1.9 TDi intermittent jerkiness when driving....

[Nessy]

Hi,
My trusty AFN TDi has now clocked up 286k miles but has begun to run roughly over the last few weeks.
It's hard to describe, but it seems to manifest itself as a jerkiness in acceleration, almost as if something is holding the car back, which comes and goes very rapidly.
The speed with which it comes and goes leads me to think it's something electrical, rather than say a fuel or a turbo issue.
This is a brief list of work done to the car recently /observations of how the car is running;

No sign of any vacuum line leaks or intake leaks.
No smoke in exhaust.
No dash lights or limp mode symptoms.
Fuel economy as per usual (50mpg on usual 80 mile daily motorway round-trip).
No coolant loss/use.
Engine temperature exactly as normal.
Fuel filter changed .
Air filter changed.
Exhaust side of turbo cleaned recently (Mr Muscle technique).
MAF sensor changed (no difference to running).
When the engine is cold the hesitation seems absent, it only appears when the engine is fully at operating temperature?

Apart from the jerkiness/hesitation the car is running perfectly normally.
No fault codes found when scanned with OBD diagnostic tool.
I'm at a loss as to what the problem could be , can anyone offer any tips?
A total stab in the dark (and yes I know I'm clutching at straws) is the electronic accelerator pedal?
I know it contains a potentiometer and as the car is getting on for 18 years old it must've been pressed hundreds of 1000s of times, would a faulty pedal show up on the OBD scanner?
Thanks!

 

[aragorn]

The throttle pedal has two potentiometers which are setup opposing each other. If one pot goes faulty or erratic, the ECU would see it, and bring on a fault lamp and drop you to limp mode.

Could be an air leak somewhere in a fuel line? Bubbles of air would probably manifest as a slight missfire?

 

[Fire26662]

If you can read live engine data using a code reader, try recording the MAF response and boost pressure. I have come across 4 AFN's in the last 6 weeks or so, where the MAF is failing or failed and they all provide similar but different sets of symptoms. The one was very intermittent and produced a misfire like symptom as the ECU will substitute a set value. A good check along with the previous reply too.

 

[Nessy]

Thanks for the replies....I didn't really think it was the accelerator,just a stab in the dark!
The problem first began about a month ago on my daily drive to work......accelerating fairly heavily out of a 50mph average speed zone the car would 'hiccup' a few times, definitely not normal behaviour.
There doesn't seem to be any particular rev range where this hiccup/hesitation occurs and it also seems to be intermittent, although a lot more prevalent when the engine is fully warm and definitely getting worse.

 

[Kevinb5]

Possibly the coolant temp sensor too,would make sense if it's only doing it when warm,my afn didn't show any fault code when the CTS was gone but would never reach 90 degrees either until I changed it out

 

[Nessy]

Cheers, I changed the CTS a few years ago when the dash gauge began giving odd readings.
The gauge reads 90 at all times though, so surely the sensor must be OK?

 

[aragorn]

there are two sensors in one package, one feeds the dash, the other the ECU. So its entirely possible for it to display 90 but be sending incorrect readings to the ECU. I seriously doubt its that though.

If you want you can view the engine coolant temp with VCDS to ensure the engine sensor is doing the right thing.

 

[Nessy]

Thanks Aragorn.
Think I've found the culprit, the N75 valve, the grey-topped 1H0906627A turbo boost pressure control valve.
After a lot of reading around found reference to the black-topped N18 valve (1H0906627, part of the EGR system which I've got blanked off on my car) up on the bulkhead being very similar to the N75 and almost interchangeable.
So I swapped them over and the hesitation/stuttering has gone.
The car isn't as nippy as it was with the grey N75 in-situ (apparently the N18 has a smaller nozzle inside the valve so it doesn't allow as much boost to build-up.....or something like that anyway) but I can live with it running like that until the replacement N75 arrives.