I get under boost fault now my new turbo has been mapped.

martin33100

Veteran Member
Joined
Oct 13, 2015
Location
UK
TDI
VW T5 2.5
Hi guys, just wanted a little advice please.
Had my bigger turbo mapped and top end power and boost control seems good but I now get an under boost fault when cruising at 70mph.
I have done a quick data log.
Column H is rpm, I is requested boost and J is actual.
This was at a steady 70mph cruise and you can see it's not under full load because the revs are staying between 2184 and 2247, I can't see why it's requesting 1.3-1.4 bar of boost with so little load?.
Limp mode was when the requested went to 1550 mbar.
 

Layerz

Veteran Member
Joined
Feb 23, 2007
Location
Leicester
TDI
Audi A4 2.5 TDI AKE
Hows your MAF output?

And have you been out in the wet this/last week. I've just had massive fun and games, fried MAF and had a holed air filter thanks to the water that managed to get in.
 

martin33100

Veteran Member
Joined
Oct 13, 2015
Location
UK
TDI
VW T5 2.5
Hows your MAF output?

And have you been out in the wet this/last week. I've just had massive fun and games, fried MAF and had a holed air filter thanks to the water that managed to get in.
I will post up the MAF logs later, they are usually slightly higher than requested low down and usually match up under power.

I have had my van 7 years firstly with just a remap.

I drove my van for a weak or so with the bigger turbo and tubular manifold on the OE remapped software and it was fine.

It only started to give the under boost code on a 70mph cruise after the tuner had adjusted the software to suite the bigger turbo.

Power is now 240hp/550nm instead of 160hp/ 425nm but I have now inherited this under boost fault at a steady 70mph cruise.
 

Nevada_TDI

Top Post Dawg
Joined
Aug 17, 2008
Location
Reno, sort of...
TDI
2001 Jetta TDI
Looking at the table 1.5 Bar equals 21.7 PSI. Subtract 15 PSI atmosphere from 21.7 and you get 6.7 PSI. Most TDI's I know of have between 4 and 6 PSI at highway speed so your log isn't "abnormal" per se. I would triple check my vacuum source and all my hoses as well as my N75 to make sure it didn't just develop a crack or something causing a loss in vacuum to the turbo actuator, causing the under boost fault.
 

martin33100

Veteran Member
Joined
Oct 13, 2015
Location
UK
TDI
VW T5 2.5
Looking at the table 1.5 Bar equals 21.7 PSI. Subtract 15 PSI atmosphere from 21.7 and you get 6.7 PSI. Most TDI's I know of have between 4 and 6 PSI at highway speed so your log isn't "abnormal" per se. I would triple check my vacuum source and all my hoses as well as my N75 to make sure it didn't just develop a crack or something causing a loss in vacuum to the turbo actuator, causing the under boost fault.
I think your looking to far down the table, 1550 mbar requested boost is after it went into limp mode due to an under boost fault.
At the top of the chart the requested boost is around 2300-2400 mbar, so after taking off atmospheric the ECU is trying to request 1.3-1.4 bar of boost at a steady 70mph cruise.
Can you see my problem now?
 
Last edited:

Nevada_TDI

Top Post Dawg
Joined
Aug 17, 2008
Location
Reno, sort of...
TDI
2001 Jetta TDI
I get it now, why is it requesting all that boost at cruise? The program is requesting way too much boost for that situation. Why is the ECU doing that? Have you checked your throttle position/sensor in VCDS? What does it say the TPS is at?
 

Layerz

Veteran Member
Joined
Feb 23, 2007
Location
Leicester
TDI
Audi A4 2.5 TDI AKE
Just to check first have you linked in with your tuner in case something was unexpectedly adjusted wrong.
 

martin33100

Veteran Member
Joined
Oct 13, 2015
Location
UK
TDI
VW T5 2.5
I get it now, why is it requesting all that boost at cruise? The program is requesting way too much boost for that situation. Why is the ECU doing that? Have you checked your throttle position/sensor in VCDS? What does it say the TPS is at?
Next time I have VCDS connected I will check the TPS, do you what measuring block channel it is?.

I have had my van 7 years now with just a basic remap on and its been fine, even driving it for a weak with the bigger turbo and tubular manifold it was fine to drive.

I only got this under boost fault at cruise after the tuner had re-written the tune to suite the new turbo.
 

martin33100

Veteran Member
Joined
Oct 13, 2015
Location
UK
TDI
VW T5 2.5
Just to check first have you linked in with your tuner in case something was unexpectedly adjusted wrong.
I have told the tuner about this issue and they have offered to come to me to put it right.

I don't have a great deal of confidence in them now to be honest, at first they couldn't re-scale my 3 bar map sensor to show above 2601 and this still needs to be done.
Also I now have this under boost fault at a steady motorway cruise.
 

ketchup

Veteran Member
Joined
Jul 17, 2013
Location
East Sussex UK
TDI
B5 Passat
I'm no expert but this looks like tuner error. Requested boost at cruise shouldn't be this high and needs to be lowered closer to actual values.

Call or visit your tuner armed with some boost logs. They should be able to straighten it out.
 

Nevada_TDI

Top Post Dawg
Joined
Aug 17, 2008
Location
Reno, sort of...
TDI
2001 Jetta TDI
OP, I am glad to hear the tuner didn't make excuses, but considering it worked before and it doesn't now-- the fault is theirs. Did you get a chance to drive it with the new turbo and no change to the tune? If so, how did it run?
 

martin33100

Veteran Member
Joined
Oct 13, 2015
Location
UK
TDI
VW T5 2.5
OP, I am glad to hear the tuner didn't make excuses, but considering it worked before and it doesn't now-- the fault is theirs. Did you get a chance to drive it with the new turbo and no change to the tune? If so, how did it run?
I fitted the turbo and drove it for a week before the software was changed and it was fine.
 
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