skafia

I have a manual GT4 with an aftermarket exhaust and I keep hearing these weird thunk or thump sounds, usually at around 3000 and between 4000-5000 RPMs, always when I let off the gas.

I was thinking it's just the Variocam engagement points or whatever because I have auto/start disabled always (which should not enable cylinder deactivation right?). But today I was testing out the cylinder deactivation and it sounds exactly what I'm experiencing described in the first paragraph (but at higher RPMs).

Is it possible for cylinder deactivation to function when the start/stop button is disabled? Is it possible for it to deactivate above 3000 RPM, especially at 4500-5000RPM? Do I have a faulty car somehow?

Larry Cable

AFIAK with A.S.S deactivated cyl deactivation is also disabled and I believe that when it is active it only activates when the engine is not under significant load - like cruising on the freeway at low rpm

Nick_CT

Cylinder deactivation is disabled if you deactivate Auto stop start. The window in which it works, in my manual GTS 4.0 anyway, is quite narrow. Never above 2,400 RPM (or 104km/h) and only under really, really light load. I think it's 2,400 RPM, been a while since i've driven. But for highway driving it basically doesn't work because the highway limit here is 120km/h so you're always driving above the max speed for it.

With the standard exhaust and valves locked open, when one bank deactivates it's sound like absolute trash and hurts your ears, you can't miss the fact that it's working.

Larry Cable

when Cyl deactivation cuts in on my Spyder it makes a noise that I image a humpback whale in labor makes ....

Nick_CT

With stock exhaust with valves closed you literally can't hear it. You can feel the lack of power, but unless you really listen out it's pretty impressive how there is no difference in sound.

skafia

I'll have to get a video but this is what I'm experiencing exactly.

Start/Stop off
Run car to 6k RPM in 3rd or 4th gear
Let off gas so it falls to 4500-5000
Really loud "thunk" from exhaust
Apply a little gas, thunk goes away
Let off gas again and you can hear it again

like I said, it sounds exactly like what the cylinder deactivation sounds like at low RPMs. I'll take it in next week to see what the dealership says.

UncleDude

do you have an aftermarket exhaust? Maybe check for tightness and the valve operation

skafia

I do, already checked twice. Valves permanently disabled. I heard it (faintly) with the stock exhaust too.

TXshaggy

How did you disable the PSE valves?

The general RPM ranges and occurring during RPMs going up/down sound like a valve issue.

Cylinder deactivation is not in play for the driving your describing.

Bob Z.

With the Soul Race exhaust, which does not have any stinking valves!

TXshaggy

Are you saying the OP has Soul Race Exhaust?

skafia

PSE disabled by disconnecting vacuum lines on my Kline. Ok I'll see what the dealership has to say.

Larry Cable

around the rpm's where the ECU opens/closes the PSE values as I recall... I wonder how (or if) your vacuum lines are "plugged" then?

skafia

SupremePower installed the exhaust and they confirmed the vacuum lines were unplugged to keep valves open all of the time.

Larry Cable

Well if Eric did this it's done right but it's not just unplugging the valves it's plugging the vacuum lines from be the engine that opens and closes them ...