You can buy the Stant cap from RA for a $2.83. It's the same Reutter branded cap as the genuine BMW cap. https://www.rockauto.com/en/moreinfo...&pt=2067&jsn=1

AM.

I have not tried it but I will. Use a long screw driver or wood dowel, push the level indicator all the way down. The red rod magnet should turn on the light when it reaches the bottom. Bleed until coolant comes out the screw level. The tank area is expansion reserved.

Mine had the coolant system replaced. I could go back to the OEM 2.0 bar cap without fear a hose or connector would fail.

I'll be ordering the 1.4 cap too. Thanks for the info AM. :bmw:
I can test how much it burps over the summer.

Lowering the expansion tank cap pressure is one of the worst and most dangerous things you can do for a BMW.

The cap does not set the system pressure, the fill level sets the system pressure. At normal operating temperatures the engine in these cars is operating around 1.0 bar or a little below, some E53's run even a little lower than that. Running a 1.2, a 2.0 or 30 bar cap doesn't change this - it still operates at the same pressure during normal operating temperatures.

A higher pressure cap raises the boiling point of the coolant. So if you are ever in a condition where the engine is overheating, a higher pressure cap will prevent boiling. And boiling always happens first at the hottest point, the engine head, which creates air pockets that prevent coolant from contacting and cooling the head when you need cooling the most.

There are exactly zero benefits to a low pressure cap and you are dramatically lowering the safety buffer built into the cooling system by lowering the boiling point.

If your cap is "burping" you have a malfunctioning cap or have significantly overfilled the cooling system.

I know the guy personally who ran that in an E39, if you mean cal45fan. He was having overheating issues and ended up removing the NPG and reverted back to normal coolant. He lives about 10 minutes from me.

Like you said if the cooling system is in good condition you should not change the expansion cap to lower pressure rating on a petrol engine which has map controlled thermostat and normal running temperature can be way above normal pressure boiling point. The lower pressure cap is a cheap fuse if you have doubts about the cooling system components but this comes with a risk of even more severe damage due to local boiling like you suggested. If you use a lower pressure cap you should mod the thermostat to be constantly on a lower opening temp. On V8 engines this can be easily done by installing the diesel engine thermostat (88 C) instead of the original map controlled (mechanical opening temp 103 C without aid of the heating element).

BMW's prior to and since the E46/E53/E83/etc. era run 1.4 bar ET caps - many using the same engines. The 2.0 bar cap does not change the boiling point of the coolant - it raises the pressure at which the cap will begin to release. I've seen no good explanation for why BMW temporarily spec'd the higher pressure cap, but my guess is that it was an attempt to be more environmentally friendly by reducing the expulsion of coolant on the ground. In reality, they made it worse due to the proliferation of exploding ET tanks common to this era, resulting in spewing large amounts of coolant all over the ground. Again, just my guess, but I suspect this is why they reverted back to the 1.4 bar cap.

Installing a lower pressure cap can help release pressure before reaching the structural limits of other cooling system components (often the ET itself). This can help reduce the chance of being stranded on the road.

Many of us on various BMW forums have been running the lower pressure caps for many years/miles with no issues other than intact ET's. In fact, not too long ago, my E53 went through several overheat cycles due to a defective water pump. I had to nurse the vehicle home by repeatedly driving it a couple miles until the needle started to climb, then shutting if off to let it cool for 30-45 minutes. The 1.4 bar cap protected the rest of the cooling system and allowed me to get the vehicle to my house where I eventually figured out that the impellor on the relatively new water pump had disintegrated resulting in no circulation of coolant.

AM.

Higher pressure increases the boiling point AND heat transfer, so all good except that the E53 has a weak spot with the coolant tank design that can rupture as pointed out.

Saying "other BMWs run a lower pressure cap" is irrelevant cause they have different cooling system volumes and therefore different normal operating pressures and a different delta between normal operating pressure and the rated cap pressure

It is also not true that BMW went back to 1.4 bar caps on all models. There are cars after the E53 era using 2.0 bar caps and there are cars in BMW showrooms right now with 2.0 bar caps.

These are sealed cooling systems. The boiling point is directly related to that pressure. Higher pressure means higher boiling point. The boiling point of water at 1.2 bar is 105C. The boiling point of water at 2.0 bar is 120C.

Of course there are many cars with caps with lower pressure than factory running around without issue... It doesn't change anything in normal operation to run a lower pressure cap. The system pressure is exactly the same in normal operating conditions, ~1.0 bar. A lower pressure cap does not change the operating pressure of the system because the cap doesn't set the system pressure.

All a lower pressure cap does is remove a critical safety buffer in the event of overheating. Overheats will happen faster with a lower pressure cap because the coolant will boil and steam is a worse conductor of heat than liquid

But, once again, the cap doesn't set the cooling system pressure. This seems to be the critical piece that people dont understand. These are not old American cars with overflow tanks where the radiator cap sets the system pressure. These are fully sealed cooling systems with expansion tanks not overflow tanks, the only variable of the system pressure you can change is the fill level. Lower fill level = lower pressure and vice versa.

In almost all cases of premature expansion tank failure I saw when I ran my shop, the owner overfilled the system because they expected it to vent like an a car with an overflow tank style system. It doesn't work like that.

I have an E39 with 140k+ miles on 16 year old cooling system components including expansion tank. I'm using it to the theory that BMW expansion tanks are unreliable and prone to explode (and that water pumps are unreliable, that hoses explode, etc). I have the fill level exactly at the mark indicated on the expansion tank. I drive that car A/C blasting in the summer in the hottest, driest climate (the most taxing situation for cooling systems) in the US. Knock on wood but I expect it'll last many more years. I truly believe BMW's get a bad rap for their cooling systems because people don't understand them, not because they are actually bad

It says a lot that in other automotive communities, cars that come with lower bar caps - HIGHER pressure caps are treated as an upgrade. A lot of automotive parts companies market per the "grass is always greener" mantra. Whatever is different is better. There are all sorts of parts like this. They will use marketing to justify whatever they are trying to sell even when it contradicts objective facts.

Brass bleed screws are another example that comes to mind. Are you sick of stripping a $2 bleed screws? Buy a $20 brass bleed screw so you can strip the threads on a $80 expansion tank instead!