Hesitation on Initial Throttle
 

I've noticed an issue with my 05 WRX (running hybrid TD04 plus supporting mods). I seem to have a bit of hesitation/flat spot on Initial throttle application.

It seems to happen after a period on the overrun, let's say when approaching a bend, if I let off the throttle for a second or two, then I would normally go for a lower gear and blip the throttle to match the revs before letting the clutch out and driving through the bend. I notice that the 'blip' doesn't lift the revs straight away like it normally does with the clutch depressed. There's about half a second where there is no response to the throttle input. The engine seems to quieten down for that split second and the revs actually drop. You have to hold the throttle for a lot longer than a normal 'blip' on a downshift before it finally barks and you get the engine speed you wanted.

I've also seen it happen when moving off from a standstill. If I try to move off with the usual "see saw", you sometimes get the same half second where the engine just doesn't respond to throttle input then finally seems to wake up.

Anyone had this kind of thing before? What's the likely cause?

Just closing this out for the benefit of anyone who has this issue themself.

The problem was two fold: a small vacuum leak and a failing lambda sensor. The initial symptoms were caused by the vacuum leak. Other symptoms that developed since making the thread were that the car would jerk when coming off the throttle gradually (going from open loop to closed loop).

Glad you've sorted it! It's good have incident and root cause confirmation, so thank you for closing the loop :D