2002 A3 1.9 tdi (130ps) overboost & limpmode - suggestions ?

[brasiliangringo]

I have summarized the fault code identified by VAG com:
17965/P1557/005463 - Charge Pressure Control: Positive Deviation

The car occasionally goes into limp mode - usually after some hammering, but not at the exact moment the engine is being gunned. Turbo behaves normally after ignition reset.

The diagnostics I have undertaken are listed below which will hopefully help you to help me narrow the problem down!

1. Checked hoses and correct hose configuration with no apparent leaks.
2. Visually inspected and confirmed turbo waste-gate/actuator rod is traveling freely as intended.
3. Tested charge pressure using VAG COM, test start/finish in 4th gear from 2000rpm to red line with no significant deviation from spec'd turbo pressure.

Any suggestions? Could it be a Sticking turbo vein (but is this still possible if actuator rod is moving?) or could it be N75 but guessing this not possible if actuator movement is correct as point 2.?

Thanks

[fourringsrus]

Yep sticking turbo vanes and yes it is still possible if the actuator rod is still moving, remember that it all resets when ignition is switched off and then restarted,this is a common situation that you are describing and usually requires another turbo,just google overboost/limp mode etc
Good luck

Stuart

[Crasher]

In the vast majority of cases 17965 is a carbon fouled turbo which will need to come off and be stripped down for a clean or full rebuild. Vet occasionally 17965 can be the Air Mass Meter (MAF), N75 valve or faulty Thrust (MAP) sensor. In a few cases I have seen the turbo actuating unit stick and this is why I prefer a full rebuild as most rebuilders fit a new capsule.

[brasiliangringo]

I just tested the resistance on N75 which was 15ohms but i suppose that should already be ruled out given ive seen the actuator rod move with full travel..

So, having read around, I strongly suspect the turbo etc needs de-coking because the car has never really been driven hard..

Limp mode is being triggered not during hard acceleration or even at the red line (when testing) but only after throttle pedal lift off. The only way i can explain this would be to surmise that sticking vanes on lift off will cause the turbo to overrun and create a deviation between spec'd and actual boost as the RPMs fall, ie the engine decelerates much faster but the correct proportion of boost at the RPMs (as they fall) is far higher given the sticking vanes, thus tripping the limp mode ?

Could that be why i see correct actual and spec'd boost levels testing under load between 2-4000rpms in 4th ?? But the over-boost is happening on the lift of stage?

[martin1810]

" ie as the engine decelerates the turbo is still boosting at a far higher engine RPM due to the sticking vanes.. ?"

That is exactly what it does because the vanes stick at the maximum boost position which is fine for maximum boost. As soon as you lift off the vanes should change angle for a lower boost but don't 'cos they are stuck.

[brasiliangringo]

Thanks so that is why i am not experiencing the problem going hard through the gears ?

[brasiliangringo]

Furthermore, assuming its just the vanes sticking slightly, what physical damage can that do apart from being an inconvenience through triggering the limp mode which can be reset with the turn of the key?

[Crasher]

It will get worse and worse until it is undriveable, in a few cases I have seen the turbo overspeed to such an extent they literally explode which makes repairs much more difficult and costly.

[brasiliangringo]

It will get worse and worse until it is undriveable, in a few cases I have seen the turbo overspeed to such an extent they literally explode which makes repairs much more difficult and costly.

Ok but mine does not overboost/speed at the redline and go into limpmode, its only after i lift off the throttle so surely that means its only over spooling on deceleration (relative spec'd boost to the the rpms being out hence goes limp), in which case not actually outside of the limitations of the turbo.. ?

[Crasher]

I had one blow on me in the car park on the overrun after I revved it up at a standstill, it filled the entire charge air system and exhaust with shrapnel and oil. It made a mess on my floor when I pulled the lower intercooler hose off. On one PD I did the exploding turbo damaged a piston.

http://i222.photobucket.com/albums/dd20/Crasher1964/oilfromintercooler.jpg

[brasiliangringo]

Nasty.. im just trying to understand this..

But if mine is not over-boosting at the redline how can the turbo over spool and blow up? Its overboosting on deceleration because of the sticking vanes which causes a time delay and consequent over boost at given RPMs on decel but its not overspooling beyond its redline (whatever it is) so how can it damage ?

[Crasher]

If the vanes are jammed open and there is sufficient gas speed, the turbo can still overspeed and then ECU may not be quick enough cutting the fuel to catch it.

[brasiliangringo]

Ive been away for a while - turned out it was a broken front exhaust section that was causing the code / over boost issue etc.. happy days, nothing too serious then