Z4 E89 30i AUTO issue

[LordLolzeye]

So, summer has arrived, and my car came back with it's "life-long" problem.

I've owned this car for 6 years now and this issue always bothered me, with no "viable" solution offered by BMW.

Each summer, when temps. go above 20 degrees, if I put the car in sport mode, AC ON ( >= 3rd speed setting on the fan),
the transmission is in 2nd or 3rd speed and I press the brakes all the way down to 0 kmph, the car will shake and then stop the engine.

Any changes to what I've said above fix the issue (example: I put the car in drive, on on 2nd speed fan setting, etc.).

I've been to the stealer and they've kindly suggested me to change the transmission and if that doesn't fix the issue, think about a new engine . I've also been to two independent BMW mechanics, both had no idea about a fix for this issue and couldn't even replicate it, even though I've given all the information on how to replicate it.

Some history from the car:
- The car is very well taken care of, in the last 2 years it got a full new cooling system (hoses, pump, thermostat, radiator, expansion tank), full new fuel system (even the fuel lines got replaced), new brake lines, new injectors, new coils, new spark plugs, has it's engine oil being changed at 5k km or 1 year, has it's transmission oil changed each 4 years.
- The car is on it's 3rd brake booster cylinder (it happened to me once and I've found out it happened twice in the past to the other owners too - the brakes get extremely stiff and then the booster cylinder blows up - funnily enough it hasn't happened in last 30k miles)

Anybody has any idea on what I should check? I'm open to any suggestion

[B21]

Have you put a code reader and /or data logged it to look at key variables to see which critters are changing ?

[javis20]

AS B21 say, you really need a Diagnostic tool to monitor what's going on. After all the money you've thrown at this car, as scanner would be a minor investment.

Anyway, it seems like your torque converter is staying locked up. What this has to do with the A/C fan setting is a puzzle. I have the exact same car. I will try doing what you described later today when the temperature rises above 20 C.

[javis20]

Just checked, mine does not exhibit this problem.

I would use a diagnostic scanner to monitor your vacuum when all this happens. It's not common to replace brake boosters, let alone 3. Be suspicious of the booster and any vacuum leaks.

[LordLolzeye]

Thanks jarvis for the test, also I have BMW’s diag software, no errors.

[matsmith749]

I think you need to datalog it when the fault actually occurs - then you can see what's happening at the point the engine cuts out.

Not codes per-se - it's the management data you need to start figuring this out.

Either that, or stop messing with the air con

It's an e89. Above 20 degrees you should have the roof open!

[LordLolzeye]

Sorry for the late reply
I will try that, nothing to lose, if nothing pops up, im just going to survive without the AC till I find a competent mechanic lol