A6 2.0tdi ultra intermittent hesitation

[Autoquip]

Good afternoon, I am new to the forum so apologies if this is posted in the wrong section. 
 

I have an A6 avant 2.0tdi ultra with the CNHA engine and manual transmission. I have an intermittent issue which is hard to explain but I will do my best to explain what is happening. On occasion when pulling away from a junction it’s like it cuts the fuel off for a split second and then goes back to normal again, it can do this either pulling away from a junction or driving through the town at 20mph or even cruising at 50mph. It doesn’t seem to have any pattern to it and I can’t “make” it happen. Sometimes it can do it once then be ok for a few days and then do it again, it doesn’t do it every time I drive it.  I’d say it seems to be when I am feathering the throttle. I have scanned for codes and there are no trouble codes present, I have tried recording live data for various sensors but I can’t seem to find any discrepancy in the readings. To be honest I wish it would just breakdown at the roadside then it’d be easier to fault find. I have been chasing this fault for about 3 months now and I really am stumped. I don’t want to light the parts cannon as I rather diagnose faults before I replace parts but I am struggling with this one. 
 

any help appreciated 

regards

Davie

[cliffcoggin]

Transient faults like that are a devil to diagnose, and as you know, replacing parts at random could cost you many times the value of the car. I suggest you get it professionally tested as some code readers are better than others.

[Autoquip]

Hi thanks for the reply, embarrassing as it is I run my own garage! I understand about the different diagnostic equipment, I am using an up to date Autel maxysis ms909 which I use on a daily basis at my garage. The only other time I have had an issue with no fault codes was a VW Amarok, crank non start with no fault codes and it turned out to be a faulty cam sensor, however I picked that up reasonably quickly as it had no reading on live data when cranking the engine. I am still recording live data whenever I drive it, when I get to the bottom of it I will post here with what I find 

[spartacus 68]

Similar post on AudiVW forum a few years ago, but it was going into limp mode. Replaced fuel pressure regulator and fuel pressure sensor. There was no update.

I wonder if the lift pump in the fuel tank might be an issue?

[cliffcoggin]

15 hours ago, Autoquip said:

 when I get to the bottom of it I will post here with what I find 

Please do.

[Autoquip]

Spartacus I had thought about that fuel pump but couldn’t work out why it would cut the fuel for a split second. I think it would have to stop for a period of time before the engine would notice it due to the high pressure pump. However definitely worth checking out. 

[cliffcoggin]

Did you get to the source of the fault Davie?

[Autoquip]

Nope not yet 😡. To be honest I’ve been busy with other stuff the past few days so haven’t really been in the car. I’ll get the diagnostics back on it probably over the weekend. I’ll keep in touch 👍

[Autoquip]

Evening folks, so might have made some progress tonight. On the drive home I was monitoring and recording live data for various sensors. My wife was in charge of pressing the “flag” button if the fault happened. This time it did happen and sure enough poised like coiled spring my wife hit the flag button. When I played the recording back, at the point that was flagged there was a dip in both the cam and the crank sensor readings! So I nudged the readings frame by frame until I got the frame before the hesitation the frame at it and the frame after. Luckily I was also monitoring crankshaft signal status as on the frame before and after the hesitation it’s reading is GAP FOUND but at the point of the hesitation it’s reading is WAITING GAP. I’m not going to get over excited for now. However, at least that gives me a starting point. I’ll remove the sensor and check the ring and flywheel condition as best as I can and then I’ll order a genuine sensor from Audi. I’ll run the car for a week or so and then I’ll come back with an update. 
 

I’d say this rules out a fuel issue, if it was lack of fuel I would still expect it to see the gap for the crank sensor. The only other possibility I can think of is that something is switching the ignition off for a split second but how unlucky would I have to be for that? (I do have a lot of bad luck to be fair). My only slight worry is that it’s visible on the graph for the cam, in my head I can make arguments as to why it would and why I don’t think it should be visible on both graphs. But let’s start with the crank sensor and take it from there. 
 

very sorry for the extremely long post, I’m not on many forums, I think they are a great source of information and I’m trying to be as informative as possible. 
 

wish me luck 🤞

 

Davie 👍

 

[Pan]

Thanks for sharing,

I serviced a car the other week and the guy described a jolt he was getting once per long drive.

Obviously a difficult thing to diagnose, particularly if it's not your own car and no fault codes. Changed fuel filter and DSG oil as part of it but he said the problem remained, occuring once on a 2 hour drive.

I asked him was it a smooth loss of power or sudden. He said it was a harsh jolt. How would you describe your hesitation?

Yes agree forum are a great source of info. These are getting on in age and mileage so great to have the info on them. 

Good luck with the sensors.

 

Brian

[Autoquip]

Hi Brian, my car has manual transmission so sort of takes that out of the equation for me. 
 

my car varies to be honest sometimes it’s very slight other times it’s quite a jolt. 
 

I'm going to try and get it on a ramp this morning, I’ll post later and let you know what I find. 
 

cheers 

[cliffcoggin]

Good luck Davie. You deserve it for the effort you put into investigating the fault. I seem to recall reading that crank angle sensor is a common fault.

[Pan]

1 hour ago, Autoquip said:

Hi Brian, my car has manual transmission so sort of takes that out of the equation for me. 
 

my car varies to be honest sometimes it’s very slight other times it’s quite a jolt. 
 

I'm going to try and get it on a ramp this morning, I’ll post later and let you know what I find. 
 

cheers 

Cheers. Hope access to them is ok!

Edited 3 hours ago by Pan

[Autoquip]

Just beats me how these cars can have so much fancy electronics in them and this sensor has been playing up for months and yet it has never put on the engine management light or a fault code. I’m just thankful it’s not a customers car because this would have taken hours upon hours of diagnosis and I would have given it back and only charged a couple of hours. Difficult to charge all the hours when you don’t have a solution.