Gio the only difference the cap does is make the "safety relief" of venting pressure if it builds up to 1.4 bar instead of 2 bar, it makes no difference on normal operation of the system.

Assuming your coolant is 50/50, not overfilled, and your system is running at normal operating temps, the 1.4 bar cap will behave exactly as your 2.0 bar cap, except if there is a problem in your system and the pressure builds it will vent at 1.4 bar instead of 2.

Normal operating of the system never gets above 1.0 or 1.1 bar, assuming what I said above is correct. If you have more water in the system than coolant, higher pressures can build. If you have overfilled expansion tank, higher pressures can build.

Personally I'd rather have the "safety relief" at 1.4 bar than 2.0 bar, so I will be putting a 1.4 bar on in the coming weeks. My expansion tank, hoses and T-stat have been replaced about 3 years ago so my system is in good shape.

Thanks Ricky , I'll go that route also. As you've read on some other thread , I replaced the bleeding screw (leaking) but I probably overfilled the tank when I topped it so the next day , the expansion tank now is leaking form a pin hole... I hope I did not damage anything else.

Would you have the part number of the cap you'll be using by any chance?
This one?
Home Page Search SiteSearch 17111742232 ES#37938 Expansion Tank Cap - 17111742232

Then put a gallon of premix coolant in the luggage compartment for any occasions when the system vents at 1.4 bar but wouldn't have vented at 2.0 bar as designed. And keep it topped up.

Or just use the correct cap.

Good point also but thats exactly the debate of this this thread. And thats exactly what i want to know from longer time users. Does it vent abnormally and need to be topped up often because of it?????

Just. Use. The. Correct. Cap.

I. Will. Sleep. On. Your.Words. Of. Wisdom. ;)

I'll be on the hunt for parts tomorrow morning (expansion tank) and will make sure to include a NEW cap.

An analogy would be that if you were worried that driving at very high speeds could wear out your cooling system sooner, you could install tires that had a lower speed rating, but cost exactly the same. They would be identical to the proper tires, but could not handle higher speeds. That way, if you ever actually drove too fast, the tires would fail before you hurt your cooling system. Yeah, that would make sense.

I don't know how often a lower pressure cap may release. But why would you ever want to find out? The vehicle would have boiled over, and be low on coolant. All that trouble to try and save a soft rad hose or cracked expansion tank that was about to fail anyway?

I understand a cooling system. But there is nothing wrong in questioning german engineering once in a while. Recalls are exactly that; admitting something is wrong and replaced by something that will help resolve a repetitive fail.
Ok , granted , there is no recall for the cap but sometimes the majority is right and german engineering is wrong. Not saying this is what is happening in this case and I will probably end up with the 2bar cap this morning.

Expansion tank failure???? What a farce!!!

I am using a 1.4 e30 cap for about a month now. Not worried about high RPM or anything leaking out. Everything is fine. All my hoses are new. New T-stat, expansion tank, all the sensors in coolant. Only thing I haven't changed is the water pump or radiator. I don't plan on changing them till I get over 100k miles.

First of all, the system hitting 2 bar while driving is just retarded, and you have more important things to worry about than a cap venting.

Second, the pressure spikes when you drive hard, then shut the car off. Not one person with has "vented" with an e39 running a 1.2 bar cap and posted about it. There were some, but they overfilled. Once they vented the excess, it doesn't vent any more.

I still have the same amount of coolant in the tank as I did when I put the e30 cap on. I keep it a bit below half plunger. Since I put it on, I checked it about every weekend before I move my car outside to work in the garage. Huge pressure relief sound when I open it up every time. PSSSSSSSSSST. I also have a tiled floor in the garage, so any "vent leak" would be very easily spotted. I mean I found my old drip leak that way too. Porcelain tile doesn't seem to absorb coolant very well, lol. Since I park in the garage after parking, that is when the pressure spike should happen, the fact that it is all dry, means the system will work just fine with a 1.4 cap.

My logic is a bit different. I think BMW is playing a cruel joke on us with the 2 bar cap. I also have this nagging sensation that is is made 2 bar solely for one reason, make us buy more parts. Especially when half the crap in the cooling system is plastic.

Again, chances of you going over 1.1 bar while driving is slim. There is a guy on e39 that ran with a gauge in line, AC, heat, over 100F outside, 1.1 is highest he went. I would rather vent at 1.4 bars when I shut the car off, than cause unnecessary pressure on all the plastic shit in the cooling system.

So I think all is good in the hood.

JCL, lets presume the pressure goes to 1.5 bar, while you are driving. Your cap isn't exactly going to open up and dump all the coolant out. It will open up to reduce pressure, but close once below 1.4. Yea you might lose some coolant, not all. But if you were running 2 bar cap, your neglected hose/gasket that you forgot about, will blow up, and will dump everything you have.

Venting will most likely happen during a heat soak after shutting it off hot. It won't likely happen while the vehicle is operating, because you have airflow, coolant circulation, and so on.

It is unlikely to happen in this weather in any case.

You really open the cap to check the cooling system every weekend? I can't remember opening my hood on any car that often, since a 76 Ford 400 cid V8 that needed valve guides in the early 80s.