That's not how it works. The pressure is there to increase the boiling point of the coolant. Modern engines are designed to run much hotter than the past for efficiency and emission purposes. Heat is energy, so the more heat that stays in the engine, the more energy is saved.

So as the coolant heats up, the pressure will build to whatever it needs to be to keep the boiling point up. If you do anything to lower the pressure below the boiling point, you will destroy your engine as the coolant turns to steam and your heads warp or causing the valve seats to drop out, or even worse, the pistons seize to the cylinder walls.

If you want to lower the pressure in the cooling system to almost zero psi without lowering the boiling point, do I what I do with all my cars now, which is switch to waterless coolant that has a much higher boiling point at zero psi than any water based coolant at even 30 psi. My cars run at close to zero psi, so I don't have blown plastic pipes or rubber hoses, and no blown water pump seals, and the reservoir floats don't get water logged and sink.

 

Lotusespritse: Correct: Pressure is needed to raise the boiling point. Since -90:s gasoline engines have been run well over normal water boiling point. When you have ~10liitre of coolant expanding from +20C temp to the +110C the expansion of water based coolant is quite high, so having 0.8 litre airpocket comparing 0.4 litre airpocket will give you suffient pressure to keep boiling point well in up to 125C cooling temp and still you will not reach high pressure state pushing seals and seams. So, you are wrong stating it would not work. Ofcource if you leave expansion tank totally empty, having airpocket somewhere 3 litre(?) then you don´t get enough pressure and your engine will be destroyed like you descripted, but i did not write that. I wrote "bit bellow max". By manufacturer the acceptable coolant level is between Low and Max. Right?

I never had issues with std coolant. I still have all original coolant components on my XJ, including water pump after 325t km (200t miles) Note: I have diesel, what runs cooler temp than gasoline engines.
I learnt all the tricks when i raced -> On racetrack only coolant allowed was water (obious reasons) and ohh boy; every hp was taken out from these poor engines and these runned on red line all the time.

Edit: Btw: Have you noted that you have heat expansion on waterless coolant as well? Its about 5-9% so your coolant level raises about 0.5 to 1cm on expansion tank from cold to running temp.

 

Paldi this is Sony2000. I placed my response into the wrong forum. I am in the XK forum but do some research in other forums. Sorry for the slipup!

 

A 50/50 mix of coolant at atmospheric pressure boils around 226 deg F. At 15 psi, it boils around 250 deg F. If the coolant needs to stay a liquid at 250 deg F (keep in mind the coolant is a lot hotter around the cylinders and heads than other parts of the cooling system), then the pressure in the entire cooling system needs to be 15 psi. Adding a slightly larger air pocket doesn't change the fact that the coolant needs to be at 15 psi in this case to keep from boiling. So you are not lowering the pressure in the system by adding a slightly larger air pocket.

Just the facts.

 

You have to understand how engine coolant sytem works. First af all: You need pressure to prevent pump blades making cavitation. If the cavitation "bubble" around blades get big enough on high rotating speeds you "drill" an hole on coolant and you are not moving anything. Second: The coolant dosent mysticly cool down on the way out of the head coolant output where the temp sensor usually are located, so what you get out from heads right to the engine temp sensor is pretty close what your coolant temp are inside cylinder head channels as long as there is constant flow. (Note: on XJ dash the engine temp is heavily tamped down. Accurate readings can be read from OBD2 port) Hottest area on heads are around exhaust ports. Around cylinder liners the coolant temp is much lower and much less coolant is guided on block than heads. Staying safe temp around exhaust ports where gas temp can be as high as 850C (1600F) an constant flow of coolant is required. Stop the flow and you are trouble no matter what you run around the system.
The coolant itself is not any hotter around exhaust ports, the effect is based on the coolant ability to constantly carrying energy (temp) away from the surface of the water channels, so when in proper operation the coolant fluid temp do not reach boiling point even on the hottest points of the heads. Running with zero pressure you might enter on situtation where your pump cavitates and an constant flow is distributed of cavitation bubbles are sent to the system. Once fluid start boiling on one place the chain reaction is rapid. Bubbles don´t have even near same ability to carry energy (heat) away from surfaces what cause more super heated surface areas boiling more coolant away etc.. The circle is ready. This is the reason why you just HAVE to stop your engine immediadly if you run out of coolant. (read exsample what happened in Fucushima)

Yes: You can easy try how much an airpocket size effect are yourself by squeesing an plastic soda bottle with 2/3 full or half full. Its just fysics. (i did not invent it, so claim Einstein )
Running engine with larger airpocket on expansion tank will effect the pressure of the system and you still reach pressure enough to be safe on Low level mark. Even somewhat under that, because manufacturers leave some marginal on desing. However: I don´t recommend to go under Low mark. Yes: These displacements are carefully calculated by engine desingers, thats why many "back yard tuners" have so much troubles with engine swaps) Of cource everything come out how you use your car. Pushing 95% of your driving time by full throttle have totally different effect for everything than using 2% full throtlle of your driving time. Exsample when highway cruising an exhaust gas temp is not even half of the full throtlle temp.

