What would cause a car to rev high when starting and immediately fall down?

[kristen_1819]

Video example here

My mk5 gti (stage 1 APPR) did this two times yesterday upon startup. The second time the battery light came on. Just put a new battery in it less than 6 months ago, but my mechanic did instal defoulers bc my O2sensors kept going out. Could it be the alternator or maybe a short circuit or weir wiring?

Thanks in advanced!

 

[ROH ECHT]

OK...obviously you aren't equipped with a diagnostic tool or VCDS.
The issue is likely caused by an air inlet leak, or it could be an engine seal leak. But could be due to something other than those.
You can only determine any of this by viewing some data; fuel trims, MAF report, fuel pressures, etc.. Or by performing a thorough smoke leak-detector test. This is of course necessary if a visual inspection fails to reveal a leak in the air inlet system.