Sarge

I have a 2008 535xiT, and I just finished a 4,500 mile road trip from Wyoming to MI/OH/IL and back. From time to time I experienced surging acceleration at full throttle at higher speeds. It would sometimes occur on kickdown to a lower gear, other times in 6th gear (in manual mode), but always under WOT or near WOT.Speeds in the 60-100 range.

This car has almost 90k on it now, but had the HPFP replaced last November at 72k, and the turbos replaced last September at 66k (due to wastegate failure). It has always been completely stock.

It starts just fine (no long crank) and generally accelerates as it should, but the surging comes and goes. It was rather harsh on one occasion, and more subtle other times.

Any ideas? If not HPFP or turbos, then vac line? I'm not too familiar with this motor, but I have CPO to 100k, so I'd like to make sure anything brewing gets addressed beforehand... Thanks for any input.

- Sarge

Persian_535i

Vac line or diverter valve maybe anyway to post sound clip and do you know if you are losing boost?

epilithic

What do you mean by surging on acceleration? Somewhat like hesitation or lack of full acceleration pressure?

Sarge

Going to wide open throttle, without hitting the final throttle detent (forces gear kickdown even in manual mode), the engine delivers 'surging' power, i.e. surging between 80% to100% power, on/off as you accelerate from say 70-100 mph. It feels like the boost is pulsing/stumbling. Generally it's a duller pulse, though one time is was much harsher/noticeable.

Temps were in the 80's and the car was fully warmed up (i.e. hours of driving, not a 'cold start'.)

Sarge

Yes, I was thinking wastegate or vac line, etc, as it seems to be dumping boost when it happens, producing surging/pulsing acceleration, where the pulses are faster than could be accounted for by spooling turbos.

Persian_535i

Pretty simple way to test if it is the vacuum line. Make sure you have a fire extinguisher if you try it just incase!

Sarge

Thanks - I learned something new!

D33ZNUT5

Thanks for the info! I am having the same symptoms in my 08 X3, hope it's just a vacuum leak