Well yes if you refer to the fact that depending on the gaseous space for expansion above the actual coolant heat expansion minimum (liquid which is incompressible) the pressure change is faster compared to temp rise if the excess gaseous volume (everything above the space needed for liquid heat expansion) is small.

Nothing about a lower pressure cap changes the required fill volume.

Once again - the pressure in normal operation is not determined by the cap. The pressure in the system at normal operating temperatures is around 1.0 bar or below.

Decreasing the amount of coolant in the system does decrease the pressure, but also provides less cooling ability due to a lower volume of coolant.

The correct fill level for any cap is the factory fill level.

It has nothing to do with required fill volume, lower pressure means more possible expansion. That volume difference in the expansion tank has nothing to do with cooling other than the effect it might have on pressure and that ends up regulating itself. Thinking that the system doesn't "normally" go all the way to cap pressure (and that is also dependant on space for gas (fill level) as Clavorion pointed out) is not the right way to look at it.
That's my last post in this thread. ;)

It is in fact the only way to look at it, because that is the fact.

You can run a lower volume of coolant to lower the pressure in the cooling system but this works the same regardless of if it is a 1.2 bar cap, a 2.0 bar cap or a 300 bar cap because the system is normally operating around 1.0 bar.

There is absolutely zero reason to mandate a lower fill level with a lower pressure cap.

In fact, if you lower the fill level with a lower pressure cap enough, it would provide venting from the cap at the same temperatures that a 2.0 bar cap vents at.

And if that's the case, the only difference in an overheat condition between the two caps is that the lower pressure cap you have less coolant for absorbing and dissipating heat.

The coolant level in the ET will not affect the cooling system's capacity to absorb heat in any meaningful way. The exact same volumes reside in the engine and radiator where all heating/cooling happens. The only place the volume is different is in the ET where there is essentially no heating/cooling happening.

The operating pressure WILL be impacted by the coolant level as a higher coolant level will result in a faster ramp-up in pressure as the system heats. For this reason, I fill my systems to the "min" level (float stick level with the top of the ET on E53), though I've read of others advocating even lower levels.

As you have correctly pointed out, the system when operating properly is running at about 1.0 bar pressure and the cap release point will have zero effect. My argument is that there is essentially no scenario where an overheating engine will climb up to between 1.4 and 2.0 bar, but then return back to normal operating temperature. If such a situation was more common, then an argument could be made to stay with the 2.0 bar cap. Realistically, once you hit 1.4 bar, you're only delaying the inevitable by continuing to hold pressure to 2.0 bar.

Regarding your post about your system holding 2.0 bar, what that doesn't take into account is the difference between a cool system vs. a very hot system. Empirically speaking, countless people have experienced cracked ET's (including me). I agree with your statement that many of these were probably overfilled, but I know mine was not. In fact, mine did not even happen in an overheat situation - just normal operation of my E46.

AM.

It is true that that the temperature in a cooling system is not equal across the entire system, nor is the pressure.

It is not true though that the coolant in the expansion tank just sits there and does not affect cooling ability of the engine. The system wants to reach a homogenous temperature and pressure and as a result of this the coolant in the expansion tank will be displaced by hot coolant, allowing the coolant from the expansion tank to mix into the system. The coolant in the expansion tank is displaced and does circulate into the system. The total volume of coolant in the system does play a large factor in the system's ability to cool properly because it determines the thermal mass in the system, which is the ability to absorb - and dissipate - heat.

The X5 has a pretty large cooling system volume so is not as susceptible to this as some other models, but on other BMW's with smaller cooling systems, running them low can and does cause overheating.

Yes, a lower coolant fill level means lower pressure. It is safer to run a little below, rather than a little above - but the correct fill level is the stock fill level.

Lowering the pressure lowers the boiling point. Lower pressure is not necessarily a good thing.

There being no scenario where it gets hot then cools down doesn't change the downsides of a low pressure cap during an overheat scenario.


Someone in the E46 world logged pressure at the cap at various temperatures and they hit 1.5 bar at about 111/112C. Which is by no means "nuclear" for an M54. I've seen them up to 120C without any damage. BMW considers it to be within normal operating temperature up to 110C.


At the mild overheat temperature that causes the system to hit 1.5 bar:

With a 2.0 bar cap, you keep coolant in the system. It isn't boiling. It isn't venting. It just keeps cooling as it was designed to do.

With a 1.4 bar cap, you'll be venting coolant. Meaning during a very minor overheat, you are:
1. Lowering the boiling point - allowing for steam pockets in the head. Less coolant contacts the head because of the steam pockets which increases the temperature at the head
2. Venting coolant - which means less thermal mass to absorb and dissipate heat. And venting coolant also decreases the pressure in the system which further lowers the boiling point (see point 1)

A 1.4 bar cap will hasten an overheat condition.

A 2.0 bar cap will allow you a larger buffer zone because the coolant is prevented from boiling so can properly cool the head and will overheat at a slower rate than a 1.4 bar cap because it is keeping coolant in the system to allow as much thermal mass as possible to dissipate heat rather than venting it onto the road.

A 2.0 bar cap gives you more time between "beginning of the overheat" and "engine is ruined"

2.0 bar is 2.0 bar, it doesn't matter whether that pressure comes from a pressure tester or from the heat from expansion of the coolant. If your logic is changing from "pressure breaks expansion tanks" to "heat breaks expansion tanks" then I don't know what to say to that, but it is not true. The thermoplastics used in these cooling systems are not the same as in your kitchen tupperware set that will melt if set near an oven.


I've busted expansion tanks too. I've busted radiators as well. Parts fail, it's normal. Saying "well it failed because the cooling pressure was too high so I'm going to install a lower pressure cap" is faulty logic - because, once again, a lower pressure cap does not change anything about the pressure during normal operating temperature.
All a lower temperature pressure cap does is compromise the integrity of the cooling system when you need it most, when overheating - while simultaneously offering no advantages during any other conditions.



The most important thing during an overheat condition is to maintain, for as long as you can, as much cooling efficiency as you can. Venting coolant contradicts that goal.

The cars that have 1.4 bar cooling caps have different cooling systems. Cars with 1.4 bar caps will be at the "dangerous" temperature at 1.4 bar. Arbitrarily installing a lower pressure cap is not an improvement in any way.

Thousands of intelligent engineers at BMW design these cooling systems with thousands and thousands of hours in on and off the road R&D. They knew what they were doing.

Recently replaced my coolant pump with one with a metal impeller. I expect the housing go crack or the bearing/seal to leak before the impeller ever comes apart.

https://uploads.tapatalk-cdn.com/202...28a892aa17.jpg

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The spot welding process that joins the impeller to the vanes destroys the "stainless" quality of the stainless steel impeller pump due to the heat from the welding process. Those pumps can rust, then separate and fall apart as a result. Change the coolant frequently enough to keep fresh corrosion inhibitors in the system if you want to run the metal pump.

The best water pump based on my experience is the modern composite impeller water pumps. I have personally put over 100,000 miles on these pumps, and customer cars have mine have cumulatively put at least a million miles on them. I have removed and replaced several rusted out metal pumps. All modern BMW's that use mechanical water pumps use composite impellers.

I change my coolant regularly.