At this moment you have propably already find out that the waterless coolant have higher expansion rate that normal coolant (5-9% against 3-5%), so your claim that with waterless coolant you can run less pressure on system is not true. Well, you can change the free air cap if really want to run zero pressure, but: Like i wrote: You need pressure on the system to avoid cavitation. I don´t recommend to run coolant system without pressure.
I am not against waterless coolant. Its good that somebody is testing it and sharing experiense for everybody. Thank you for that.
It might help lot of problematic XJ/XF/F-Type etc gas engine owners. (diesel engines dosent seem to have similar issues with cooling system)

 

Which is why I don't use the ECO start/stop. I don't want high internal engine temps and no coolant flowing.

 

Exactly. You also lose oil pressure when the engine shuts off, so the Eco mode puts more wear on your engine, especially the already-fragile timing chain tensioners. I press the ECO disable button before I even put my seatbelt on nowadays.

 

Agree. Its very bad after fast driving on autobahn etc where you can keep up ~230kmh for an hour or so, and your exhaust manifolds are really red hot when you stop. Heat will transfer backwards very quickly to the heads. Only very latest and few manufacturer have added electric waterpumps to keep coolant flowing during Start-Stop. Not sure if ever any manufacturer made such on oil circulation?

 

This is common on heavy machinery applications, but I don't think any passenger car manufacturer has done so. The life cycle for a passenger car isn't long enough to justify the added cost and complexity.

 

Cavitation is a non-issue in these car cooling systems because the 50/50 coolant has a much lower vapor pressure than water, and the system is closed, which means any water pushed out of the pump pushes water back into the pump. So, the car will never have enough negative inlet pressure to cause the vapor pressure point to be reached with 50/50 coolant. Please watch the video at the end to see how much negative pressure it took to get plain water to cavitate in the pump. And please also note that while cavitation took place as the inlet pressure became more negative, the pump never stopped pumping in the demonstration.

With regards to your theory that the coolant is the same temp at the temp sensor as it is at the combustion chambers can be disproven by opening up the radiator cap while the coolant shows 190 deg F on the OBDII port readings and watch what happens. At 190 deg F, the coolant is around 30 degrees below the boiling point, yet there is far more pressure in the cooling system than thermal expansion of the coolant could ever cause. That excess pressure comes from the coolant around the combustion chamber being above the boiling point of the coolant. Please don't burn yourself doing this test!

And here's the huge point that will help you understand how an engine cooling system really works: I can drive our cars with waterless coolant for an hour in 100 deg F temperatures in Texas, pull into the driveway, pop the hood and remove the coolant reservoir cap immediately, and only a baby's breadth of pressure will come out. This real-world test disproves everything you said.

And also look up how waterless coolant has a much lower vapor pressure than 50/50 coolant, which is already much lower than water. Why is that important? You will definitely not get any cavitation running waterless coolant with its much lower vapor pressure, not that you will with coolant or even plain water.

Our 3.0L supercharged gas Range Rover Sport now has 150K miles and has been running Evans waterless coolant for the last 100K miles and 6 years with zero issues. I just got back the Blackstone report on it, and it came back with excellent wear metals and no coolant in the oil.

 

Lotusesprite: The video you sent is exsactly what happens when you run coolant without pressure. On video the low pressure point is made by valve. You got same low pressure points at the end tips of the coolant pump blades. The bubbles are bigger the faster pump rotates and when this cavitation bublles get big enough you have bored an hole on the water. (read why submarines can go faster in greater depths without cavitation - or read why commersial house heating system is pressurised) So the video prove i am right.
Its good if you got waterless coolant working. Great. However: The expansion rates etc i wrote are from manufacturer specksheets, so without myself testing it i can´t say if your experiense of ability running without pressure is true or not. At this point i believe your findings, even its against the given specks.(maybe your vehicle have an issue, what you fixed with waterless coolant?)
About coolant temp coming out from heads: Unfortunally i am right. The coolant coming out from heads are in fact pretty much accurate what are the temperature of metal on heads. You can even measure this with ease by IR temp meter. (well, not very easy on XJ engine bay, but basics are same regardess the vehicle/engine) Study bit more about engine physics as well: The combustion chamber/piston is not the hottest place, because incoming new air / fuel cools these somewhat down ei every intake cycle. Hottest area on heads are around exhaust ports. If you measure the temp from radiator cap, you have already somewhat bellow. Top hose and top part of radiator are hot and already past some enegry to the air around. You are wrong: The coolant pressure on heads are same than you have in radiator. There is no difference. (fluid physics)
I really recommend that YOU learn how engine coolant system works. (i have degree of vehicle engineering and been building succesfull race engines about 35 years)
In idle of motorwway cruising the waterflow is slowed down by thermostad because othervise the coolant flow is too great to engine reach its operating temp. (everybody knows what happens when thermostat is stuck open) On this scenario the circulation is slow and hot coolant is cooled by radiator very well to the outside air temp. Exhaust temps are around +350C (+660F) Only maybe 35% of coolant capasity is so called hot. Only top part of radiator is hot. The engine oil and trasmission oil heat exhangers dont swap too much heat since both oils are also cooled on oilpans by airflow. On supercharger the inlet pressure are low and not much heat is generated and transferred to the coolant. -> Coolant system pressure is low because there are not much hot coolant in circuitry = not much fluid expansion by heat. You follow?

