poly n moly! Have
Join Date: Feb 2014
Location: United Kingdom
Theres no min/max boost position par say! The VNT works in a range of movement/position to spool the turbo ready for boost. At high revs the VNT opens wider to control over speed, at lower revs the VNT shuts momentarily to reduce boost and control overboosting ( this is often done with the EGR open).
A worn VNT will give poor control to the shutting of the VNT and thus overboost warnings and surging of power whilst at a steady state throttle position. Shortening the VNT rod a turn or two can often restore the VNT to it's proper operation range, but cleaning the sensors, inlet manifold, boost leak checking, inj leak back analysis, rail pressure check, high pessure pump pressure stage check, air in fuel line check, VNT solinoid lines/vent filter check will need to be done after a quality fuel cleaner is used and new fuel filter fitted. This lot can be done in a mornings work and will ensure your JTDM will not wreck its DMF or smoke the streets out. Hope this helps.
As a rule of thumb, shortening rod brings quicker spool and more control sometimes into overboost, lengthening rod gives later boost and less control. So unless you know what a smooth/correct running JTD should feel like, you may have trouble telling which way the rod needs to go, or weather another problem is effecting motor control.
Most motors can be sorted, it's finding someone who can, or learning the tell tale signs/values on the live computer OBD yourself. All good fun.