Turbo problems? Graph included...

[mickmcvw]

2002 Allroad, 2.5TDI AKE engine, 6-speed manual box, about 120k or so on the clock, never been remapped as far as I know, no error codes or engine light, own it <1 year, driving fine(ish!)

Have never been overly happy with the acceleration for what's a 180bhp engine. It always seems particularly slow picking up the revs at the low end and the turbo doesn't seem to kick in until about 2000 RPM. I know it's a very heavy car and driving all 4 wheels etc., but expected more for some reason. I'm wondering do I have a problem with the turbo or its associated components and seeing as I have VCDS, I figured some plots might help out. Problem is that I'm not sure what's normal and what's not...

The graph shows engine RPM, requested intake pressure (mbar), actual intake pressure (mbar) and N75 duty cycle (%) (the duty cycle uses the right y-axis). Time is along the bottom in seconds. This was taken in 3rd gear flooring it from about 1200 RPM to red line. Atmospheric pressure was read as 996mbar.

Lots of questions now:

1. Why is the N75 so slow to respond to a boost request? I thought it would be able to change the duty cycle from 90% to 25% a lot quicker than it does. Is this the normal response profile needed to spin up the turbo?

2. There's a huge initial lag between requested boost and delivered boost, it's about 4 seconds before it gets what has been asked for. Again, is this normal or the result of the N75 valve operation?

3. What's the max boost delivered by a stock turbo on this engine? My calculations on this show it to peak about 22psi before dropping off.

4. The overshoot on the boost is normal from what I've read but should it settle a lot quicker than it is trying to?

5. When the engine is redlined about 4500 RPM, should the boost delivered be constant and matching the requested? According to a lovely plot on the TDI section of the Ross-Tech website, it should match. It looks like it's dropping off considerably but maybe a function of the rev limiter?

My thoughts so far:
a) Vacuum pump problem.
b) Vacuum leak - maybe but haven't found anything and hoses are pretty new.
c) Boost leak - also possible particularly as the delivered boost is dropping off to about 3psi at redline. Not found it yet though.
d) Messed up remap - No clue, have no experience with remaps at all.
e) Faulty N75 valve - Doubt it, boost seems to follow what the N75 is being told to do.
f) Turbo trouble...
g) Combination of the above.
h) Perfectly normal behaviour in an 8 year old engine - Not my preferred outcome.

Anyone spot anything obvious from the graph? Anything else I should plot/log to try identify the problem further?

Thanks!

[mickmcvw]

34 views and no reply, man I was hoping for some enlightment on here!

Another plot to add to the confusion, this one is almost the same as the last but held it redline for about 10 seconds or so. Delivered boost is dropping way off, down to almost nothing, and the N75 valve seems to be trying to do something to recover but to no avail...

Can anyone help?

[zollaf]

somethings not right. you seem to have most of it covered though by checking the boost pipes and vacuum hoses. i would double check these though, and the intercooler for any leaks. other than that, possibly a map sensor or thrust sensor at fault.

[a8 tech]

check the turbo waste gate control arm for free or excess movement by simply running vcds output test to confirm if there is to much free play or insufficient movement due to soot build up on the vanes and depleted vacuum being lost at the accumulator due to possible actuator rod movement.
I do not use vcds log graphs as I prefer to use a mitivac or inline pressure tester as well as vas pc for a pre set turbo test from 1500rpm 3rd gear paying attention to air drawn and boost output as third gear full load is the best test.

apply a vacuum to the intake system to check for leaks so it reproduces boost pressure limits to the pipes and ensure airmass values are not above the limits.

N75 is just a switch and they rarely fail but are frequently replaced.

Basics

correct air and fuel

correct valve timing

correct boost pressure

ensure freeflow for exhaust gases

disable egr if suspect

ensure engine and fuel temp sensors are in order from cold and monitor coolant from cold to 80 by using mvb and uniform upwards reading of 2 to 3 Celsius and not up then down.

Forgive me for ignoring the graph but i simply do not trust them as there just a toy in my eyes where as mvb are more to my liking and again vacuum and boost pressure testers

One last note, don't be so pushy for an answer its free but only stays free by members within the motor trade offering help and use the thanks button or don't ask again

[mickmcvw]

Thanks for the replies guys, wasn't trying to be pushy, apologies if it came across that way. Only added the second post today once I'd seen the strange behaviour when staying at redline...

Zollaf, I'll have to do as you suggest and work through all of the hoses/pipes again as a starting point. I did this as a first check but maybe I missed something, perhaps an intercooler leak?

a8 tech, a lot of detailed info there (hopefully I can figure it all out!) but very much appreciate the reply. I don't have a mitivac but do have a vacuum pump and guage so at least something else I can do to check what's going on.

I understand your dislike for the VCDS plots but while I had 10 minutes this evening, I ran the same test on my wifes Passat. Completely different engines I know but talk about different looking graphs! The specified and actual track each other so much nicer. I can throw this one up too if anyone wants to see something different...

Thanks again!