Staffan

I replaced the stock Bosch CBV with a unit from Forge yesterday. What a difference!
Higher boost between shifts and better low rpm performance.

I would recommend any 964 turbo owner to check their CBV. More info troubleshooting here: <a href="http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic&f=25&t=000080&p=" target="_blank">Hows your CBV (compressor bypass valve) </a>

My 2 cents to troubleshooting is to put the car in 3rd gear, accelerate from 3.000 rpm, at 4.000 rpm release the throttle, you should now here a "psst" sound. If not the CBV is very likely gone.

A question, when I first bought the car there was this "jerk" when releasing the throttle, which I understand is "normal". Over time this behaviour have dissapered. Now, after replacing the CBV the "jerk" is back and it is so violent that I "have to" put the clutch down when releasing the throttle to balance the car (just like it was when I bought the car). I am starting to think that this isn't normal.

Can I conclude anything from that this behaviour was gone while I had a failed CBV ?

To all 964 turbo owners, does your car get "really out of balance" when releasing the throttle at lets say 5.000 rpm ?

Thanks in advance
Staffan

SonnyV

Staffan,

I used to have the same problem as you described when let off the throttle after a hard acceleration at high rpm. As I understand that this behavior is normal.

When I had my car's major service done last Oct, I had the deceleration valve relocated from the front (standing at the rear looking toward the front of the car and it is closer to you) to the rear of the engine( near the throttle body and toward the firewall) and this behavior is now a lot less noticeable and smoother. I still have the original CBV on my car.

Hope this helps,

Sonny

Kevin Ross

Staffan

I can report similar behaviour. Mine too has become less noticable. I am going to get it replaced at the next service.

Staffan

Sonny, do you know how this "deceleration valve" works ?

Is it located just in front of the throttle body, or did your mechanic move it their. I guess I should ask, where is it located on a stock car ?

SonnyV

Staffan,

The stock deceleration valve is a large (about 4") round copper in color, that looks like a round disk. In stock form it is under the intercooler on the left side a little bit and nearer to you, if you stand at the back of the car looking to the front of the car.

My mechanic, Lee Rice, recommended that I should move this valve to a new location, near the throttle body, and further into the engine and to the right.

I am not sure of its function, but it helps with the deceleration and allows better recovery (faster boost) when you are back on the gas.

Hope this helps,

Sonny

Staffan

Thanks Sonny, that must be that "thing" that is in the way when trying undo one of the hard to reach bolts to the intercooler.

I took the car for another spin today, w/o thinking about it, I am smoother when releasing the throttle so the "jerk" is less violent know. But I will talk to my mechanic about this valve when doing the 80.000 km service.

Thanks again
Staffan