My apologies if I'm covering old ground here, but this problem is a bit mystifying and I haven't found a similar problem in my search of the forum.
I'm getting a knocking sound that comes and goes from what I believe to be the VVT system. I've noticed is a slight difference in performance when the knocking is present; the throttle response seems a bit slow and it takes more pedal travel to get decent acceleration. When the noise is quiet, the car seems to be raring to go and it jumps with very little pedal travel.

Could this be related to the VVT system, or is it something else?

Thanks,

Max

 

It may or may not be related. What I can say is that I can sometimes hear my wife's 2006 XK8's VVT system knocking slightly at cold start-up if I am standing outside when she starts her engine. The sound goes away within a few seconds. I consider the noise to be normal and am not concerned about it....

 

Is the noise worse at low rpm, high load (going uphill)? If so, check the fuel trims instead...

 

some of the 'knocking' might be the solenoids engaging and disengaging? how loud is the knock? in my experience working on the VVT system on my machine (which I have not been able to get right), there is not a lot of possibility for knocking from the VVT units themselves. and, if you hear or heard it knocking, it might me quite violent. grinding metal to metal is the sound I fear. there is a lot of force behind their function with what looks like a giant gear that engages and disengages at its innards....

could it be, do you get the feeling, they are quickly swinging from fully retarded to fully advanced ---- and that is the sound you hear? if so,,, I would still be looking to the solenoids...

 

Thanks for everyone's input. The sound doesn't seem to be rpm sensitive; I can hear it at all rpm's when its present. And there doesn't seem to be any rhyme or reason for its coming and going. What I'm considering doing is disconnecting each of the solenoids in turn to see if I can notice a difference in sound/performance. Forgot to mention, all of the chain tensioners and guides were replaced when I bought the car a few years ago.

 

Have you eliminated the canister purge valve? It is in the driver side wheel well, behind the liner, firewall side. It is computer controlled, and comes on (click-click-click) and off every 60 sec or so, I believe. Heard mostly at idle, clicks a few times a sec, independent of road speed or rpm.

 

No, but I need to check it. Noticeable gas smell when I first open the garage door which I believe is a symptom of a bad purge valve.