CTT hesitation and rough acceleration on part throttle

[lewisweller1982]

Hi Guys, back again for more punishment..........

Quick info and background 2006 CTT with E81 power kit from new, 111,000KM, just had head gasket failure and full strip down and repair with a load of new parts, all the plugs, Coils, filters, oils coolant thermostat,  temp sensor, purge valve, vent pipes, etc etc all replaced. 

Got the car back and its still runs like a bag of Sh*t.

 

Idle is good rests at 500rpm approx. and stable, with wide open throttle high rpm and it goes pretty good, but part throttle low rpm is dog rough lumpy, sluggish and pants, is this normal for the V8 TT? Surely not!

 

No fault codes, I use 99 octane fuel, put some STP fuel injector cleaner though it a few tanks no change, compression test was good even before the heads were done and boost pressure reads 0.6-0.7 ish.

No vac leaks or boost leaks I can detect.   

 

I did discover my Secondary air pump was clogged with soot hence the check valve was stuck open and basically has destroyed the SAI pump, so I blanked this off because its a cold start emission device for the cat only. Made no difference in the rough running though before or after, just the pulsing noise is gone as no exhaust leak effectively now.

Is it possible the SAI pump impellor could clog the Cats??

 

Really running out of ideas. HELP ME PLEASE

[lewisweller1982]

BINGO SOLVED IT>

 

 I'm back and after a lot of head scratching and no fault codes PIWIS or Durametrics and no help from the "German car specialist garage" who said it was normal to have a Cayenne turbo drive like a total pig I found the answer!!!

 

The car would hesitate and felt sluggish at low rpm, and when you accelerated and let off the braking was terrible (when I slipped to neutral the braking was good so not a brake vacuum servo leak), the engine is fighting the brakes which in fact got me thinking on the correct thought pattern to find the problem.  

The only reason the engine would fight the car is if the boost built up wasn't able to divert around the turbo, the Diverter valve obviously does this function and after some reading up the Bosch ones can be suspect. so I took the plunge and bought two new from Porsche Bosch 710 P revision.  

Whilst fixing the steering rack leak I whipped off the passenger wheel arch cover and swapped the DV, but when I checked it was already 710 P revision and actually did function when test manually, so I then started to think about the vacuum piping to the DV. I blow down the vacuum pipe and heard a leak in fact 3!! 

the vacuum pipe to the driver side was melted and leaking at the nylon "T" located under the Throttle Body inlet area, chopped that bit off and reconnected, tried again and still could hear leaking, traced it the rigid vacuum pipe which runs along the left bank injectors area to the Change over valve.

As you can see from the pictures its melted in two places.

What throw me off is I didn't understand the function of the DV and the Change over valve exactly, yes the DV will open when throttle closes to prevent turbo stall and surging which damaged the turbo and gives turbo "lag", but also the change over valve is boost pressure when accelerating and vacuum when off throttle. Most technical data I read didn't disclose this detail.

 

So my understanding now is:

On throttle the changeover valve is positive pressure open to the DV to assist in holding the boost preventing the DV brass spring from compressing then lift off the change over valve will switch to crankcase vacuum to pull the DV open to by pass.

This explained why my CTT would have crap low rpm acceleration and boost with hesitation and kangaroo de-acceleration and engine fighting the brakes to stop as the boost had nowhere to escape to.

 

Maybe someone can explain this sequence more accurately or correct me on this info, I would welcome it.

 

 

 

  

[ekstroemtj]

I am glad you found the problem. Are you talking about this parts?

Found it in onother forum:

http://www.6speedonline.com/forums/cayenne-955-957/18559-hyperboost-hx-diverter-valves-installed.html

[lewisweller]

Yes DB Diverter valves, the one mentioned in the link are aftermarket but essentially the same.

[ekstroemtj]

May i ask you how you fixed this vacuum hoses? You cut the damaged parts and how u connected the 2 ends?

[lewisweller1982]

ekstroemtj

 

The pipe is available at my local Hydraulic/Pneumatic supplier for like $3, it was 6mm OD and 1-2mm thickness so 4-5mm ID. Polypropylene I think, but don't quote me. I used heat shrink tubing to sheath the pipe where it touches or runs near the engine so like the whole length really, I reused the clips that I popped of the old pipe.

To gain access remove the Secondary Air Pump and the coil packs silver colour plastic cover T30 torxs and then unplug all 4 injectors, then slid the change over valve at the rear of the engine of it little bracket. this allows you to unclip the clamp and disconnect the pipe. the other end is connected to the Nylon "T". If you get that far then replacement is straight forward reversal.