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2.5tdi Turbo issues - P1556 error

bxl_lad

bxl_lad

Its not easy being green...
Messages
670
Location
Brussels, Belgium
Vehicle
T4 PopTop
Morning all,
having some issues with my 2.5tdi (ACV). Turbo not kicking in and struggling uphill...

Had a similar issue a few years ago, replaced 'some' of the vacuum tubes at the time and this seemed to have solved it for a while.
But the issue is back. Have gone through and replaced the rest of the tubes now, no success.

Have the following error in VCDS(Lite):
Screenshot 2024-03-24 100439.png

Does this error tell anything obvious? (Have trawled the web and it seems to be quite an ambiguous error that could point to a number of things that could cause it...).

Recently I had changed the air filter and doing so I moved the MAF sensor (I think the turbo was kicking in correctly up to that point, but can't say hand on heart because we hadn't been using the van for a while and as you know if you're just trundling around town then you don't really notice the turbo much) - I know @sidepod recently posted about his MAF sensor being easily buggered, could this be the same thing? But before i splash out on a new one, does the above error code tally with a buggered MAF sesnor, or is this a red herring?

Any ideas? Don't want to go changing out (for example) N75, N18 and MAF if there could be another issue and these parts are ok.
How to test these components (easily, don't have all the equipment!)...

My gut is telling to go and change the MAF as this is really the only thing i potentially interfered with...
Otherwise I'll send it off to the garage i guess...

Over to you guys and your wealth of experience and knowledge! :thanks
 
Starter for 10.

 
Try un plugging the MAF and see if it makes any difference. If not then it’s a safe bet it’s duff.
 
Try un plugging the MAF and see if it makes any difference. If not then it’s a safe bet it’s duff.
If its unplugged what difference would I likely see? Forgive my ignorance, but is it more likely that the turbo kicks in without a MAF connected?
 
Unplugged, It’ll be flat as a fart from the start.
 
If your version of VCDS supports adaptive testing you could run that. It’ll go through each control function/unit and you should be able to tell which unit is faulty.
 
So thinking about this, the ACV motor has a waste gate turbo as opposed to a variable geometry (VNT) as fitted on the AXG.

So full boost should be called for from idle with the waste gate popping off at high rpm to dump boost - I think.

Have a look a see what position the waste gate is in with the engine off. Fire it up and recheck the position. I’m not sure what the position should be with engine off but it might give you a clue as to the N75/air leak situation is.

Reading other online stuff suggests it could be a loose/failed actuator rod on the waste gate.
 
Just went trough a similar thing, but with the AXG 151pk motor and the big turbo; after a hard ride, motor not completely cooled, the turbo went down quickly after restart. Fault: turbo pressure too low. Seems that the adjustable vanes of the turbo were completely loaded with dirt (soot?) and the whole thing blocked when still warm. Garage dissambled and cleaned the whole turbo, now very smooth ride. But your problem could be something completely different... just trying to help.
Also check the waste gate valve - if it's loose, could also be a problem.
 
Unplugged, It’ll be flat as a fart from the start.
Unplugged it while it was idling, no discernible difference...
took it a spin round the block without the plug connected, in all honesty if i didn't know it was unplugged I wouldn't have known any different. If you push me for an answer then i'd say there was a very slight difference, but nothing striking.
Should the difference be night and day?
Incidentally, no warning lights on the dash plugged or unplugged
 
I’ve had a few go over the years on various VWs and yes, it’ll barely pull its own weight when unplugged. It’ll rev but just no guts.
 
An unplugged MAF won’t bring up a CEL.
 
I’ve had a few go over the years on various VWs and yes, it’ll barely pull its own weight when unplugged. It’ll rev but just no guts.
So what's your opinion on what's happening with mine?
 
Well it does sound like a MAF based on your description of plugged in/unplugged being no different.
That said, I think a faulty MAF would throw a different code.

I’m not home at the moment but tomorrow I can unplug my MAF and see what code VCDS throws.

This is why I asked if your version of VCDS allows testing.
This activates the N75/start of injection valve/egr/anti shudder etc and you can tell by engine note which one does or doesn’t have any effect.

If you have a hand held vac pump you could test the waste gate to make sure it’s working.

Are the brakes ok? Just thinking vacuum pump failed or on the way out.

I have a spare N75 and MAF. Happy to send them over for you to test. That said, MAFs fail periodically so it’s not a bad thing to keep a spare- the point being, it’s not the end of the world if you buy one and it’s not that.

Just recheck all of the vac hoses. There are several and two different diameters.
There’s a small filter and one way valve as part of the hose system. Check those.
 
Well it does sound like a MAF based on your description of plugged in/unplugged being no different.
That said, I think a faulty MAF would throw a different code.

I’m not home at the moment but tomorrow I can unplug my MAF and see what code VCDS throws.

This is why I asked if your version of VCDS allows testing.
This activates the N75/start of injection valve/egr/anti shudder etc and you can tell by engine note which one does or doesn’t have any effect.

If you have a hand held vac pump you could test the waste gate to make sure it’s working.

Are the brakes ok? Just thinking vacuum pump failed or on the way out.

I have a spare N75 and MAF. Happy to send them over for you to test. That said, MAFs fail periodically so it’s not a bad thing to keep a spare- the point being, it’s not the end of the world if you buy one and it’s not that.

