Mass Air Flow

[Taxiboy]

Since I bought my Passat B8 2.0tdi 150 in May last year I’ve not been happy with it due to various issues. Some were fixed however the car has still been annoying by been intermittently noisy, slow and just basically rubbish to drive yet would be nice and smooth during DPF regens and pretty refined. But now I have noticed poor fuel consumption over the past 2 months and getting a considerable bit less per full tank. I tried disconnecting both O2 and NOX sensors and there was no difference so that rules them being faulty. Also put injector cleaner in and still no difference.. However I have disconnected the Mass Air Flow meter and the car suddenly feels much faster, quiter and fuel consumption has improved too..
Does anybody know if replacing the MAF will solve my issues. For the time being I will have to reconnect MAF so the car can do active DPF regen. I was getting no fault codes either obviously until any sensors were disconnected. Anybody know if a faulty MAF can be faulty without any fault codes.

[Crasher]

Disconnecting the O2 and NOX sensor and that making no difference is not a test, in VCDS there are proper tests for them and for the AMM. Have you checked the DPF ash and soot loads?

[Taxiboy]

No I’ve not had it into garage yet. It does it’s DPF regens twice per full tank so that appears to be normal. Originally I did think it could have been an injector issue as the car felt sluggish and poor mpg but with the MAF sensor disconnected the car is much better so perhaps rule out the injectors.

[Mittens247]

Have you tried cleaning the MAF with electrical contact cleaner? What sort or mpg or range are you currently getting and how much has it fallen? Could some of it be winter related as mpg and range will detiorate in colder months. With no short journeys and just me driving, I was getting 55mpg/800 miles range in summer. In winter, the wife uses the car as well, as she likes the heated seats. With the added short journeys we are only getting 45-50mpg and about 650-700 miles range.

[Taxiboy]

I reconnected the MAF sensor and left it 2 weeks ago. Since then the car has driven better and mpg has improved and getting between 560 and 600 miles per tank which is better than before where I was lucky to get around 530 to a tank. It still has the tractor noisy engine/sluggish symptoms intermittently although not as frequent as before but it still annoying though. It also ran rubbish one morning last week when it was cold just noisy and crap to drive then once the outside temp went from 0c up to 4c it improved and was fine rest of the day. Might be worth cleaning MAF anyway.

[VAG-Abound]

Do you know whether it might have a rubbish engine remap on it? That could cause various problems.

Assuming not...

During regen the engine runs differently to get the exhaust temperature up. EGR off, altered injection timing, less air via throttle valve management and changed turbo pressure are the main things I think.

Has EGR system been checked? I don't know that type of engine, but if it is like ours a leak on the EGR pipes would let boost pressure out when the EGR valve opened for example. And that wouldn't happen during DPF regen if the valve was closed. Pulling the EGR valve control might help determine that.

Moving to harder work - I read an interesting thread elsewhere recently where it was reused / stretched injector bolts causing rough running. Has anyone checked injectors, their seals, injector loom (although it'd be unlikely for that to never play up during regen if it had intermittent breaks), etc?

[Taxiboy]

I wouldn’t have thought there would have been any remap done as car was bought with 30,000 miles on it and appeared to be ex company lease car.. The diesel specialist I took car too said injectors were fine and couldn’t find any problems. Considering it’s great to drive during DPF regen could well be down to the EGR. Due an oil change shortly and will ask for the EGR to be checked. But with no fault codes the garage answer is it would show up but will be saying it needs checked. Can the EGR valve control solonoid be disconnected? Would that keep EGR valve off/closed?

[Taxiboy]

Took the car to the diesel specialist and it is the glowplug that’s done. That explains the diesel knock mainly after I start from cold. Whether it improves the car overall I doubt it but never know. Also I wonder if a manufacturer software update might be worth getting done. Was told EGR valve is fine. After I cleared the pending fault code the car actually drove well all day.

[mickeybo]

Map sensor check 5v feed on tract 3 check the insulation between tracts 5 & 6 check tracts 2 & 5 if the voltage is not approximately 0.6vs plus/minus0.1 replace sensor these maf sensor plugs are a common insulation fault

[Taxiboy]

Thats for that I will give that a try. My Engine management light on this morning both pending and stored code from the Glowplug but also a pending code P20EE- SCR Nox Catalyst Efficiency Below Threshold Bank 1.. Is that an Adblue issue does anybody know?

