Strange problem 2.5TDI FWD

[VagCabby]

Hello, i have a very strange problem that i have never come across before, being my first diesel car im not 100% sure on the throttle system, had the car a year with no issue at all. Over the past week i have been having this strange issue when cold, if i accelerate and then let off the accelerator the car will carry on accelerating for approx 1 sec without me touching the throttle! once the car is up to temp it seems to be fine. I have also noticed that the car is very under powered at the moment and seems to be lacking torque but then suddenly at about 3k rpm the torque kicks in massively! Again when the car is up to temp this doesnt seem much of an issue. Its doing my head in. Could this be a throttle position sensor problem? i have run VCDS on the car and no issues are flagging up apart from a brake pad warning which is due to a broken brake pad sensor wire which i broke today Any help would be great.

Cars done 145k, full service history and has been serviced by me every 6k for the last year and most recent service was done last month including, air, fuel, oil filters and oil

thanks
Jamie

 

[madmike01uk]

Have you cleaned the throttle body?

 

[adamss24]

Have you cleaned the throttle body?

No throttle body as such on the v6 tdi engine, they onlu have a anti shudder valve fitted just before the EGR valve/intake manifolds...

 

[PAULF]

This sounds a little like my B5 AFB V6 TDi engine.
Does the 'acceleration' only happen at low revs?
Going up my dirt track, if I am going slowly, the engine RPM is below minimum idle sometimes. When I lift off the throttle, the ECU now takes it's signal from its own zero setting, which is higher than the current engine speed so it raises the RPM to minimum and the car accelerates a little. When it is cold, the idle RPM is slightly higher so the effect is more noticeable.

I am also sure (Chris may be able to chip in here) that the fuel timing is advanced via the advance solenoid/cold start valve in the pump depending on the temperature. This seems to give a louder 'diesel bark' than when warmed up, and also coincides with less low down power.
Unfortunately I have been unable to find much info on the cold start valve position/duty cycle yet.

This phenomenon may have become noticeable now due to the recent cold weather, or possibly you have an issue with your temp sensor.

 

[VagCabby]

This sounds a little like my B5 AFB V6 TDi engine.
Does the 'acceleration' only happen at low revs?
Going up my dirt track, if I am going slowly, the engine RPM is below minimum idle sometimes. When I lift off the throttle, the ECU now takes it's signal from its own zero setting, which is higher than the current engine speed so it raises the RPM to minimum and the car accelerates a little. When it is cold, the idle RPM is slightly higher so the effect is more noticeable.

I am also sure (Chris may be able to chip in here) that the fuel timing is advanced via the advance solenoid/cold start valve in the pump depending on the temperature. This seems to give a louder 'diesel bark' than when warmed up, and also coincides with less low down power.
Unfortunately I have been unable to find much info on the cold start valve position/duty cycle yet.

This phenomenon may have become noticeable now due to the recent cold weather, or possibly you have an issue with your temp sensor.

Mine does it anywhere in the rev range, if I just blip the throttle at 2k rpm the car will accelerate by itself upto 2100rpm then decelerate as usal. It's weird its almost like I'm putting my cruise control on and then using my cruise stalk to accelerate the car for a second then turning it off!

 

[Az]

I'm wondering if it could be the needle lift sensor on injector number 3, these 2.5's are quite common for this. I had to have mine replaced, not a cheap part at around £300 plus fitting and coding (the needle lift sensor is part of the injector so the whole injector has to be replaced)

Does it sometimes 'race' on idle? At around 600rpm? And then settle itself after about 10 seconds? Or does it still want to pull away at low speeds on its own? If so, that's signs of the lift sensor. It doesn't harm the engine when it goes but can or will lead to wrong fuel reading which will result in poor mpg.

 

[RobMorris]

This has happened to me a few times over the last few weeks.
Hasnt done it for a few days.. come to think of it since I change my coolant temp sensor.. - coincedence?

 

[VagCabby]

I'm wondering if it could be the needle lift sensor on injector number 3, these 2.5's are quite common for this. I had to have mine replaced, not a cheap part at around £300 plus fitting and coding (the needle lift sensor is part of the injector so the whole injector has to be replaced)

Does it sometimes 'race' on idle? At around 600rpm? And then settle itself after about 10 seconds? Or does it still want to pull away at low speeds on its own? If so, that's signs of the lift sensor. It doesn't harm the engine when it goes but can or will lead to wrong fuel reading which will result in poor mpg.

Well i noticed on the way home from work today the idle seems to be at about 1100 rpm for a good while, it took 10 mins of driving untill it idled properly at around 800rpm. im going to scan the car again and see if anything come up. I did have 3 leaky injector pipes prior to this which i have just replaced and all the leaking has stopped so i don't know if this has anything to do with it......

 

[VagCabby]

Well Rob i think you were right! started the car just now and the engine management light was on, plugged in VCDS and there it was, coolant temperature sensor fault! i only changed this 3 months ago but i bought an ebay jobby. Im well happy as for the last 2 months my temp gauge has been reading 70 degrees and then dropping to about 60 with the heating on, i was sure it was the thermostat. I have just gone to my Audi dealer, bought a genuine sensor and bam, temp gauge reading 90 degrees and my car is no longer doing the silly throttle thing and it also feels like its gained 60hp! haha very happy. £40 well spent.

 

[RobMorris]

Result!

 

[Az]

Nice! That's a touch!

(oh by the way I meant 1100rpm not 600rpm in my earlier post! So used to driving trucks all day that idle at 600rpm)

 

[VagCabby]

Nice! That's a touch!

(oh by the way I meant 1100rpm not 600rpm in my earlier post! So used to driving trucks all day that idle at 600rpm)

haha i thought 600rpm sounded very low! but yeah genuine CTS has sorted all my little engine issues i had including my poor starting in very cold temps which i thought was my glow plugs!

 

[Az]

That's good then mate, did you have the idle/pulling on its own with your temp gauge reading at 90 degrees as well?

 

[VagCabby]

Cheers fella, before i changed the temp sensor my temp gauge very rarely got to 70 degrees, let alone 90. Since i have changed the sensor my temp climes to 90 as it should and i am getting none of the pulling with the throttle let off . Fuel consumption is back to how it used to be averaging around 35mpg on my daily commute to work through towns. for the last 2 weeks the car has been averaging 23mpg!!!!