Paul,
I've just fitted a HKS SSQV to my Evo 6 (because the better half likes to go around scaring people with the noise
) and have noticed that on occasions the idle will fall off.
After thinking about it, this is my reasoning as to why it happens and what you can do to stop it.
The valve will open under a certain differential pressure between the plenum chamber and the boost pressure. The differential pressure that this occurs at it based upon the strength of the spring in the valve.
When the valve opens normally on throttle close, you have a large difference in pressure, the valve opens and stays open until the boost pressure is relieved.
Due to the fact that you have a large vacuum in the plenum chamber at this point, the valve won't close particularly quickly and so if you go straight to idle after the valve has lifted the valve remains open for a short time and you get extra air leaking backwards through it and causing the idle problem. This will occur with either type of DV but with the OEM item, it doesn't matter because the air has already passed the airflow meter and is accounted for in the fuelling supplied by the ECU.
Strengthening the spring by adjusting the screw may help this by closing the valve slightly earlier and not allowing as much air to leak back in but what I have found is that when you are coming to a stop and idling if you coast in-gear up to a point at which the revs are less than 2,000 rpm and then go into neutral you don't get the idling problem because the dump valve is closed and cannot leak air backwards.
Therefore your options are as follows:
1. Tighten the spring fully and see if this cures the problem of the valve remaining open when there is a large vacuum in the plenum.
2. If this doesn't work then try running around and before idling, coast in-gear for a short time to make sure that the dump valve is closed before going to idle.
Hope this helps
Andy
