[burrcat1]

Thanks to all who have helped with all my DIY's in the past. I've read all posts, I think, with regards to this problem. Any advice for the next step would be appreciated.
2010 328xi E90 N52
RPM fluctuates between 600 and 1400 only with brakes applied. With no brakes RPM returns to normal - about 600. There is an engine code 2D15 air mass sensor, measurement.
Items accomplished as general maintenance; valve cover gasket, valvetronic gasket, intake manifold gasket, crankcase vent hose and two smoke tests to confirm no vacuum leak (although I could not find the PSI needed to do the test - I did it at 6 psi)
Items accomplished in trying to correct this problem and engine code 2D15; brake booster vacuum pipe with check valve and washer, MAP sensor and mass air flow sensor.

Thanks to all - please advise as to what should be done next.

[marvinstockman]

Check vacuum level when you apply the brakes.

[burrcat1]

Thanks marvinstockman
I will do the vacuum test today and let you know the results. My real job gets in the way of fixing this car!!
So far the brake pedal movements appear normal, no hard brake, prior to start and after start normal brake movement etc.

[burrcat1]

Vacuum reads 25 PSI, when brakes are released vacuum drops to 20 PSI and readily goes back to 25 PSI. If I rapidly pump the brakes I can get the PSI down to 5.
Not sure what this is telling me but it seems normal.

[Andystobbs]

How are you getting a reading that high? 1 ATM is 14.7psi absolute pressure. Sounds like a brake servo (booster) issue to me. MAFM getting confused by unmetered air entering via vacuum leak.

[burrcat1]

Sorry about that - not PSI it 25 in Hg

[burrcat1]

That would be about 12.28 PSI

[lowrydr310]

Originally Posted by Andystobbs

How are you getting a reading that high? 1 ATM is 14.7psi absolute pressure. Sounds like a brake servo (booster) issue to me. MAFM getting confused by unmetered air entering via vacuum leak.

I'm confused by all this talk of vacuum leak and unmetered air and brake issues. E90 has no (rather little to insignificant) manifold vacuum because of valveteonic (throttle plate normally open - just used as a backup). There's a standalone timing-driven vacuum pump that is used for the brake booster.

[Andystobbs]

Is that the case on all US market engines?

[burrcat1]

lowrydr310 I'm confused with the vacuum issue too. It is a stand alone vacuum pump. My knowledge extends from having to replace the vacuum pipe, with check valve, from the brake booster. The gasket in the brake booster was replaced too. I was hoping this would not only correct the hard brake issue, which it did, but the rise in RPM only when starting a cold engine and only with the brakes applied. After approximately a minute or two the car returns to normal RPM and runs great. If I keep the brakes applied during this warm up the car fluctuates from 600-1400 RPM. I have not been able to fix the problem - it's frustrating.

[Andystobbs]

Ok I had thought maybe the 328 did it differently, an engine which we don't see in the UK. However it is still valid because the vacuum pump exhausts the air which it scavenges in creating a vacuum into the engine. So if it is constantly drawing air its basically the same as pulling the dipstick out at idle.

[burrcat1]

Thanks Andystobbs. Looks like I'll have to replace the brake booster, something I did not want to do.

[Andystobbs]

Do you not have any fuel trim fault codes?

[burrcat1]

Only fault is 2D15 Air mass sensor, measurement range

[Andystobbs]

Bit odd, would expect something to be going lean. How long does it take for the ECU to re flag the AFM code after a delete?

[smass]

I replaced an N52 starter today and forgot to plug the air mass sensor back in. When I pressed the brakes, the engine RPM went up and down. Once the air mass sensor was plugged back in, the RPM issue was gone. So your code may be related to the issue.

[burrcat1]

After deleting the code it comes back after the first start. The air mass sensor has been replaced with OEM and taken it out a few times to double check.