Just recheck all of the vac hoses. There are several and two different diameters.
There’s a small filter and one way valve as part of the hose system. Check those.
I've only got the VCDS Lite, so not sure if I have the access to those functions??

Brakes seem fine.

Thanks for the offer, but the shipping costs to Belgium from UK probably mean its cheaper to just buy one (thanks Br...t!). But like you say, can't hurt to get one in, so will order a new MAF and see how that goes.

Question (yes another), is it the case that the 'norm' state is that the vacuum is opening the waste gate (so removing the vacuum tube will release it) - i guess what I'm asking is if the engine is idling and i remove the tube, will i see it move, or does the engine need to be under load to see that?

I guess i need to head under the bonnet again to check those tubes. I did pull off the top tube of the N75 and there was suction on it.

So I'll order the MAF anyway and see if that improves anything - Thanks!
 
Question (yes another), is it the case that the 'norm' state is that the vacuum is opening the waste gate (so removing the vacuum tube will release it) - i guess what I'm asking is if the engine is idling and i remove the tube, will i see it move, or does the engine need to be under load to see that?
That’s what I’m not sure of. Does vacuum open or close? Gut feeling says the waste gate is sprung closed and vacuum opens it. If this is the case then I guess you won’t see any movement from engine off to engine on at idle. Movement would only occur at max allowed boost when vacuum opens the waste gate in order to dump boost.

This doesn’t make it easy to test without VCDS controlling the N75 at idle.
On mine (VNT) the actuator moves as soon as the engine is started, so kind of the opposite.

Another option if you have one (or get a cheap one) is to plumb in a temporary vacuum gauge from the N75 into the cabin. Drive the van and watch for vacuum.

It could be the diaphragm in the waste gate actuator has failed so everything is ok but the vacuum is leaking inside the turbo. Again a hand vac pump would confirm. Connect it to the waste gate actuator and see if it moves. You could run the engine and bypass the N75 to confirm this.
 
Not sure if you sorted your van. Ross Tech suggests low boost. See here http://wiki.ross-tech.com/wiki/index.php/17964/P1556/005462

Low boost can be caused by leaking vacuum hoses, too much air getting into the turbo inlet. So first check hoses. Replace if needed. Check actuator is not stuck. Should move by applying a little pressure. I am tall enough to reach over intercooler and move by hand.

To help pinpoint problem of low boost check measuring blocks 3 8 11.

Low values in measuring blocks 3 doesn't mean your MAF needs replacing. It might mean that too much unregulated air.

So read the following

https://www.vwt4forum.co.uk/threads/2-5-tdi-power-problem.263448/ info in link will walk through process of setting up VCDS.

Use chrome to translate. Pay attention to advise on checking turbo inlet as this can be a source of unmetered air.

https://www.vwt4forum.co.uk/threads/how-to-test-g70-air-mass-meter-aka-maf-sensor.87513/ this link is also useful in testing the MAF sensor with a multimeter

https://www.t4forum.de/forum/index....ust-acv-98-was-kann-ich-noch-prüfen/&pageNo=1 use chrome again to translate

My advice is read all the above. The turbos are long lasting so don't jump to replace.

Also a blocked cat can cause low boost pressure. Any rattling noises from the cat?
 
Update:
so, have swapped out the MAF for a new one, rechecked and clamped all vacuum tubes. I checked the waste gate valve with a large syringe and that seems to function as it should.
Checked the blades turn in the turbo entrance, they do.
But still no turbo kicking in.
My VCDS Lite shows no more errors and when checking measurement 011, there appears spikes when apply acceleration - Lite only allows you to graph one measurement at a time, and actually I'm not really sure what it is I'm supposed to be seeing here...

Anyway, I've done all I can in my pay grade, so monday its off to the garage to let someone competent have a look at it. Will update when I know more...

Thanks all for your tips and advice, but I feel I've reached my limit of trying to fix this myself.

If i'm to guess, WelshWestie's point about the DPF could indeed be a cause (or certainly not helping), my van was fitted with an aftermarket DPF back in 2014 (previous owner in Germany) and my guess is that its never been serviced since...
 
The verdict is in, seems the turbo is buggered after all...
Garage will be replacing next week. Not cheap, but hopefully that solves it - fingers crossed!
 
Curious about this. What part of the unit failed? It must be the actuator diaphragm as there are no moving parts in the actual turbo (apart from the spinny whizzy round bit).
If this is the case then keep the old one and get it refurbished. There are numerous places out there that will repair them.
 
Apparently its going to be getting a refurbished one...
In all honesty I'm at the mercy of the garage now. I've taken it to one that's been previously recommended on here and have to hope they're not pulling a fast one, but he claims to have tested everything and narrowed it down to being the turbo itself. Will have to trust in that.

I had wanted to solve this one myself, but just don't have the garage space or indeed time to mess with it more that I already had - without the right tools/equipment it becomes a game of trial and error to find the right thing to fix...

I guess the proof will be in it running correctly once i get it back. Fingers crossed!
 
Finally back from the garage...
New turbo and they replaced the catalytic converter - goes like the stink now! Seems also to have fixed its smoking problem...
Now we're ready for camping season to begin :cool:

If you're in Belgium and want someone to take care of your T4 (or T5 for that matter), then I can recommend "Garage Van Den Broeck (Paul)", in Heist-op-den-Berg.
 

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