Diag Logging

mixxy

mixxy

Member
Can anyone with fairly tech knowledge shead some light here
I still have a rough start on my A2 TDI ( AMF ) its a bit smokey on start up ( White smoke )
I have gone through various test etc and still have soo much to test

Once the car is warmed up it seems to be OK although it feels like its holding back, I replaced all the small vacuum pipes at the rear of the car which made it fly for a while but gradually slowed again

Doing some Diagnostics I appear to have around 7psi of boost at idle , Overboost ?
So would assume this could cause the car to overfuel at startup and thus white unburnt diesel

Even with the n75 unplugged I have 6-7psi of boost, could it be the boost sensor ?
My brothers AMF Polo reads 0.5 - 1.0 vac at idle which I understand is correct
Is there a boost sensor somewhere to clean or replace

If i unplug my MAF sensor I get 21.0 vac at idle !!?
With the logs I have it could diag a bad MAF, I cant attach a XLS on here but one reading exampe on the motorway would be
Engine RPM(rpm) Mass Air Flow Rate(g/s) Turbo Boost & Vacuum Gauge(psi)
2751 67.16000366 17.80368996
2817 88.05999756 27.8257885
2817 82.44000244 24.17903137
2861 82.44000244 24.17903137
2884 83.68000031 24.15689659
2884 83.13999939 23.59826088
2901 83.13999939 23.59826088
2913 84.62000275 24.0515976
2913 84.19999695 23.70041275
2911 84.19999695 23.70041275

Im still unsure except to sort this overboost out
Then check timing ( loging is saying -3.5 ) - this could be cam sensor
 
Possibly the wastegate actuator is stuck (open)?

If you can make the trip, I'd suggest you book youself into see Wak at VAGCheck - he's down here in Hounslow - he totally sorted my overboost problems due to split plumbing, and also sorted my cousin's overboost due to bust intercooler.
 
At idle I can't imagine there would be enough exhaust gas flow that allows the turbo to generate 7psi boost pressure.
The turbo wastegate should be closed at idle. The N75 valve only opens the wastegate to reduce boost so disconnecting would have no effect on the boost reading as you found.
There is a boost pressure sensor (MAP) on the aftercooler ( http://www.a2oc.net/forum/showthread.php?14973-MAP-sensor&p=113626#post113626 ) and the ECU compares the signal from this with the (air into turbo) MAF readings (plus throttle position sensor etc) to determine the correct fuelling
Your diagnostic log suggests the readings from the MAP are way out which could indicate a faulty sensor as you thought. Since you can datalog, I assume you can run a diagnostic scan so I'm not sure why the faulty sensor was not picked up.
It would be worth having a VagCom scan as confirmation before spending money on a new MAP sensor.

Cheers Spike
 
I dont have any fault codes stored with Vag com, unless I force one by disconnecting the MAF etc
I Would have taken the car into Audi if I had somthing to give them
I will continue my logging for now and try giving the map sensor a clean as a first step

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I think I may have the answer
Would the MAP sensor also been known as the intake manifold pressure sensor ?

I've only just started using this logging software which names things different to Vag com ( Torque ) so just making sure Im not going down blind alleys but I get a constant 37PSI!!! from the intake sensor, Im not aware of a sensor near the inlet so currently assume its also the MAP sensor near the intercooler

So deffo going to look at that / clean that this weekend

EDIT Think I've answered it myself - Intake manifold pressure sender / G71 sensor / MAP sensor - they're all the same thing.
Might fire up Vag com at the weekend to confirm and check block 010
http://www.vwt4forum.co.uk/showthread.php?t=84104
 
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For reference, I checked my MAP sensor with VagCom and it reads atmospheric pressure (nominally 1 bar / 14.7 psi ) with the engine off

Cheers Spike
 
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