There were several posts about this feature in the CRD. I will try explaining how, when and why it is used.

When
After a decent acceleration and the pedal is released very fast (cresting a hill, etc.). There are several inputs in the logic that must all be true: boost above ~1.5 bar, rpm between 1700-3800, rate of pedal change above 150%/sec are the main ones.

How
When the throttle tip-out is present the ecm commands the vanes to open all the way and at same time the EGR valve is opened for about 1/2 second. The vane reaction is fairly slow, but the electric EGR valve opens very fast.

Why
Lets say P1 is pressure in airbox, P2 is pressure in intake, P3 is pressure in exhaust manifold and P4 is pressure after turbo in exhaust pipe. With the quick pedal release the desired boost is down to 1.0 bar (atmosphere), yet there is residual boost in intake P2 varying from 1.5 to 2.0bar, the P3 is less than this due to no fuel injected. The excess pressure in P2 intake has nowhere to go and has pulses that push back on the compressor wheel. Keep in mind the compressor is still spinning above 100,000rpm and basically pushing air into a wall. This can cause flow separation in blade tips leading to a surge (whoosh, whoosh, whoosh sound). When the flow separation occurs the rate of change on compressor wheel speed can be abrupt. Over time it can lead to low-cycle fatigue failure of compressor wheels and most likely the forces can be transmitted throught the bearings.

This all occurs very fast and the pressure differentials are momentary.

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Providers of wholly developed aftermarket solutions for the KJ CRD.
(248) 977 - 9531

When I blip the throttle, especially at slow speeds, I hear a honking sound from the turbo side of the engine right after I release the pedal. Is that normal in regards to what you just explained?

Thx

Thanks for the Explanation Keith.

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05 Liberty CRD Sport with Leather 115k
GDE ECO, No EGR, no FCV, (system gone)
Oil Drain plug P/N 090-039
K&N Air Filter, 2nd gen Fuel filter head
NEW Rockers & lifters
Mobil 1 0w-40
Stant 13519 195 deg inline Thermostat.
0 281 002 845 MAP
Etecno GX3123 7v Glow Plugs
Advance Gold 800CCA Battery
Traded it for a Quality VW TDI

This website is awesome. I'm glad that I have a GDE tune. Thanks Keith. My turbo is safe, and I am glad to know that I can safely blow people away at a light (It always helps that they never know we're racing) and let me foot off the pedal whenever I feel like it.

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2005 Black KJ Sport CRD,
GDE Full Torque ECO TUNE, GDE Transmission ECO Tune,
Samco British Racing Green Hoses
Build Date 1/05
200k miles.
Original Owner

Thanks GDE for that comprehensive information. I did not appreciate the three parametre (IFs) required to cause surge protection to work. Aftermarket diesel BOV systems (like Turbosmart's) that just rely on one of these, most often throttle sensor voltage changes, would seem to be more prone to dumping pressure at the wrong time. Obviously, once a GDE tune is running the cause of EGR clogging is removed, but shouldn't a good cleanout be required at that time to make sure the valve stays healthy?

Am I correct in saying that my 2002 2.5 CRD with a fixed-vane turbo will not have any Turbo surge issues as the dump valve will simply open...regardless of whether or not the EGR has been disabled?

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2002 Export CRD 2.5 Sport