[Crasher]

That means the Selective Catalytic Reduction system is not working as well as it should, the Reductant (AdBlue) is what the cat uses to reduce the temperature at which the soot particles will combust and turn into ash, it may just need a forced regeneration, it could need cleaning or the injector may be faulty which causes a buildup of urea crystals on the monolith that sets like concrete so the SCR unit has to be replaced. We use an endoscope down the injector port if we suspect a blockage.

[Taxiboy]

Better keep an eye on that but I wonder if the my failed Glowplug on first start of the day has caused it. The code never came back after I cleared it and I have engine off and restarted lots of times today and no codes. Also had a long job which meant going over country roads. For the past number of days the car has been good to drive and a fairly refined engine with good acceleration. Today it done an active DPF regen which took the usual 20 mins or thereabouts. I did notice before it a rather gruff sounding engine (similar to how the car has always been) and to be quiet and fast again during regen. Afterwards for the rest of my day the car drove great and refined and even with air con on I had to overtake a car and later a tractor and car in Sports mode was a flying machine. That’s the best the car has ever been. Can’t explain why it’s got better although I did pull out the Turbo Pressure sensor and connected it back the other day. See how it is tomorrow it’s got a mind of it’s own.

[Crasher]

The glow plugs are used in combustion strategy and not just cold starting and could cause a rise in soot emissions which could upset the DPF but not on an SCR car YET in my experience.

[Taxiboy]

Interesting crasher. Perhaps this has been my problem all along. For months now diesel knock was noticeable especially when cold but it’s only this week the first code has shown. The SCR code has not returned so far. Car due oil and filter change very shortly so will get the garage to fit the new Glowplug while it’s there or do it myself. Might get some DPF doctor sprayed into the DPF to give it a good clean although the previous model was a lot easier to get into at the pre car O2 sensor.

[Crasher]

Don't use snake oil treatments, just top quality fuel such as V Power.

[Taxiboy]

Replaced Glowplug myself at the weekend which I got from VW dealer. Also stuck V-Power in it on Sunday (half a tankful). Car has been driving well much better acceleration and quieter. However did notice today that diesel knock noise has returned but this time acceleration is still good. No codes showing though so will see how it is rest of the week.

[Crasher]

Once it has got clean V Power through the system it should improve, it takes a while for the ECU to react to the new fuel. One snake oil treatment I do use and have had some results with is Red Line 85 Plus (https://www.ebay.co.uk/itm/372726195108?hash=item56c83663a4:g:zg0AAMXQJ3xRc1-D)

[Taxiboy]

Thanks I could give it a bash. Today the P20EE code has returned and the car is noisier due to that. One thing after another with this car.

[Crasher]

I have a feeling the SCR is blocked, it is becoming a big problem, if you let it go too far the urea crystals set like concrete and thats the end of the SCR unit.

[Taxiboy]

If that’s the case it would need to be professionally cleaned. I do wonder if the NOX sensor is faulty perhaps causing this issue. Know a few drivers both with Superb and Passat never had issues with SCR.

[Crasher]

We take the injector out and check with a borescope, if the SCR become blocked with Urea crystals, it cannot be cleaned and has to be replaced.

[Taxiboy]

Will be booking it in for oil service next week hopefully there’s space. Will also get them to check the injector. Should have got shot of the car months ago but too late and a lot of money for something a few years newer now. This could end up a costly one.

[Taxiboy]

Well had the car in 2 garages one for service and later another one for DSG oil change. The SCR codes has never returned. Both garages say the car is fine both claim no fault codes so it must be fine. Yet the car sounds awful quite often and even at tickover the ticking noise is getting noiser again. Both Nox sensors seem fine along with O2 too. MAF and MAP also seem to be reading correctly. Thrown enough money at it. I was in a Skoda Superb last week 19 plate and same engine (manual only difference) and it was night and day to my own car.

[Pytrak]

Hello. Any updates on your car? Did you sort the problem? I’m afraid I have the same issue with the noise and scr fault.