if you're certain that there's no vacuum leak then I'd say to remove the air intake so you can see / reach the throttle and clean it out with some brake cleaner and a lint free rag
and its also possible that the fuel purge valve may be sticking and so flowing fuel tank vapour when it shouldn't so try disconnecting it to see if that helps
and the best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.
thanks for the extra information
I'm wondering if you have a faulty throttle position sensor or faulty pedal sensor if you watch the throttle butteryfly whilst your assistant slowly pushes the accelerator pedal down , then the butterfly should open smoothly and slowly with no sudden movement
thats a good find, the ICV should be quite heavily sprung loaded so its held on its seat so it sounds like it might being moved about by air motion more than it should
OK if the ICV is blocked and there's no vacuum leak I wonder if the issue is with the PCV / breather allowing air in when it shouldn't
I'd suggest tracing the breather pipe back from the cam cover to the inlet and consider replacing the PCV
well that's possible
but if there was cracked manifold then I'd hope that the vacuum leak test with the lighter gas would show this up?
however if the manifold crack opens up with heat then this could be possible
have you tried doing the vacuum test with a hot engine?
is this the plastic manifold version?
If it is then check along the seams for signs of leakage - what might be worth a go is running a couple of strips of heavy tape along the seams to seal it, just for a trial