So it seems my mk4 has a boost problem. Under high load e.g. flat out in top is will accelerate smoothly up until a certain point and the power will come in quick 'waves' as you hold the pedal down. I don't have Forscan unfortunately, so I did my best to look at it with Torque. I found that the car is fine up until it tries to make more than about 15psi of boost at which point the boost will rise and fall. I should have turned logging on... But my passenger took screenshots of the boost graph while it was happening.

The MIL it not lit up, but I scanned it anyway and found a pending P2566 Turbocharger boost control position sensor circuit intermittent, web lookup suggests faulty boost control position sensor, dodgy connection or wiring issue.

Question is: is it possible something else is causing the sensor to appear faulty (sticking turbo vanes?) or is it simply a case of checking the wiring and possibly changing the sensor?

Just an update, the MIL lit up for the first time today. Torque shows the P2566 code so I just cleared it for now.

Also, a friend of mine suggested a boost leak can cause spiking, or possibly the EGR valve as the culprit. Any credence to this?

I'd go for something simple first...check your boost pipes for possible splits, which are quite hard to detect as they tend to close up except under boost pressure. 

18 hours ago, nicam49 said:

I'd go for something simple first...check your boost pipes for possible splits, which are quite hard to detect as they tend to close up except under boost pressure. 

There is that. Wonder how the heck I'd DIY diagnose that one...

Surest way is to remove them and inspect them closely, squeeze and twist them... Most likely ones to split  are close to egr that get hottest.  (the rubber outer splits, but the mesh lining holds it together, so its not obvious there's anything wrong) 

On 6/18/2021 at 8:13 PM, nicam49 said:

Surest way is to remove them and inspect them closely, squeeze and twist them... Most likely ones to split  are close to egr that get hottest.  (the rubber outer splits, but the mesh lining holds it together, so its not obvious there's anything wrong) 

Got it! Thanks, I'll give that a go

Not had chance to check yet, life etc. has been in the way... I did however get chance to check it with Forscan, since I got the laptop working. Log is:

 

===PCM DTC P2566:00-24===
Code: P2566 - Turbocharger Boost Control Position Sensor A Circuit Intermittent

Status: 
 - DTC Maturing - Intermittent at Time of Request
 - Malfunction Indicator Lamp is Off for this DTC

Module: Powertrain Control Module

===END PCM DTC P2566:00-24===

===OBDII DTC P2566-P===
Code: P2566 - Turbocharger Boost Control Position Sensor A Circuit Intermittent

Status: 
 - Pending - malfunction is expected to be confirmed 

Module: On Board Diagnostic II

===END OBDII DTC P2566-P===

===SASM DTC None===
Successful DTC reading, no error codes found

Module: Steering Angle Sensor Module

===END SASM DTC None===

===ABS DTC None===
Successful DTC reading, no error codes found

Module: Antilock braking system

===END ABS DTC None===

===HCM DTC None===
Successful DTC reading, no error codes found

Module: Headlamp Control Module

===END HCM DTC None===

===BCMii DTC None===
Successful DTC reading, no error codes found

Module: Body Control Module

===END BCMii DTC None===

 

You don't know how old some of those are, as there's no time stamps, so just clear all errors and see if any return. 

1 hour ago, nicam49 said:

You don't know how old some of those are, as there's no time stamps, so just clear all errors and see if any return. 

Done that a few times. The P2566 returns each time it has boosted above about 15psi.