P1556 Charge Pressure Control: Negative Deviation

Grumple

Grumple

Member
Hi All,

My 75 TDI keeps randomly losing power. I've guessed it was a turbo issue for a while, and a scan by @PlasticMac has found the error in the title.

This has been an intermittent issue for some time, and there seems to be no telling when the power will drop off. It can happen suddenly after driving along fine for minutes/hours, or it will literally start with limited power.

I've read through some relevant posts, but it seems that 'positive' deviation is more common than 'negative', so wondering where to start.

A couple of pics of the EGR valve area from either side:

20231028_114445.jpg

20231028_114506.jpg


It's an incredibly tight area to get to, so before I start taking the scuttle panel off it'd be good to have an idea of where to start.

Are all the tubes 3.5mm?
Should I replace the mucky EGR valve or just block it? (I imagine there will be differing opinions)
How do I go about checking the intercooler?

Thanks in advance!
 
First thing to look at is the turbo intercooler. Good chance the lower end cap has cracked / split and the internal rubber seal has been blown through.. If so time for a new intercooler. Here is a picture of typical end cap split / crack and seal exposed..

1698504473247.jpeg
 
Not a diesel person, but I think it means that the turbo has not provided the boost that the ECU requested.
As it's very intermittent, I'd suspect control side rather than pipework. The N75 controls boost, and I reckon it's worth checking the small bore connections to it, as described in the post @timmus made long ago. Good luck.

N75 / Boost Pressure Regulator Location

Hey guys, I am going to have a look and see if I can find out what is causing this limp mode problem after I get to 70mph later and it seems the biggest culprits are the N75 / Boost Pressure Regulator and the MAF. The MAF I can see easily (and will clean it) but where should I be looking...
www.a2oc.net

Here's a pic, courtesy of @johnyfartbox and @Skipton01

TDI lacking power

That's a nice offer skip, I will pm you.
www.a2oc.net

Mac.
 
audifan said:
First thing to look at is the turbo intercooler. Good chance the lower end cap has cracked / split and the internal rubber seal has been blown through.. If so time for a new intercooler. Here is a picture of typical end cap split / crack and seal exposed..
Click to expand...

Thanks. Is it possible to check the intercooler without removing the bumper etc...? I'm getting the tracking done on Wednesday so might be able to have a look when it's on their ramp.
 
audifan said:
First thing to look at is the turbo intercooler. Good chance the lower end cap has cracked / split and the internal rubber seal has been blown through.. If so time for a new intercooler. Here is a picture of typical end cap split / crack and seal exposed..

View attachment 114774
Click to expand...
Would that sort of leak give a, (very), intermittent fault though? I'd think a leak of that size would give a virtually permanent fail state.
VCDS Report:
AUZ7Z0C2373082
1 Fault Found:
17964 - Charge Pressure Control: Negative Deviation
P1556 - 35-10 - - - Intermittent
Mac.
 
Depending on the size of the crack / split will vary the amount of pressure air lost especially if the seal has not been blown through. May only leak at high temp or under vibration etc.
 
Wasn't sure how to get a better pic of the intercooler, so hoping this will do?

1000016571.jpg


It doesn't look broken/split to me, but maybe I'm not looking at the right bit...

If I take this to a garage and explain what's happening, how easily will they problem solve/find the issue?

I'm kind of at my limit with the car now, so if this is getting expensive I'll be selling it as there's other issues that need attention as well.
 
Grumple said:
Wasn't sure how to get a better pic of the intercooler, so hoping this will do?

View attachment 115885

It doesn't look broken/split to me, but maybe I'm not looking at the right bit...

If I take this to a garage and explain what's happening, how easily will they problem solve/find the issue?

I'm kind of at my limit with the car now, so if this is getting expensive I'll be selling it as there's other issues that need attention as well.
Click to expand...
Those end caps look ok to me?

