI? Since I also notice that the coolant in the overflow bottle always stays at ambient temperature.

This is another reason why I'm concerned that you've got air in your system. Generally, as the car warms up, of course... the hot coolant will escape into the overflow tank, and it will fill up. If there's air in the system... the coolant still expands, and when it cools down, it sucks in some coolant leaving it emptier after it cools down.

When your car is cooled off, pop off the thermostat housing and check to make sure it's full. Otherwise... normal pressure should be reached at 195 degrees.

quote Originally posted by 82-T/A [At Work]: This is another reason why I'm concerned that you've got air in your system. Generally, as the car warms up, of course... the hot coolant will escape into the overflow tank, and it will fill up. If there's air in the system... the coolant still expands, and when it cools down, it sucks in some coolant leaving it emptier after it cools down. When your car is cooled off, pop off the thermostat housing and check to make sure it's full. Otherwise... normal pressure should be reached at 195 degrees.

Yeah that would make sense, theoretically if the leak leaked coolant as it heated up, then with less coolant in the system but open to the air, it sucked air into the system through the leak, leaving the system with an air bubble that then leaks air when it heats up, essentially acting in place of the expansion tank.

Very Likely Top "leak" will come back when dirt etc currently plugging it fails.

Can be iffy/bad lower intake or gasket(s) because coolant flows thru it a least 1 spot. Intake May block other coolant holes in the heads even if doesn't flow.

quote Originally posted by lateFormula: Now as to the second question, system pressure is not temperature related. The system is pressurized by the water pump on the engine. Total system pressure will change based upon engine RPM.

Wrong.

The Water Pump on a vehicle only pushes/pulls water thru the heater loop & the rad when T-stat is open.

System Pressure is Totally Thermal Generated & regulated by the Rad Cap on 90+% of vehicles. 15-16PSI Rad Cap make most of Boil Point way above 212°F/100°C w/ minor help from Ethanol Glycol as Coolant on most vehicle or "safe" Propylene Glycol on some others.

IOW Car Coolant system = Pressure Cooker for food. Rad Cap = Whatever Weight/Screw the Pressure Cooker uses to keep pressure w/o Blowing up the thing. "in old days" PC used a screw to set pressure. Many now use a Weight that Wobble at Working Pressure so people using them just have 1 pressure w/o think about it.

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[This message has been edited by theogre (edited 03-18-2024).]