I'll try to keep this short, but I'm really at my wits end on this. Any help from all of you would be greatly appreciated.

I have an issue with something that causes an odd noise and a blip on the throttle under certain circumstances. It does this all the time, every day, since I got the car back in October. The car was stock then. I have since added a FabSpeed Gen 3 Moderate exhaust and new plunger style DVs. These upgrades have not affected this issue at all. The intake is still all stock.

It sounds like this; under normal acceleration (no boost showing on gauge) typically in third or 4th gear, say 2500-3000 RPM, once you slowly back off the throttle to maintain speed, I get this change in the resonance of the intake/exhaust that kind of sounds like a butterfly valve slowly closing. It takes exactly 3 seconds and when it closes, it drops the RPM by about 50. It goes from a louder, buzzing noise (almost drowns out the exhaust) to a more muted sound. I can replicate this every day, but it does not seem to happen when the engine is stone cold. It also appears to be more pronounced when the ambient air temp is warmer, say above 40 degrees, vs. when it’s colder.

It is very annoying and falls under the category of drivability I guess, but my tuner and a reputable dealer have spent time attempting to diagnose it and come up empty. We put it on the dealer’s engine analyzer and came up with no indication of any problem anywhere. No CEL either of course.

The dealer mechanic, who works on their 911s and has for years, suspected it might be a valve in the intake plenum T that is worn and is either closing slowly (or too quickly ) He rode with me so I could duplicate the problem and it's very noticeable to a trained ear.

Now, that said, after researching a little further here, I actually do not see where there is a valve in the intake T. I'm basing this on the nice photos of the new "Big Mouth Intake Plenum" offered by Vivid. Am I wrong on this? If there is no valve there, we’re back to square one. We both agreed it was probably not the throttle body, because that would be a much more pronounced change in RPM and would affect drivability considerably.

What say ye? What else could this be? Wastegate sticking? Doubtful right?

Are there other valves in the intake I’m not aware of? This is driving me nuts!

Thanks!!

 

Sounds difficult to diagnose especially over the net....I would at least clean the TB. There is no valve in the TB TMK, but there is an intake distributor with a flap. Wastegates operate with boost, so this is not likely an issue, but perhaps there is a small leak somewhere?

 

Where is this "intake distributor with a flap"? This is what my tech was referring to, but again, when I look at the pics of a stock splitter, all I see is a hollow tube on the back end with the TB on the front.

 

Intake plenum shock wave! My two cents....

 

Could this be related to the VarioCam when it changes from low to high output valve timing?

 

I am trying to see something mechanical after the throttle body and nothing as per parts cat.

 

here is a pic. I guess depending on which book I look at it is called differently. From the PET it is TB (#1)

 

isn't it the throttle body Wross?

 

When I got a problem like that, I always look at what was done before that specific problem. The exhaust and the dv's ??? Can it be related to the diverter valves? Personally I would like to look at this possibility first.

 

+1 imo

 

Maybe a loose component in the muffler? A lot of these have a resonance in this rpm range. My guess could be nonsense.

 

This problem was occurring before I installed the exhaust and DVs. I was hoping one or the other would cure the problem, but it did not.

I'm curious about the vario cam timing issue. My tech mentioned that as well. That sounds more serious?

 

Oops sorry! But if you got no fault code... the variocam is monitored and controlled by the ecu.... And if you got a Durametric it can be test easily with the "drive links"

 

You haven't had any CELs so there isn't a VarioCam problem. That is just the noise that the VarioCam makes and it's normal.