By: Dean Futrell
First off for those that don't know ISV is short for Idle Stabilizer Valve , every water cooled VW with fuel injection has one, its purpose is self explanatory: it's there to keep the idle stabilized at all times. On the G60 it has double duty as it serves as a bleeder for boost if it goes over a certain PSI. So in turn it also sucks in a lot of crap, and needs to be cleaned periodically. For reasons unknown to me they also crap out altogether on occasion.
When the motor is idling, the ISV regulates air flow from the bypass tube to the intake manifold to control idle speed. Under boost conditions, the ISV can reduce boost levels by bleeding boost pressure from the manifold into the bypass tube. One unfortunate side effect of the stock set-up is that the bled air being returned to the inlet of the G-60 is hot (from having already gone through the G-lader at least once), and increases the charge temperature (which reduces power output and increases chances of detonation).
The ISV re-route (aftermarket mod) by-passes the ISV but the ISV still serves its purpose of stabilizing the idle. ( or not if it's crapping out) I thought this might have been why my boost was reading low at idle, but when I called Francis at Motortechnik he said this is normal there should only be boost under acceleration at the manifold.
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