Now: Lets take some power out from the engine: We put vehicle on rolling road (dyno) or some Autobahn (or race track run) for lets say 30min with 75%-100% of full gas. (most commersial vehicles never seen this kind of usage) Hand up any forum member ever done this with their vehicle?
Now the thermostat is fully open. Exhaust temp is constant somewhere +860C (+1200F). Great engine roar fill the air and big smile on drivers face. Coolant do not boil on heads because fast coolant flow and (rising) pressure of the system. The coolant flow transfer heat rapidly away from heads. Main radiator are complete hot down to the bottom and propably about +20...+30C degree (+70F...+85F) warmer than outside air coolant are coming out from bottom hose of radiator to the engine/heat sources. This means the radiator can´t keep up, but in most cases its still Ok, because cooled fluid is still much colder than the heat sources. The transmission and engine oil temp rises and oil to coolant heat exhangers are very hot too.
By my experiense almost any std commersial vehicle enter near so called heatsoak state where 90% of coolant running the circulation is so called hot. (hot i mean above +55C / +131F) Including supercars, like Ferraris, Lambos, etc.. Note: Exsample Veyron have 10 radiators and have thermal ability to run max 15min with full gas. Gasoline tank will last 12min, so its Ok.

Now issues:
Most problematic event happens on supercharged engines when air to coolant interchangers can´t keep up because the incoming coolant is already hot and hot intake air enter on engine. This will rise the exhaust temp to the point where ECU will start to turn power down to prevent totally meltdown. (and naturaly hot air have less oxygen) On turbocharger engines this is not the case, because most use air to air intercoolers, but turbochargers get very hot by exhaust gasses and because turbos are lubricated by engine oil, the engine oil get very hot and need to be regulated by the ECU too. If too high oil temp is reached the power output will be regulated as well by ECU to prevent catastrofic collapse of engine oil pressure.
Needless to say that the pressure differense on these two scenarios when only 30% of coolant is hot (expanded) or 90% of coolant of is hot is really different. You agree?
I have been running engines up to the destructions in test bench for years to test different combinations of radiators, waterpump blade speeds, types and shapes, hose-/portworks etc fo find most economical combination of weight, powerloss and cooling ability to take max out of the engine without blowing it up on track. Have you?

I am not out of fight. More helping fellows run their engines without issues. I think you are up to that too?
Like i wrote: Its good that you test waterless coolant and write experiense out of it.
However: Thre are some facts on engine physics what are good to know and understand too.
On everyday usage the engine should be running without any issues with normal coolant (you used term 50/50, what is not nessesary the actual mix rate in every coolant, but near enough that everybody understand) even with time to time full throtlle events in everyday traffic. (with these poweroutputs they last 5-30sek instead of minutes)
Unfortunally x351 gasoline engines suffer weak parts, plastic piping and waterpump. These all are under pressure, either by heat expansion, or water flow itself and we all know what happens if and when they fail.
My recomeditation are that change to metal piping, keep coolant level on low side (at least don´t overfill) and change the coolant in 3 years period. (the coolant have also addictives helping corrosion etc)

 

I am sorry, everything you said is nonsense. I am not going to continue to try to argue with something that is so convinced they are right when they are completely wrong. Have a good day.

 

Have good day for you sir as well.
Maybe one day you understand more about vehicle cooling. In fact any liquid thermal exhange system.
(i have learnt basics by study / school, so the info is not from my head)

 

Coolant is leaking 6 months after the tank was replaced! I had to top it off to the full line again this morning. 2nd time in a week. It's apparently from a hose on the front driver's side *(right side looking from front of the car) not a "gusher".