This was mine when it had blown;
IMG_8294.jpeg
 

Attachments

  • IMG_8294.jpeg
    IMG_8294.jpeg
    660 KB · Views: 84
Yes what is visible certainly does not look like a split end cap ( at least on the inner face ). Once you check the outer face same location on the intercooler you can probably rule the intercooler out. Next to check is the MAP sensor on top of the intercooler as the cheap aftermarket ones have a tendency to blow the cap! After that carefully check each air pipe from the turbo all the way to the EGR valve for cracking, splits, holes and tightness of fitting and come back to us.
 
audifan said:
Yes what is visible certainly does not look like a split end cap ( at least on the inner face ). Once you check the outer face same location on the intercooler you can probably rule the intercooler out. Next to check is the MAP sensor on top of the intercooler as the cheap aftermarket ones have a tendency to blow the cap! After that carefully check each air pipe from the turbo all the way to the EGR valve for cracking, splits, holes and tightness of fitting and come back to us.
Click to expand...
Thanks mate.

So, any other way to see the other side than taking the bumper off?

What exactly am I checking on the MAP sensor?

And I've looked before but get a bit confused with other terms people use, but is the EGR valve where the silver UFO/pork pie hat looking thing is?
 
If you look down the front of the condenser to the left is the front face of the intercooler. You could also if wanted remove the lower grille from the bumper and look up with a torch and mirror.

On the MAP sensor the genuine ones have part numbers and a few other things printed on the flat cover of the sensor. It is this cover that can split or even get blown off but usually only on aftermarket crap sensors.

Yes your very first picture does indeed show the EGR valve and is fitted directly onto the end of the air pipe.

On the AMF engine the EGR ( exhaust gas recirculation valve ) is combined with the ASV ( anti shudder valve ) the BHC and ATL engines have two separate valves.

In this picture below ( I know the bumper is off ) the arrow shows where to look down the front behind the bumper to see the intercooler front face.

1701377925011.png
 
I have no idea about access, but check the small bore pipes that run between the intercooler outlet, and the N75 control valve.
The N75 is reporting the charge pressure deviation, (via the ECU), so that any leak on that tube will be seen as a charge pressure deviation by the N75 valve.
It could also be the N75 itself, but leaky pipe is a first suspect.
Hopefully, someone can help describe it's location.
Mac.
 
Last edited:
PlasticMac said:
I have no idea about access, but check the small bore pipe that runs between the intercooler outlet, and the N75 control valve.
The N75 is reporting the charge pressure deviation, (via the ECU), so that any leak on that tube will be seen as a charge pressure deviation by the N75 valve.
It could also be the N75 itself, but leaky pipe is a first suspect.
Hopefully, someone can help describe it's location.
Mac.
Click to expand...

Hi Jay,
Have a read of this thread, and follow the link in post 8.

limp mode no power help

hi, after years of trouble free motoring in my little a2 it has now decided to go into limp mode now and then, normally after the car has been running and done a few mile it loses all power. turning the ignition off and on resets it and it works a little bit then goes again so i presume its in...
www.a2oc.net www.a2oc.net
Mac.
 
Intercooler isn't looking quite so tidy on the other side - not the easiest thing to photograph, but hopefully visible enough to guage if it looks problematic:

1000018784.jpg

1000018790.jpg
 
PlasticMac said:
Hi Jay,
Have a read of this thread, and follow the link in post 8.

limp mode no power help

hi, after years of trouble free motoring in my little a2 it has now decided to go into limp mode now and then, normally after the car has been running and done a few mile it loses all power. turning the ignition off and on resets it and it works a little bit then goes again so i presume its in...
www.a2oc.net www.a2oc.net
Mac.
Click to expand...

Thanks Mac, will take a look 👍
 
Yes you have found the problem. In your last pictures the second one clearly shows the end cap has split and the black rubber seal is visible. You need a new intercooler as I suspected.
 
You must log in or register to reply here.
Back
Top