T6 204ps DSG - P2099 Underboost

[Catagonia]

Hello folks,

Recently had a persistent fault with my T6 BiTdi 204ps 4motion DSG.

Either rumbling along in high gears asking for revs low down, or going up hill, it will stutter and go into limp mode with the coil light. Happens frequently on the motorway (three times on the M25 today). Ignition cycle clears the fault and it drives fine afterwards.

EGR was blanked off, and the DPF drilled out last year with a very light remap; but this was well over 8k miles ago.

Fault finding wise I've done the
following:

Filter and MAF cleaned.
Smoke tested
Vacuum test with the actuator movement checked for both turbos.
Switched around the actuator vales between turbo A and turbo B... just to make sure one of them wasn't faulty.


I have to admit there isn't a whole lot of literature on the BiTdi 204ps, so I'm not 100% on how the second turbo is introduced- ie is it a butterfly valve mechanism in the exhaust manifold. So I'm wondering whether turbo B is knackered.

I'm stumped so any help with be greatly appreciated!

 

[Big Al]

I can guarantee there will be a procedure to follow to determine a fault with 1 or more of the turbos. Cant 2nd guess these things.

 

[catfood12]

If no other fault codes, like charge temp too high, it's time to get the gauges out to see if boost is low, or if sensor issue, or if it is temp related. I guess by smoke test you mean checked for leaks, split hoses and the like ? There was another thread on here about low power and a schematic was posted of the turbos, pipework and sensors for the bi-turbo

 

[Catagonia]

I found those schematics in the end, very useful indeed.

There would have to be some pretty serious temp increases to lose that much boost pressure though surely?

Unrelated I have just serviced the van - filters and oil (Inc fuel filter) and it has imediately thrown a MAP sensor fault and check engine light. So I'm wondering whether the fault may lie with a faulty MAP after all.

Disappointing for a still relatively new van with sub 50k miles. Cost me an absolute fortune to buy new aswell, with all of these issues happening outside of warranty

 

[DXX]

There would have to be some pretty serious temp increases to lose that much boost pressure though surely?

P1V1/T1 = P2V2/T2

 

[Catagonia]

Given there is an in and out temp sensor across the charge cooler I would have hoped that it would pick up the temp spike there? Are you suggesting along the lines of a blocked / inefficient cooler?

 

[DXX]

Given there is an in and out temp sensor across the charge cooler I would have hoped that it would pick up the temp spike there? Are you suggesting along the lines of a blocked / inefficient cooler?

I’m not suggesting anything, merely pointing out the calculation needed to answer the boost pressure question that you asked. Due to space restrictions I would imagine that VW designed the charge air cooler with very little excess capacity for the standard engine map.
The only medium that could contaminate the charge air cooler is oil, this would affect the heat transfer rate but as the likelihood of oil contamination is quite high it really should be designed into the capacity.
Your problem as you described appears to be power from the lower rev range but with high fuelling. Is it the same with a cold engine, where the charge air coolant is cold?
Regarding the issues happening outside of warranty, I don’t see how that affects the situation as you have modified the map, EGR and DPF, VW would simply wash their hands of it.

 

[Catagonia]

Interesting stuff, for the cost of a charge cooler flush I might considered that. Although its mainly at motorway cruising speeds / temps, it has raised its head from cold start before.

VW were terrible unfortunately, I initially had an issue with the DPF kicking into persistent regens last year to the point that it got so hot it melted the undercarriage cover. Reported within the last month of the warranty, couldn't get an appointment with VW dealer due to the first round of lock downs, they refused to honour any warranty as they said it hadn't been seen by them within warranty.

 

[catfood12]

What are you using to read the codes ? If VCDS/Carista check the charge temps against the ones I posted in the other thread. I can't see the charge cooler being blocked, you'd be well down on power all of the time. If you eliminate charge temp, then that's one thing to tick off. If not, it could be charge cooler water pump/circuit issues.

Next would be air leaks, MAF etc I'd suggest. I have a 204, so let me know if you want any stats for comparison.

 

[Catagonia]

Just started getting fume smells in the cab, and a big puff of white smoke from underneath the turbos / DPF area on start up and turn off.

I know the egr pipe has been an issue on these 204s, but has anyone had an underboost from it leaking enough. From what I understand at the highest boost pressure they are just running off the bottom turbo, so I big enough exhaust side leak could prevent it from boosting enough.

Other than that I'm wondering if it's a cracked manifold / DPF flange. I've had a good look at the egr pipe and it actually looks like the improved version, and I can't see any obvious leaks or soot.

 

[Catagonia]

Update, in the spirit of not adding to the long list of forum problems that never have a solution.

Exhaust leak from the DPF / Turbo joint was a complete red herring, a £7 metal compression gasket had failed. No issues there.

EGR delete plate removed, returned to main dealer, ecu flashed back to stock and dealer diagnostic pathway undertaken. They identified it as a blocked EGR valve, which surprised me as It had been blanked off and coded out over a year ago - with the caveat that the physical blank plate had only been put on the intake manifold side, ie: coming out of the EGR. EGR flush at dealership sorted thr problem out, so the valve must have been coked up into a semi open position - affecting very low and top end boost, ultimately leading to an underboost at maximum boost levels. I'm going to plate the EGR off on the exhaust side going forwards.

Van is running strong as an ox, albeit with the odd massive plume of white smoke coming out of the exhaust when it is attempting to do its regens - just needs to go back to have the dpf mapped out again.

FYI: the master tech at the unnamed commercial VW Centre I used actually turned out to be someone I knew from a separate life years ago, and he shared a fair bit of knowledge with me. They are having a phenomenal number of returns for EGR faults, with the majority simply being solved with flushes, and around 30% permanently fouling DPFs within 20k miles. VW are giving out a lot of flushes and replacements as goodwill, even well out of warranty, when customers stamp their feet. From what the dealerships have said to each other it appears that the EGR / DPF system for the 5.1 and 6 was developed during the height of the emissions cheat heyday, and VW always knew that the system would repeatedly fail, if used as it was purported to be used. It has been muttered, albeit unsubstantiated, that once the homologation had been given that they could simply map EGR resuscitation down to miniscule levels with no-one ever being the wiser. Post emissions scandal the 5.1, most notably the 180ps vans, were the first to get hit - and when VW were forced to change all of their new vehicles back to the tested mapping the EGR deficiencies made themselves known in a big way, with the well known engine failures etc.

Not meant to be a boogie story, just sharing what I've heard. YMMV, but words to the wise the chap mentioned that newly all commercial fleet vehicles (eg British gas, AA etc) all come back from their 2 year lease deals having never had an oil change, with nearly completely unused EGR coolers and with the same tank of ad blue they were delivered with... so make of that what you will!