think i must be the only person around who actually updated/kept their tvis rather than getting rid of it. the problem is the rubber diaphram is stuffed in 90% of the older cars, and so the system doesn't work.... and toyota dont sell a replacement unit....

On the TVIS its default setting (ie disconnected vaccum tube to actuator) is open flaps.  On mine, it was obviousy not releasing the vacumn, and was hindering top end power, so my diaphram in the actuator would have been ok. 
 
If you diaphram is leaking, then your problem would be the other way, flaps dont close (at all or fully) at low revs (or whatever the exact strategy of the ecu is - I believe its more than just revs related switching).
 
How will you know if yours works?  Well if its an issue like mine, disconnect it, the flaps will be open and go for a drive, you will notice it on your first full throttle blast to redline.  Like I said, on my car the difference was HUGE.
 
Cheers

On mine, I could get it to open by driving in first gear at say 2500rpm at a constant speed for 5 seconds or so, then full throttle.  It wouldnt open always at the same revs, but did open everytime I did this.  If I just used full throttle while rolling off the clutch it wouldnt open.

artymr2
On mine, I could get it to open by driving in first gear at say 2500rpm at a constant speed for 5 seconds or so, then full throttle.  It wouldnt open always at the same revs, but did open everytime I did this.  If I just used full throttle while rolling off the clutch it wouldnt open. 

This has been my experience exactly, and I always believed it was either the butterfly shaft or solenoid sticking.  Having removed and inspected the T-VIS plate and given the solenoid a good workout on the bench, I found that it was all working perfectly, the vac tank had even held a good vacuum for days after it was removed!  Using an RPM switch as I have to override the ECU above a set RPM (I've set 3800 as a start point) will eliminate this seemingly counter-productive behaviour of the ECU, and as MuMan detailed above will allow the T-VIS to provide the performance gains it's capable of.
 
I guess like the apparent limiting of boost in low gears that Celica owners report, this T-VIS behaviour may have been designed to limit drive train torque in an effort to reduce warranty costs.  History seems to have shown us though that this isn't really necessary!

guys
just one question, if going standalone what difference does it make if the TVIS is disabled. couldn't the bottom end torque that's lost always be made up by tuning the aftermarket ecu appropriately or even by using an appropriate boost controller to bring on boost quicker avoiding waste gate creep thereby kinda equating it to the same as having a tvis
what's your take on this. im also thinking about taking the TVIS off for ease of tuning on a PFC

/\ yeah thats what im trying to avoid, tuning for both sides of an rpm switch point. then comes getting the switch and wiring for it to work etc etc.
if i can get my turbo to spool quicker than stock then iil be happy to get rid of the tvis which should be possible with a decent boost controller.

I doubt you guys are going to have much success improving turbo spool if you disable the T-vis.
As cogs explained, the T-vis effectively increases manifold runner velocity, which in turn improves VE, or more accurately cylinder filling effiency and this in turn improves turbo spool.
Should you disable the T-vis, an efficiently sized turbo will take longer to spool regardless of optimised tuning, as precise tuning of ign & EFI values will improve power by increasing combustion efficiency &  raising PCP closer to TDC, but won't effect VE.
Better to leave it operational and enjoy the increased torque and quicker spool it provides when working correctly, stock ECU or standalone.
 
Should you decide to take manual control of the T-vis as I've done (no choice on a gen3 -:)) you can set an RPM switch point, using an rpm switch that has hysteresis (or ECU2/3), and include a simple boost over-ride circuit, or pressure switch, to open at a selected point according to the  spool characterists of the turbo your using. This way the operation of the T-vis becomes predictable and more beneficial, closing off idle and opening prior to VE peaking.
 
I tuned this arangement basically with the T-vis locked closed up to the switch point, and locked open after, with some enrichment at the switch point itself. This worked out very well, and delivers good power. Recently using 20% bio-flex, it worked out exceptionally well, so might be a good starting point if you want to experiment. Down side is your removing the T-vis operation as an effective safety strategy, but then I'm not convinced its all that effective as such, and more likely used simply because it's there.
Also don't forget USDM ECU's and JDM's are different, the nominal switching point for a USDM is 4400rpm, where as JDM is 3800.
 

IGT is the signal you intercept for timing control.  IGF is just a feedback signal that the ECU monitors to determine whether the igniter and coil are functioning correctly.