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.

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