The o-ring may look perfectly fine to your eye, it's the thermal cycling at higher temps when it's put under stress and allows coolant to slip past that rubber seal. And they are fairly cheap, the cap used to be under $10 but I think they've now crept up in price to the mid-teens. Stuart's car had this same issue about a year ago, the new cap fixed it and I've had the same issue on at least 3 different cars.
Bruce
21 GMC Sierra - mine
16 X5 M-Sport - wife
11 M3 - 1st son
04 Tundra - 2nd son
17 X1 M-Sport - daughter
Former (mo/yr sold):
15 335xi M-Sport (8/18)
11 335is (8/17)
11 X5 AW/blk (11/15)
04 330xi blk/gray (7/15)
06 ZHP SGM/blk (6/15)
08 335xi blue/blk (4/15)
04 ZHPci MB/blk (2/14)
04 ZHPcic IR/Alc (10/13)
01 540iT anthra/blk (7/13)
04 ZHPci TIAG/blk (9/12)
03 540i M-Sport (9/12)
01 325ci (3/12)
02 325iT 5MT (10/11)
05 ZHPci (7/11)
03 330cic (2/11)
00 323ci (10/10)
Matt/Matthew/Belvin
2003 ZHP Sedan, 6sp, Jet Black/Sand with Black Cube
*Amber Army*
Or if you want, get a shop to do a corner balance on the car
BAV Stage 1 - BlueBus - UltraGauge - V1 - Orion V4's - ARC-8's - Brembo 996
TTFS Engine/SMG Tune/CSL 255 - SGT Headers/Sect 1/SCZA - Evolve Eventuri
VIS XTS CF Hood - CF Lip/Console/Diffuser - OE CSL Trunk - BWS 500S 9k/11k
The ZHP has been tucked away in the garage for two days straight now, she's clean and staying inside, away from the contractors wreaking havoc on our house!
2011 E90 Lemans Blue/Bamboo Beige M3- Harrop Supercharged IG: lmb_zcp
2013 F25 Alpine White/Chestnut X3 35i- Family Hauler
2006 R53 British Racing Green/Panther Black Mini Cooper S FOR SALE! DM for details!
Past: 2010 X5M, 2014 328d, 2003 330i ZHP, 2004 330i ZHP, 2012 X3 35i
WTB!! 2009-2012 E91 328i Touring Contact me with info!
2005 IR ZHP Alcantara 6MT l Bimmian phone mount l Khoalty Angel Eye's l Khoalty Fog's l AFE Stage 1 Intake l AFE Dynamic Air Scoop's l CDV Delete
Besian Sealed VANOS l Flat black Kidney Grills l Magnaflow Cat-Back
Agree with above, but it still could be a hairline crack in tank or hose leakage. Start cheap.